Rain Catchment Tarp Yield Calculator

When it’s raining onto your tarp, all that rain appears free and plentiful. Seems like it will fill up your containers right away. Guess what? Most of the time it doesn’t. Most of it gets absorbed by the ground, splashes around, or was never positioned well in the first place. So before your water hit the bottom of your container, you’re probably losing half of it.

Enter your tarp size and how much rain fell, and the calculator above do the math for you. You won’t have to guess at conversions and coefficients anymore. It’ll show you just how far off your actual drinking water is from total rain you collected.

How to Calculate Your Rainwater Harvest

The biggest problem with how most people estimate their collection area is that they measure the whole area of fabric instead of just the footprint it would project. For example, if you’re using a A-frame shelter, the sides cover much more area than what’s on the ground. Rain comes down vertically so only what casts a shadow will catch rain. That makes a huge difference in how many liters you’ll collect.

The tool can accepts custom, rectangle, or triangle input depending on your view so you can adjust accordingly. Again, that little detail make a big difference if you want to get the most out of a quick storm.

The kind of material you use will also have a big impact on how much runs off top. Composites made from Dyneema and vinyl are both pretty slick, so they tend to shed water. Mesh setups and waxed canvas rig will allow water to run right through them. They can also absorbs moisture into the fabric, reducing your yield. Each fabric has a specific runoff coefficient listed in the reference table. Keep that in mind, a super thick tarp could appear full but be holding gallons within the fibers. This will become more apparent as it begins to run off after a couple minutes.

Physically, whether it sets up well depend on pitch and edge capture. If there’s too much pitch (like a flat tarp), then water will pool, resulting in overflows and splash loss. Steepening the sides improve drainage by sending water towards a corner or gutter, but it reduces projected area. How fast do you want it flowing? And how large an area should be collecting it?

Do you wish to filter initial runoff? By diverting the first flush, you eliminate the initial dirty water carrying pollen and other dust. This can save gallons, but run the risk of contaminating your supply with hard-to-remove debris.

The last limiting factor in the system is storage capacity. You can catch hundreds of gallon in a heavy storm, but if your barrels only hold thirty, the rest runs to waste. The calculator will cap your usable output based off your tank limit. It reminds you that you need to balance how much you collect with how much you store.

Evaporation occurs in heat and there are always spills when transferring. It’s smart to plan for a safety margin. It grounds your expectations in what you can actualy keep as opposed to what fell from the sky.

Collecting rain is all about keeping it simple, keeping waste down and keeping expectations in check. At once you’re battling impurities, surface tension and gravity. This is where knowing your true harvest can make all the difference: whether to hope for more rain or to pack along some extra water. It’s not guessing anymore; it’s a plan that works.

The next time a storm rolls through, you’ll be able to count on exactly how much you’ve stored up for when it does. That peace of mind counts for something. And so you find yourself standing under the tarp once more; but this time you know where each drop goes.