Rainfall Calculator
Calculate how many gallons of water a roof catches from a given rainfall amount, for rain barrel and cistern sizing.
How this calculator works
A roof is a giant funnel. Every square foot of it that faces the sky catches rain, and that rain runs down into your gutters. This calculator takes the size of that funnel and the depth of rain falling on it, and converts the result into gallons you can plan storage around.
Formula: Gallons collected = Roof catchment area (sq ft) × Rainfall (in) × 0.623.
Worked example
A 1,500 sq ft roof catching 1 inch of rain:
- Catchment area: 1,500 sq ft
- Rainfall: 1 in
- Gallons: 1,500 × 1 × 0.623 ≈ 934.5 gallons
Notes
Use the roof’s footprint area, not the sloped surface area, as your catchment area. Rain falls vertically, so what matters is the horizontal area it lands on when viewed from above — a steep roof and a flat roof with the same footprint catch the same amount of rain, even though the steep one has more actual roofing material.
Real-world collection runs below this theoretical maximum. Gutters can overflow and spill water onto the ground during heavy downpours instead of channeling all of it into storage. Some water evaporates off the roof surface before it ever reaches the gutter. And many systems use first-flush diverters, which deliberately discard the first bit of runoff from each storm — the water that carries dust, pollen, and debris off the roof — before letting the cleaner water through to storage.
How to use
Measure your roof’s horizontal footprint in square feet and enter the rainfall depth in inches for the storm or period you’re planning around. The calculator returns the theoretical maximum gallons collected, which you can then size your rain barrel, cistern, or storage tank against.
A theoretical maximum. Real-world collection is typically 75-90% of this figure due to gutter overflow, evaporation, and first-flush diversion — size your storage with some margin.
Frequently asked questions
How many gallons of rain can I collect from my roof?
Multiply your roof's catchment area in square feet by the rainfall in inches, then multiply by 0.623 — the gallons captured per square foot of catchment per inch of rain. A 1,500 sq ft roof catching 1 inch of rain yields about 934.5 gallons in theory, though real-world collection is somewhat lower.
Which roof area do I use — the flat footprint or the sloped surface?
Use the roof's flat footprint area, not the larger sloped surface area. Rain falls straight down, so what matters is the horizontal area the rain lands on as seen from above, not how much roofing material covers that area.
Why do I collect less than the calculated amount in practice?
Several things reduce real-world collection below the theoretical maximum: gutters overflowing during heavy downpours instead of channeling all the water, evaporation off the roof surface before runoff reaches the gutter, and first-flush diverters that intentionally discard the initial dirty runoff before it reaches storage.
How big a rain barrel or cistern do I need?
Size your storage against your catchment area and typical rainfall per storm, then add margin since only 75-90% of the theoretical maximum is usually captured. Check your local average rainfall per event and multiply through the same formula to see how much storage a typical storm would fill.
Estimates only. Verify quantities with your supplier before purchasing.