Gallons Of Rainwater Collection From A Tarp (How-to Calculate)

Rainwater collection. How many of you do this? When it rains, there’s a stunning amount of water that falls from the sky! Rainwater collection adds up very quickly for every square foot that you’re able to capture from water runoff.

To put it into perspective, I’m going to use an ordinary tarp (various sizes /dimensions) to give you an idea how much rainwater that you could potentially collect with that tarp.

Note: A great prep item is an ordinary tarp (lots of uses!). Even a small 5×7 foot tarp can collect a lot of rainwater!

For example, by using a few lengths of cordage, tie up the tarp corners to some tree branches (or use support poles -sticks), slope the tarp downwards, and form a ‘V’ such that any rainwater will channel to a runoff point (into a container).

Even just 1/10″ of accumulated rain will produce rainwater collection of nearly 2 gallons with that small tarp!

Rainwater Collection Tarp

Update: Since the original publish date, I’ve noticed a product (tarp) specifically designed to collect rainwater. It’s available in various sizes. The corners are tied up, and the center has a standard garden hose connection to hook up your hose – routing the water wherever you wish. Clever…

Heavy Duty Drain Tarp
(view on amzn)

Rainwater collection tarp with garden hose attachment

How much rainwater can you collect? I’ve calculated several examples of tarp dimensions to give you an idea – as well as various amounts of total rain. The formula is written below (plug in to your own spreadsheet).

Gallons Of Rainwater Collection From 1/10″ Rain

(2 gallons) 5×7 Tarp
(3 gallons) 6×8 Tarp
(7 gallons) 9×12 Tarp
(9 gallons) 10×14 Tarp
(12 gallons) 12×16 Tarp
(17gallons ) 14×20 Tarp

Gallons Of Rainwater Collection From 1/4″ Rain

(5 gallons) 5×7 Tarp
(7 gallons) 6×8 Tarp
(17 gallons) 9×12 Tarp
(22 gallons) 10×14 Tarp
(30 gallons) 12×16 Tarp
(44 gallons) 14×20 Tarp

Gallons Of Rainwater Collection From 1″ Rain

(22 gallons) 5×7 Tarp
(30 gallons) 6×8 Tarp
(67 gallons) 9×12 Tarp
(87 gallons) 10×14 Tarp
(120 gallons) 12×16 Tarp
(175 gallons) 14×20 Tarp

How Much Rainwater Collection From A Roof

Rainwater Collection Systems
(view on amzn)

Let’s say you set up a rainwater collection system for the roof of your house (adapting to the gutters), and let’s say that the roof footprint is 28×40 feet. A 1 inch rain event will yield an incredible 700 gallons of water from that footprint! (just plug in your roof footprint dimensions to the formula below).

Downspout Diverter

The point being that it doesn’t take much to be able to collect LOTS of rainwater. By keeping in mind that an ordinary tarp can be used for rainwater collection. And, a downspout diverter from your roof gutter can be used for LOTS of rainwater collection from your roof. It could be designed to collect into a barrel (with an overflow runoff, and a hose spigot mounted near the bottom for using the collected water), or any sort of collection container (they make some big ones). Maybe use to water your garden too…

How to Calculate Gallons of Rainwater Collection


14.44 cubic inches in a cup
1728 cubic inches in a cubic foot (12 x 12 x 12)
119.7 cups per cubic foot (1728 / 14.44)
7.5 gallons per cubic foot (119.7 / 16)

Example: 1/10″ of rain on a 10 by 10 foot surface

CONVERT SQUARE FOOTAGE: 10 x 10 = 100 square feet
CONVERT INCHES TO FEET OF RAIN: (1/10) / 12 = 0.0083 feet
CALCULATE CUBIC FEET OF WATER: 100 x 0.0083 = 0.83 cubic feet
CONVERT TO GALLONS: 0.83 x 7.5 = 6.2 gallons

Generic rainwater harvesting formula is as follows:

(plug the formula into your spreadsheet of choice…)


X*Y = tarp size (or any dimension, such as your roof)
R = inches of rain

I suggest that you filter that accumulated rainwater before drinking it. As you know, I highly suggest the Berkey countertop water filter systems. I use it for all of my drinking water needs.

About The Berkey Guy – Authorized Dealer

Illegal To Capture Rainwater?

I originally published this during 2018. I don’t know the extent at which they may have changed since then. If anything, I suspect that it has probably gotten worse…

Amazingly, apparently in some areas (states) it is illegal (or restricted) to do rainwater collection. Water has become ‘big business’ and along with it comes regulations. Yep, water from the sky may belong to the government in your location…

I believe there are rainwater harvesting issues in the following states:

Arizona | Arkansas | California | Colorado | Hawaii | Illinois | Nevada | New Jersey | North Carolina | Ohio | Oklahoma | Oregon | Rhode Island | Texas | Utah | Virginia | Washington |

[ Read: How Will You Get Water From Source To Home After SHTF ]


  1. I live in Texas and have rain collection all over in plastic barrels even some for the wildlife with R panel roofs that drain to the barrel then to a metered trough. I was just in Hawaii, they have water catchment tanks just about on every house.

    1. Think twice of using a tarp. While good in thought you have to consider the following.

      The following is just me opinion. Only you can decide for yourself.

      1. In a situation where you need it the color will matter. Bright color like yellow will call attention.

      2. You need to take the wind in a storm into account. You will need to stake it up somehow and tie it down so the wind doesn’t grab it and blow it away.

      3. It has to be wide enough and high enough for you to drain the tarp either while it is raining or after.

      4. Where are you going to drain it too? If your going to go into a 55 gal drum you may run into issues getting all of the water out of the tarp and into the barrel. The siphon principal.

      So these are some thoughts you need to take into consideration before getting one.

      Good luck

  2. I have 2 thirty gal. barrels that my gutters drain into. I use this to water the garden. I have 150 gal. stored in barrels and several cases of bottled water for emergencies. If it gets bad enough the rain barrel water can be made potable and it is free. I am considering putting a third barrel on the gutters. I have access to diesel exhaust fluid (DEF) barrels. DEF is a urea solution. Does anyone know if this is safe to catch water to use on the garden?

    1. Hey car guy, looked up the MSDS for DEF on and you should be okay! Lots of info there on this fluid. It is just urea and water. If I remember correctly, urea is also used as fertilizer… Best of luck with your project!

    2. I have 10 55gl def barrels that I use. They are food grade and from my research they would be ok for drinking water

  3. You are actually ‘legally allowed’ to harvest 110 gallons of rain water per household in Colorado. So much for the big city progressives on the Front Range trying to control everyone, have to keep everyone tied to the municipal water systems to keep them afloat!
    Do not see the ‘rain barrel police’ out much on the western side of the state. Maybe a few people doing rain dances, (with everyone’s approval). Very dry here. Delta and Montrose counties in extreme drought. Glad we have major irrigation with class 1 rights.

  4. You can catch rainwater in Texas, it is perfectly legal. But it has to rain first. We are in dire need of rain in our area right now. When digging up potatoes, there was no moisture in the soil at all. It was completely dry all the way down. We are going to set up some type of rain water system. Hopefully we will get some rain soon!

  5. I was told by a waste water employee of a local packing house that whenever there is a 1 inch rain fall at the plant, they will get approx 1 million gal of water from 22 acres of flat surfaces ( parking lots, roofs, roads etc ).

  6. No need to buy a tarp, when you change your shower curtain save it. I have four saved now. They have grommets on one end to easily tie it to a pole.

    1. That is using your noggin sir. Some of the old ones were really tough, like vehicle floor covering tough. The bad things about tarps, especially the ‘cheapies’ is the sound of flapping in the wind.

  7. I live in the Toledo Ohio area, our problem is that there is way too much rain. I mean we get a LOT of it. I’m unaware of any problems collecting it. I don’t see hardly anyone collecting it. I do handyman / home remodeling and I’m in and around a lot of homes every year. In the last year I have only seen 2 rain barrels. Both homes were upscale homes.

  8. We have 2, 1550 gallon tanks and 1, 1100. Without them we would never have a garden or orchard.

  9. Good reminder, i need to get the roof on my shop and house connected to our pond
    Have about 3800sf between the house and shop and will feed into my reservoir nicely, reservoir is about 40x 75 and 7’ deep, been just feeding it from the county line but am looking at ways to reduce the bills. Is mostly for farm irrigation but in a pinch is good for the homes on our property

  10. Collecting rain water is easy as pointed out a simple tarp can give you gallons of it.

    But to turn it into drinkable water you need to filter out bird droppings and other nasty things.

    Look up how to make a first-flush device, these are easy and inexpensive to make, then a sand filter would be good before it even gets to your Berkey filter.

    1. Its funny ya know, i grew up on the big island on a remote ranch, we only had catchment water, huge redwood tanks, all open except for the huge roof over them that also coincidentally acted as the catchment, big old zinc coated galvanized corrugated roofing, with lead covered galvanized roofing nails, those tanks had all manner of debris floating in them, dead rats, mynah birds, bird poop, you name it,
      I think our filter was a bull durham bag tied to the spout on the kitchen faucet

      1. I’ll bet as a kid many of us drank water from a garden hose. How did we ever survive.

        But still I want to filter out bird droppings from any rain water I drink.

  11. What a positively GREAT article !!! :)

    water is my biggest issue in the preps dept.

    We have several shepherds hook in different heights. The tarp eyes hook right over the hook and they can be spaced wide at the tops height and spaced narrower at the lower end of the collection point. The tarp can be secured at the lower end to a tent stake to keep it from flapping about.

  12. Alright, next question. What water containers are MOVEABLE and will be able to store the water collected from these tarps ? Is a five gallon bucket (maybe daisy chained to several containers) the way to gain this water ? Or is there a better solution ? What I have pictured in my mind is a pair of 6″ PVC pipes (closed end – plug receptor opposite end) secured to a wheelbarrow frame. Pour the water from the buckets into the pipes and when ready, wheel over to your permanent barrel or other container of choice. Two four foot lengths of pipe would contain an amount of water that is transportable. 20 gallons is 165 pounds, more or less. Wouldn’t want it to get too heavy.

    1. Pop bottles are the way to have transportable water in a weight-manageable way.

      You don’t travel with large containers of water. It’s just too heavy to do so.

      Get a good water filter so you can gather it along the way if you feel bugging out is the way to go.

      Here in Northern Ohio you don’t need to carry more then a gal of water in your auto or a quart on you as it’s all over the place.

      If you feel the need to have lots of water on hand in a potential bug-out get a good size container that will fit in your auto. Let the cars engine do the work.

      Or get a motor-home and keep its tank(s) topped off. My motor home has 2 100-gal water tanks in it. That’s a lot of water if you use it sparingly and replenish along the way it will last a good while. But then where do you expect to go? And ask yourself about ways to gather water along the way.

    2. I have had good luck with the F-Style 5-gallon jugs sold by Uline, which tend to be cheaper than the jugs sold in stores for camping. You can tow two or three in a garden cart, but in a pinch you could use a kids wagon.

  13. Nice set up!! Looks like you painted those totes to reduce algae build up? Or to protect the plastic of the totes from UV?

    What sort of water pump system? I expect PV panels and 12 volt pump?

    Details Please RWwt! Thanks

  14. Total capacity is 500 gallons. 64 sq ft of roof should net me 40 gals per 1 inch of rain. I finished it this weekend. Plumbed it in and added cattle panels on the sides to keep the heifers from breaking my plumbing. Total cost for totes,plumbing, paint, new wood and I already the tin was $300. I am using a 9v auto timer that attaches to your garden hose to water when I am not there. I am still working the total # of seconds out so the flow will equal 1 gal per tree. So far it looks like 1 minute since this is a gravity fed system. Difficult to do since the first closest tree seems to be getting the most water. No pump only the 9v to replace every few months. Once full it will support 10 fruit trees on an automatic schedule of every 3 days with 1 gal of water for each tree. My calculations have it lasting at least 3 months without rain before I have to add water or pray for rain. We normally have sporadic rain of less than 3” during our summers in Texas. This is an experiment and my first year with it. I’ll keep this updated on how it goes.

    I do not have running water on my place and use a similar system off my barns. Supplies all my drinking water and salads water for the cattle. I have pumps on those since I have power in that part of the pasture.

    1. A real simple solution that would waste some water, but I think would work is to get a soaker hose that you wrap around the drip line of the trees and go from tree to tree. If you have something like a small orchard, you could step it up in complexity and cut shorter lengths of soaker hose with male and female HT ends with solid hose pieces from tree to tree for distribution with no/less water waste. I think that might even out the water distribution and you could leave it in place seasonally (unless you’re concerned the cattle would mess with it.)


  15. Rwt not sure just how cold it gets where you are. That detail would help. I would look at a two stage system given your reasonable desire not to spend a lot of money.

    1st Mother Earth News has a Solar Stock Tank Warmer that works well. You could build it to add a Solar Heat Grabber (Also Mother Earth News) to boost the amount of heat gained in deep winter but would need some Cattle Panels to keep your beef off them. In the non winter months you would have to disconnect the heat grabber and open a divert panel on the stock tank warmer as not to offer your cattle Tea for Dinner :-)

    2nd Stage Solar Stock Pond Aerator. Again need volume of tank and worst case weather (IE how many below 0 days in a row) to size it. I Google them and WOW what prices I saw…

    The Basics of a Battery tied Solar system is a Solar Panel to a controller to a battery to the inverter to the item to be powered. At the junction of the battery – item you can add timers as needed. If you stay 12 volt you do not need an inverter (MY Preference).

    My BIGGEST concern with a Battery tied system is that batteries do not respond well to freezing. I have built top and side insulated battery boxes 18 inches underground with some success as the ground temperature insulated from winter can keep them above freezing in most situations.

    Looking forward to your reply.

    1. BTW Mother Earth News sells USP sticks with ALL there Back Issues for a reasonable price and a LOT of great Hippie Level older working plans and salvage plans there.

      The usefulness of this information in a searchable format made me Faraday Cage a Laptop and Solar Recharger for it. If you want to know how the more recent Back to the Landers did it this Data Stick is it.

      Hopes it helps someone

      1. NHM
        The Mother Earth archive is definitely a good one,
        The other one that would be worthwhile would be JWRs archives from Survivalblog, lots of real good info on there on all sorts of topics.

        1. Nailbanger I have seen the JWR has done some archival from Survival Blog but I cannot find out if it was searchable as scrolling through pages looking is tough.

          So do you find it easy to use? Price?

        2. NHM
          From what i know, it is easily searchable just like his site or his store

  16. RWT so your not talking about a stock tank like us Eastern Farmers think of but a Fish Pond. Well minimum freezing is good low O2 saturation for your fish not so good, Do you have algae bloom issues given the cattle at the stock pond? That would be a subject for the Ag College nearby to answer. In fact given your TAX Dollars support that Ag College I bet they have some insight as to Solar Stock Pond Aeration.

    Your a little outside my skill set but if you relook those Videos and can determine the specs on the Solar Panels used and the actual air pump used we could reverse engineer it. For example if the air pump is a 110 volt unit and you can figure out the HP then we can figure out the inverter needed and how much battery capacity you need for 3 hours of bubbling in the wee hours and recharge needed for next night. Thus the Solar Panels needed is figured out.

    If the video is an Advertisement then getting the brand name and make could get us the information on HP and Voltage of the pump from the Internet.

    Do you have reliable night time winds above say 5 MPH? There are wind units just for the job of aerating fish ponds.

  17. RWT As far as your Fruit Tree situation an Renology 100 or 200 watt prepackaged system (depends on how much sunshine you get there in cloudy NH it would be 200 watt) wired per instructions (note Renology has You Tube videos for set up Use them) to a 12 volt RV water pump.

    I prefer a small Sureflo Diaphragm pump solid easy to rebuild and that pump tolerates minor voltage variations well (cloud shade etc.). A good pump that is reasonable in price. The actual plumbing and timer you seem to have well in hand.

    1. RWT too fast on the post button. That cold protected underground battery box I described above would work well in preventing the Texas Sun from over cooking it if that is a problem.

      Only visited Texas, a LONG drive to anywhere :-)

      1. NH. I found a no pressure auto timer for rainwater barrels that worked well enough. I have to be concerned about making everything cow proof. Thank goodness I don’t have any bulls as that is an entirely separate problem. This added cost all the time and has kept me from running anything solar in the pasture. The cows are not malicious eco warriors. Just big dumb and curious. Most of mine are pasture art that want their necks scratched. Right I have only lost 1 fruit tree in our 73 day drought with 100 degree temps. I believe it was lost due to an ant bed issue. My 12 other trees have survived the drought, freezes and grasshoppers each year. I have to expand my system as my goal is to have 30-40 fruit tress with them all producing over different months. Currently have 2 pear. 5 peaches (3 variety’s) 2 apple 1 fig and 1 pecan. Planting different ones each year when I catch them on sale. Very difficult to have produce apples in the TX heat and bad clay soil.

        I have expanded my entire off grid water system with over 7100 gallons I can make potable. I now added county water and ran 3200’ of water line last year so I have 3 different water sources. County, rain harvest and a 3/4 acre stock tank that holds approx 2.5M gallons.

  18. water is crucial for drinking and cooking, but don’t forget to factor in your needs for sanitation, ie, washing your butts off. personal hygiene is going to be a real thing if the water runs out long term, especially in the summer.
    if we don’t keep ourselves reasonably clean it can affect our health, fungal and bacterial infections can happen in a matter of weeks. if we get sick then we will become a burden to others and less than useful.
    i take a good bath once a week whether i need it or not. : )

  19. water is crucial for drinking and cooking. but don’t forget to factor in hygiene. hygiene is as just as important. if you don’t keep yourself clean you can get sick and ill from fungus’s and other nasties. if we can’t keep our health, then we become a burden to others and worst than useless.

  20. Thanks for the update Ken. Washington state lets each county control rainwater harvesting. As you can imagine, the dry counties east of the Cascades have more rules than here on the wet west side. In the county I moved from a couple years ago it’s now strongly encouraged, and homeowners are advised and sometimes assisted with installing rain gardens that catch the water and wick it into the soil. That’s because as the county became more populated thus more paved the stormwater control system was overwhelmed. . . .
    The catchment amounts you list, that’s for a full-face tarp, yes? Any way to calculate with a sloped or slack tarp? Thanks.

  21. I gots my huge stock of dehydrated water for the end times.
    Just add water..?

  22. Arkansas has a regulation that rain water catchment systems be designed by a registered engineer. It’s a health regulation. Lived here 15 years, in the Ozarks.

    Wife asked me today what we would do for sanitation if the power went out. She’s of a mind that our entire modern society will collapse. She may be right.

    I lived in Texas for 50 years. No regulations.

    Outfit in Dripping Springs, TX sells decent priced catchment kits. Thinking about visiting my son in Austin and checking them out. They also have filtration kits (sediment, U.V., etc.). Plastic-Mart.

  23. It was entirely illegal to capture rainwater in Utah, until 2011. The state lost a lawsuit to a car wash, claiming that the water falling out of the sky is also government owned. So they now allow rain water harvesting, limited to 200 gallons unless you register your water rights with the state.

  24. I collect a small amount of rainwater from my asphalt shingled roof to water my potted plants, decorative and food type. Is there pollution with petroleum products from the shingles contaminating this water?
    If there is, would allowing the water to “rest” for a while to allow any petroleum products to evaporate
    from it? Or should I boil the water or do something else to it before using it? The water has a slightly yellow tint
    and i’m wondering if that is a sign of petroleum contaminating the water. (I have no sense of smell) If the color is indicative of petroleum, it does not kill mosquitos as they eventually will thrive in the barrel!!
    I can see how a tarp collection would be much better than from a shingle covered roof since the only contaminants would be bird poop, insects, tree sap, etc.
    I live in a hot, arid environment. In an average year we get 12 inches of rainfall.

    1. More likely that the yellow is bird poop. People say not to use water off of an asphalt shingle roof, but I never noticed that the plants cared. If I was going to drink it, yes I would purify it first

  25. Tips and tricks …….I came across this awhile back….the 32 gallon hyper tuff plastic garbage cans at wallyworld are on sale for like 13 bucks… sit the can up outside and take the lid which is curved turn it upside down and drill several holes in the center handle (which is now the lowest point on the lid) attach a piece of fine screen wire over the holes, and place the lid back on the can upside down and bingo you have a water collection unit. I have used some of these cans for ten years outside and they are still going…tough tough tough.

  26. Another way to catch rain water, I use my freestanding outside clothes line which is five strand plastic coated wire the span is 16’ long by 50” wide….place a plastic tarp over the lines, and drape the downward at one end to funnel the water towards my previously mentioned plastic garbage cans, I keep the tarp in place with a lot of clothes pins, and after a storm or use I can easily remove the tarp for my next usage.

  27. Illegal to harvest rainwater is a new one to me.

    What about those who build a small pond that collect runoff water?
    My garden absorbs more water than grass or a driveway?

    Sounds like a “law” that has not been challenged. Water should fall under mineral rights and most all landowners have that.

    1. Idaho for one, if you didn’t register a pond on your property by the deadline, that pond is now the property of the state as is all rainwater as well.

  28. BamaMan, Rainwater harvesting rules vary from state to state, county to county, and municipality to municipality. Folks need to look up their own.

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