Rain Barrel Harvest Calculator — How Many Gallons of Water From Roof Runoff?
Wondering how many gallons of water your roof runoff can produce? Enter your roof's catchment area and rainfall to estimate how much rainwater you can harvest per storm, per month, or per year — and how many 55-gallon rain barrels it would take to hold it. Results update instantly in your browser.
Enter your numbers
Use the roof's flat footprint (length × width), not the sloped surface — rain falls on the horizontal projection of your roof.
Efficiency covers losses to gutter overflow, first-flush diverters, roof wetting and leaks. 85% is a typical default for a decent system.
Your results
| Gallons per inch of rain | – |
| Rain barrels to hold one event (rounded up) | – |
| Estimated annual harvest | – |
All results are estimates. Efficiency and rainfall vary — these are planning numbers, not guarantees. Barrels assume standard 55-gallon rain barrels.
How to use this calculator
Measure your roof's catchment area as the flat footprint of the part that drains to your gutters — roughly length × width of the roof above each downspout you plan to use. Then enter the rainfall you want to plan around: a typical storm (e.g. 1 inch), a wet month, or your average monthly rainfall.
Leave collection efficiency at 85% unless you have a reason to change it — a new, tight system may reach 90–95%, while an older or partial one may sit near 75%. The calculator updates instantly as you type, and the Calculate button re-runs the numbers.
Use the "barrels needed" result to decide how many rain barrels to chain together, and compare it to what your garden actually uses per week — most homes only need enough storage to bridge dry spells, not to catch an entire storm.
How it works
Gallons per inch. One inch of rain falling on one square foot of roof equals 144 cubic inches of water (1 ft × 1 ft × 1 in). Since one US gallon is 231 cubic inches, 144 ÷ 231 = 0.623 gallons per square foot per inch of rain. The calculator uses: gallons = roof area (sq ft) × rainfall (in) × 0.623.
Efficiency adjustment. No gutter system catches everything. Water is lost to wetting the roof surface, first-flush diverters that dump the first dirty gallons, gutter overflow during heavy bursts, and small leaks. The gross gallons are multiplied by your efficiency setting (85% by default), so a 1,000 sq ft roof in a 1-inch storm yields 623 × 0.85 ≈ 530 gallons — an estimate, not a guarantee.
Barrels per event. Gallons per event ÷ 55, shown to one decimal plus the whole number of 55-gallon barrels you'd need if you round up. The annual estimate repeats the same math using your annual rainfall figure: annual gallons = roof area × annual inches × 0.623 × efficiency. All outputs are estimates — real collection varies with storm intensity, roof material, and how clean your gutters are.
Frequently asked questions
How many gallons of water come off a roof in 1 inch of rain?
About 0.623 gallons per square foot of roof area. So a 1,000 sq ft roof collects roughly 623 gallons from 1 inch of rain at 100% efficiency. Real-world systems lose water to gutters, first-flush diverters and leaks, so multiply by your collection efficiency (around 85% is typical) for a realistic estimate.
How many 55-gallon rain barrels do I need?
Divide the gallons per storm from this calculator by 55, then round up. A 1,000 sq ft roof in a 1-inch storm harvests about 530 gallons at 85% efficiency, which would fill roughly 10 standard 55-gallon barrels. Most homes can't use or store that much from one storm, so size your barrels to your garden's weekly watering needs rather than a whole storm.
Why does the calculator default to 85% collection efficiency?
No roof-and-gutter system captures all the rain that hits it. Water is lost to evaporation and wetting the roof surface, first-flush diverters that discard the dirty first gallons, gutter overflow in heavy bursts, and small leaks at joints and fittings. 85% is a common planning assumption; use 75% if your gutters are old or partial, and up to 95% for a new, well-maintained system.
Is rain barrel water safe to drink?
Generally no — not without treatment. Roof runoff picks up dust, bird droppings, pollen, asphalt granules, and possibly metals from roofing materials. Untreated rainwater is fine for watering plants, lawns and gardens, washing cars, and outdoor cleaning, but it should be filtered and disinfected before any drinking or cooking use. Check your local regulations before drinking it.