Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Gallons collectible
897
Gallons per inch of rain
897
Barrels needed to capture it all
17
Overflow past first barrel
842
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter roof footprint area (sq ft).
- 2Enter rainfall amount (in).
- 3Enter collection efficiency (%).
- 4Enter storage capacity (gal).
- 5Read your gallons collectible on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
Every square foot of roof collects roughly 0.62 gallons of water per inch of rain that falls on it, since one inch of rain over one square foot equals 0.623 gallons. Multiply that by your roof's horizontal footprint (not the sloped surface area) and a collection efficiency factor that accounts for splash-out, evaporation, first-flush diversion, and gutter overflow during heavy storms, and you get a realistic harvestable volume. This is the standard sizing calculation used by rainwater harvesting installers to size cisterns and by stormwater engineers to estimate runoff reduction from a rain barrel program. Most residential setups run 75-85% efficiency; a well-designed system with first-flush diverters and screened gutters can hit 90%.
Worked example
Using the values the calculator loads with:
Inputs
- Roof footprint area: 1800 sq ft
- Rainfall amount: 1 in
- Collection efficiency: 80 %
- Storage capacity: 55 gal
Results
- Gallons collectible: 897
- Gallons per inch of rain: 897
- Barrels needed to capture it all: 17
- Overflow past first barrel: 842
What each field means
Inputs
- Roof footprint area (sq ft)
- The roof footprint area used in the calculation, measured in sq ft. Starts at 1800 sq ft so you have a working example on load.
- Rainfall amount (in)
- The rainfall amount used in the calculation, measured in in. Starts at 1 in so you have a working example on load.
- Collection efficiency (%)
- The collection efficiency used in the calculation, measured in %. Starts at 80 % so you have a working example on load. Accepted range: 30–95 %.
- Storage capacity (gal)
- The storage capacity used in the calculation, measured in gal. Starts at 55 gal so you have a working example on load.
Results
- Gallons collectible
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Gallons per inch of rain
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Barrels needed to capture it all
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Overflow past first barrel
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why use roof footprint instead of the actual sloped roof area?
Rain falls vertically, so the amount collected depends on the horizontal area it lands over, not the pitched surface area of the shingles. A steeply pitched roof has more material but the same catchment as a flat roof with the same footprint.
What lowers collection efficiency below 100%?
Wind-driven splash-out during heavy rain, evaporation off hot shingles before runoff reaches the gutter, first-flush diverters intentionally discarding the dirtiest initial water, clogged gutters, and overflow once storage is full all cut into the theoretical maximum.
How much rain does it take to fill a standard 55-gallon barrel from an average roof?
On a 1,800 sq ft footprint at 80% efficiency you collect about 897 gallons per inch of rain, meaning even a tenth of an inch (roughly 90 gallons) can fill a single 55-gallon barrel — most systems need multiple barrels or a cistern to capture a full storm.
Is harvested rainwater safe to drink?
Not without treatment. Roof runoff picks up dust, bird droppings, and roofing material residue (especially from asphalt shingles), so it's fine for irrigation and non-potable uses but needs filtration and disinfection for drinking water in most jurisdictions.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Rainwater Harvest Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/roof-rainwater-harvest
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/roof-rainwater-harvest" target="_blank" rel="noopener">Rainwater Harvest Calculator — RevenueLab</a> (2026).</p>
Source: [Rainwater Harvest Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/roof-rainwater-harvest) (2026).