Rainwater Harvesting Calculator
Litres or gallons a roof collects from its plan area, a rainfall depth and your runoff coefficient, less first flush, with a monthly table and tank check.
The yield a catchment gives per rain event, per month and per year, and how many days it covers a demand — from your rainfall records and your roof, with the coefficient stated.
Example: A 100 m² roof in 50 mm of rain at a runoff coefficient of 0.85 yields 4,250 L (1,123 US gal) — 28 days at 150 L/day. In imperial: 1,000 ft² × 1 in × 0.85 = 530 gal.
One square metre, one millimetre,
one litre.
The yield formula, what the runoff coefficient covers, and how the tank check should be read.
Yield
Rain falling on a roof is the plan (footprint) area times the depth of rain — the slope of the roof does not change how much falls on it. One millimetre on one square metre is exactly one litre, which makes the metric arithmetic direct; in imperial units an inch on a square foot is 1/12 ft³ = 0.6234 US gallons, the “0.623 rule” of the harvesting manuals. Multiply by the runoff coefficient and subtract any first-flush diversion and you have the net yield.
The runoff coefficient is yours
Not all rain reaches the tank: some wets the roof and evaporates, some splashes or overshoots the gutters, and porous or vegetated roofs hold more. The coefficient bundles those losses and depends on the roof material, gutter design and rain intensity; published values differ by source and are usually ranges. Take the figure from your roofing or harvesting standard, or measure it. A coefficient of 1 is flagged as lossless.
Monthly table and tank check
Give twelve monthly rainfall depths from local records and the page lists the net yield per month and the annual total. First-flush diversion is deducted once per row, so for a month with many events the table understates that loss — a note says so. The tank check is deliberately simple: the days a single yield covers a daily demand, and the storage needed to bridge a dry spell of the length you specify. Proper tank sizing balances daily supply and demand through the year (a water-balance model) and is beyond this page.
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SOURCES
- Texas Water Development Board, The Texas Manual on Rainwater Harvesting (3rd ed., 2005) — yield = area × rainfall × efficiency; 0.623 gal per ft² per inch (method only; coefficients not adopted)
- NIST Special Publication 811 (2008), Guide for the Use of the SI — gallon (US) 3.785 411 784 L, cubic foot 28.316 846 592 L
Last reviewed 19 September 2026. How results are checked: How we verify.