Gallons of Rainwater Collection from a Tarp or other Square Footage

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 in perspective I’m going to use an ordinary Tarp of various sizes to give you an idea how much rainwater that you could potentially collect.

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

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).

Tip: Drape and secure tarp onto a piece of gutter, tilt gutter slightly and dump into container. This almost eliminates the affect of wind flopping the tarp around and missing the container.

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

Gallons Of Rainwater Collection From 1/10″ Rain

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

Gallons Of Rainwater Collection From 1/4″ Rain

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

Gallons Of Rainwater Collection From 1″ Rain

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

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!

The point being that it doesn’t take much to be able to collect LOTS of rainwater. By keeping an ordinary small tarp in your kit you will not only have the ability to make a simple shelter, but it will provide the ability for rainwater collection if you need it.


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:


X*Y = tarp size
R = inches of rain

EXCEL Spreadsheet Rainwater Collection Calculator:
Rainwater Collection

Rainwater Collection Systems

Illegal To Capture Rainwater?

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 |


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  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.

  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. Reading there may be regulations regarding rainwater harvesting here in Washington state got my dander up, so I looked it up and apparently it’s mostly related to reduced stormwater fees for commercial buildings that utilize rainwater harvesting. Here’s a link for anyone from other states interested in rainwater harvesting regulations:

    Phew! Now I can put my pitchfork back and storm the castle another day!

    It’s amazing how much water falls for free from the sky! Capturing it makes all the sense in the world.

    Great information!

  4. Yertle the Turtle has nothing on the .gov. It’s gotten better, since they approved a limited amount of rainwater harvesting as long as you register with the state. RME. I wish they’d approve gray water harvesting, but they treat it like raw sewage with settling tanks, annual inspections, massive fees, etc.

  5. 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.

  6. Ken – That state list from the site that you referenced (NCSLdotorg) included both bad and good issues. Some are states that promote the capture of rainwater and the reuse of gray water. Oklahoma passed a new law in 2012 to further encourage those uses of water.

    CD in Oklahoma

  7. 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!

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

  9. 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.

  10. 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.

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

  12. 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

  13. Ken maybe my reading comprehension is a tad off today but with your numbers above what is the TIME of that rainfall to collect that amount of water? I am assuming an hour of say 1/4 inch rainfall on a 10 X 20 tarp would more than fill up my 40 gallon Olive Barrel?

    Thanks for the work you put into MSB.

    1. NHM
      Pretty sure he is using it as a cumulative number, ie, if you get 1/4 inch or if you get 1/10 of an inch or 1 inch
      Generally when measuring rainfall its cumulative and not time related unless you are using it for a reference for intensity, ie 1” per hour vs 2.5” per hour or like what we had a few weeks ago with 3.5” in an hour, man that sucked

    2. I think its about volume, not necessarily time. If it rains 1/4 inch in an hour to fill your barrel. Then If it rains 1/8 inch per hour, it would take 2 hours to fill the same container.? I’ve had a 175 gallon tote overflow during a 15 minute down pour. And also had it take a couple days to fill with only light rain. I might not be understanding your question correctly tho’. If so, my apologies. 👌

  14. 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. Another thing, Harbor Tools puts out monthly flyers that have give-away things. One of these things is a 4 ft by 6 ft tarp. Go to the store and register to have the free catalog sent to you. Every month they have flashlights, tarps and a few other things you can get for free with any other purchase.

    2. 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.

  15. Kent
    Is there a method to post a pic ? Wanted to show my water collection system from the Ranch. Use it to water a dozen fruit trees on an auto-timer

  16. 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.

  17. 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.

    1. 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

  18. 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. Rwt sound good except that first tree getting the most water.

      Have you looked into a Renology 100 watt system and a direct (no battery) 12 volt pump?

      I set up a mobile system for a friend with a shallow well to be a back up water system when power fails. He could not set it up fixed as it was a community style well but wanted the ability to pump several gallons a minute when needed. I know he used it to drain a pool and as an emergency sump pump (I suggested he sanitize that pump before drinking water).

      Sureflo pumps with a repair kit are really good. All pumps need repair or replacement with grit etc.

    2. A slight upward kink in the hose going to the first tree may mitigate that. Look at the flow rates and put a rock under the first hose to see how it affects the flow rate.

  19. NH Michael
    I could use some assistance on that build out. I need a small pump for an aerator I need in my stock tank. Any assistance on the build is appreciated and I am more than willing to connect on email to discuss. I haven’t found a 12v pump with a low draw that doesn’t blow up my budget. I am also willing to add a battery as I would like it to run for a few hours at night. I need to keep it below $300 total for electronics only if possible.

    I will likely place a screw into the end of the water line so I can adjust the flow.

    1. 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

    2. 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.

    3. 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 :-)

  20. NH Michael
    Minimal feeezing in South Texas. We normally only have a few nights below freezing each year. My thought is to use an air pump to aerate the tank. Tank is 10’ deep and total of 3/4 acre. ROM 2.5M gallons from my math. As long as I have some aeration it will help the fish survive the hot summer months.

    The reason for the battery is to configure the pump to run from 3-6 a.m. every day. This is the time of the least oxygen in the water and when the fish need it the most.

    I’ve tried a few sites to determine exactly what I need with a solar set up and they all provide a different widely ranging answer. I’ve seen some
    Videos on line that appear to have a solution but they start at $500 and are solar day time use only. I figure if I am going to do it I want to do it once and correctly.

  21. I do have an 80 amp hr battery – new from auto zone I just picked up. I know it is not ideal however, I have so many items that need batteries when he offered it to me for $40 I couldn’t pass it up. Sooner or later I’ll have something that needs a battery, sprayer, tractor, extra for the travel trailer or run my CPAP off it.

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