rainwater collection gallons with a tarp

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 |

source: NCSL.org