Rainwater Catch Calculator

How to use the rainwater catch calculator

1. Choose the tarp size. The sizes are the same as in the tarp pitch planner: 2 × 3, 3 × 3, 3 × 4 or 4 × 4 m.
2. Choose the setup: a catch corner (one corner pulled low so everything drains to one point), a V-fold (a gutter-like funnel) or a flat tarp with several drip points.
3. Give the rain either as an intensity – light, moderate or heavy – or as your own figure in millimetres per hour, for example from a forecast.
4. Set how long it rains and how many minutes of first flush you discard before you collect.

The result shows the litres collected, the litres left after the first flush, the time until a 1 L bottle and a 5 L container are full, a bar for each rain intensity and a table comparing the three setups under the same rain.

Worked example: a moderate shower on a 3 × 3 m tarp

A 3 × 3 m tarp pitched with a catch corner, moderate rain of about 5 mm per hour for two hours, and a first flush of seven minutes. The calculator returns:

• 45.0 L collected before the first flush (9 m² × 5 mm/h × 2 h × 50 %)
• 42.4 L usable after the first flush; 2.6 L are discarded
• A 1 L bottle is full after 10 minutes, a 5 L container after 20 minutes – both including the first seven minutes
• For the same tarp and setup: 8.5 L in light rain (~1 mm/h) and 84.8 L in heavy rain (~10 mm/h)
• Setups compared under the same rain: catch corner 42.4 L, V-fold 33.9 L, flat tarp 21.2 L

Look at the heavy-rain figure: at about 45 L per hour you could fill a 5 L container every seven minutes. The tarp is not the limit – your containers and your attention are. Have a second pot ready before the rain gets heavy.

A smaller case: a 2 × 3 m tarp, flat, light rain for one hour. It collects 1.5 L and 1.3 L after the first flush; a 1 L bottle takes 47 minutes, and five litres would need more than three hours – longer than the rain lasts, which the result tells you.

How the catch is calculated

The physics is simple. A layer of rain 1 mm deep over 1 m² is a litre. So the gross catch is the tarp area in square metres times the rain in millimetres. For a 3 × 3 m tarp that is 9 L for every millimetre. The calculator multiplies the area by the rate in millimetres per hour and by the hours, then by an efficiency factor for your setup.

These factors need a plain explanation. The guide on collecting rainwater does not give percentages per setup. It says only that the arithmetic assumes a nearly flat tarp, that all the water reaches one point and that nothing is spilled, and it advises to halve the arithmetic and expect clearly less. So the catch corner is set to 50 %, which is that advice. The V-fold at 40 % and the flat tarp at 25 % are conservative assumptions of this calculator, not measurements. Your own tarp, wind and fabric will differ; use the results as a planning range.

The rain intensities come from the same guide: light rain is up to about 2.5 mm per hour, moderate about 2.5–10 mm per hour, heavy more than about 10 mm per hour, and the exact limits differ a little between weather services. The calculator uses 1, 5 and 10 mm per hour as representative values, the same rows as the guide’s table for a 9 m² footprint. If you know the actual figure, switch to millimetres per hour.

The first flush

A tarp that has been up for a day carries dust, pollen, needles, insects and resin. The first minutes of run-off wash that off, so you let it run away. The guide suggests the first 5–10 minutes of steady rain; the calculator starts at seven. It subtracts rate × minutes ÷ 60 from the run-off. In very light rain, wait until the water runs clear rather than by the clock. If the rain stops before the first flush is over, the result is zero – and that is correct.

What the result does not cover

• Wind and splash. Gusts blow water past the pot and shake pooled water off the fabric. Neither is modelled.
• Dry or new fabric. A dry tarp soaks up some water before it runs; a waxy new one may bead and run immediately.
• Tilt. The area used is the full length × width. A steep pitch has a smaller footprint than the fabric, a flat one almost none.
• Overflow. The result is how much falls into your system, not how much you actually store. If you cannot swap containers fast enough, water is lost.
• Rain that varies. A real storm is not constant. The calculator assumes the same intensity for the whole time.
• Dirty catchments. Under trees the water is canopy drip, tinted and full of debris. The calculator assumes open sky.

Safety: treat the water before drinking

Rain caught cleanly from a clean tarp is one of the lowest-risk wild water sources, but it is not sterile. Fabric, containers and hands add germs. Boil it, filter it or disinfect it before you drink. The guide to water purification explains which method fits which situation, and boiling is the most reliable one. Do not count on rain as your only source – rain you wait for is no plan.

Practical tips

• Pitch the tarp nearly flat on three high points and pull the fourth corner low, to about knee height. That is the simplest setup – see the setups in the rainwater guide and the geometry in the tarp shelter guide.
• Lead the water down a short cord into the pot so it does not splash. A smooth, clean drip cord does it.
• Wipe the catchment side of the tarp with a wet cloth if it is dirty, and keep it away from the smoke of the fire.
• Keep a second container ready: in anything but light rain a 1 L pot fills in minutes.
• If you are short on water, remember the other sources: the guide to finding water shows how to look for streams and springs, and the water needs calculator tells you how much you need per day.
• Do not lower the shelter so far for water that it stops protecting you.