Rainwater Harvest Calculator
Gallons of rainwater a roof collects per storm and per year, from footprint, rainfall depth and collection efficiency — with first-flush sizing.
Rainwater Harvest Calculator: with the default inputs, collected from this storm is 530 gal.
The plan area the roof covers, not the sloped surface — a steep roof catches the same rain as a flat one of the same footprint.
One inch is a solid soaking storm. Check your local record for what a big one delivers.
The runoff coefficient: 0.85 for metal or asphalt shingle, 0.75–0.8 for tile or gravel, plus losses to splash, wind and the first flush.
Your 30-year normal, from the NOAA climate normals for your station. The US average is about 3 in a month.
- If nothing were lost
- 623 galThe full theoretical yield at 100% efficiency.
- Collected per year
- 20,347 galFrom your average monthly rainfall × 12.
- Collected per month
- 1,696 gal
- Gallons per inch of rain
- 530Your roof's yield per inch, after efficiency — the number worth remembering.
- First flush to divert
- 12.5 galPer storm, before the tank fills.
- Days of supply per month
- 0
- Mains water displaced per year
- $0.00
Assumptions
- One inch of rain on one square foot is 0.6234 US gallons (0.08333 ft³) before losses.
- Roof footprint, not sloped surface area, is the catchment area.
- Collection efficiency bundles the surface runoff coefficient with splash, wind and first-flush losses; 0.85 suits a shingle roof with good gutters.
- Annual yield assumes rainfall spreads evenly across the year; a real system is limited by dry spells, not by the annual total.
| Rainfall | Gallons collected | Litres | 55-gal barrels filled |
|---|---|---|---|
| 0.1 in | 53 | 201 | 1 |
| 0.25 in | 132 | 501 | 2.4 |
| 0.5 in | 265 | 1,003 | 4.8 |
| 1 in | 530 | 2,006 | 9.6 |
| 2 in | 1,060 | 4,012 | 19.3 |
| 4 in | 2,119 | 8,023 | 38.5 |
One inch of rain on 1,000 ft² is 623 gallons before losses — a rain barrel is full in the first three minutes of a real storm.
How this is worked out
The formula
Volume = roof footprint × rainfall depth × collection efficiency In US units: gallons = ft² × inches × 0.6234 × efficiency (1 ft² × 1 in = 0.08333 ft³ = 0.6234 gal) Annual = average monthly rainfall × 12 × gallons per inch First flush = footprint × first-flush depth
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Roof footprint
- The plan area the roof covers, not the sloped surface — a steep roof catches the same rain as a flat one of the same footprint.in ft², m², yd² · 0 or more · defaults to 1000
- Rainfall in one storm
- One inch is a solid soaking storm. Check your local record for what a big one delivers.in in, mm, cm · 0 or more · defaults to 1
- Collection efficiency
- The runoff coefficient: 0.85 for metal or asphalt shingle, 0.75–0.8 for tile or gravel, plus losses to splash, wind and the first flush.a percentage · from 1 to 100 · defaults to 85
- Average monthly rainfall
- Your 30-year normal, from the NOAA climate normals for your station. The US average is about 3 in a month.in in, mm, cm · 0 or more · defaults to 3.2
- First flush to divert(under More options)
- 0.02–0.04 in of rain is the usual first-flush allowance — that water carries the roof's dust, pollen and bird droppings.in in, mm · 0 or more · defaults to 0.02
- Daily water use from the tank(under More options)
- Irrigation or household use. Leave 0 to skip the days-of-supply figure.in gal, L, m³ · 0 or more · defaults to 0
- Price of mains water(under More options)
- From your water bill. Leave 0 to skip the savings estimate.in dollars · 0 or more · defaults to 0
What you get back
- Collected from this stormmain answer
- If nothing were lost
- The full theoretical yield at 100% efficiency.
- Collected per year
- From your average monthly rainfall × 12.
- Collected per month
- Gallons per inch of rain
- Your roof's yield per inch, after efficiency — the number worth remembering.
- First flush to divert
- Per storm, before the tank fills.
- Days of supply per month
- Mains water displaced per year
What this assumes
- One inch of rain on one square foot is 0.6234 US gallons (0.08333 ft³) before losses.
- Roof footprint, not sloped surface area, is the catchment area.
- Collection efficiency bundles the surface runoff coefficient with splash, wind and first-flush losses; 0.85 suits a shingle roof with good gutters.
- Annual yield assumes rainfall spreads evenly across the year; a real system is limited by dry spells, not by the annual total.
About this calculator
A roof is a rain gauge with a very large collecting area, and the arithmetic is simpler than people expect: one inch of rain on 1,000 square feet is 623 gallons. Everything else is losses.
How to use it
Enter the roof's footprint — the area it covers on the ground, not the sloped surface. This trips people up, but it's right: a steep roof and a flat roof with the same footprint intercept exactly the same rain. If you're only plumbing two of four downspouts, enter the portion of the roof that drains to them.
Collection efficiency is the runoff coefficient plus system losses. The Texas Water Development Board's rainwater manual puts metal roofs around 0.85–0.95, asphalt shingle around 0.80–0.85, and concrete tile or gravel lower, because those surfaces absorb water and shed it slowly. Then subtract for splash-out at the gutter, wind-driven rain that never lands, and the first flush. 0.85 overall is a fair planning number for a shingle roof with decent gutters.
First flush matters more than the volume
Between storms, a roof accumulates dust, pollen, leaf litter, bird and rodent droppings, and grit off the shingles. The first fraction of an inch of rain washes all of it into the tank. A first-flush diverter — a standpipe that fills and closes before flow reaches the tank — throws that water away. Size it for 0.02 to 0.04 inches of rain, which is about 10 to 25 gallons per 1,000 square feet. It is the cheapest part of the system and the one that decides whether your tank stays clean.
Sizing a tank
Look at the results two ways. The storm figure tells you how fast a tank fills: a 55-gallon barrel on a 1,000 ft² roof overflows in the first few minutes of an inch of rain, which is why barrels are a gesture and cisterns are a system. The annual figure tells you what the roof can supply, and dividing it by your daily use gives days of supply. The binding constraint is almost never the annual total — it is the dry spell between storms, so size the tank for the longest gap you need to bridge, not the yearly volume.
Before you plumb anything
Rainwater harvesting law varies by state and by city: some places offer rebates, a few restrict collection, and most regulate any connection to indoor plumbing. Roof water is not potable without treatment designed for it. Irrigation, toilet flushing and laundry are the usual uses, in that order.
Frequently asked questions
▸How many gallons of rain does a 1,000 sq ft roof collect?
623 gallons per inch of rain if nothing is lost, or about 530 gallons at a realistic 85% collection efficiency. The constant is 0.623 gallons per square foot per inch.
▸Do I use the roof footprint or the sloped area?
The footprint. Rain falls vertically, so what a roof intercepts is set by the area it covers on the ground. A 12/12 pitch roof catches no more rain than a flat roof with the same outline.
▸What is a first-flush diverter and how big should it be?
It's a standpipe that captures and discards the dirtiest first water off the roof, then lets the rest through to the tank. Size it for 0.02–0.04 in of rain — roughly 10–25 gallons per 1,000 ft² of roof.
▸Is a 55-gallon rain barrel worth it?
For hand-watering a garden, yes. As water storage, no — a 1,000 ft² roof fills it in the first three minutes of a one-inch storm and the other 570 gallons go down the driveway. Cisterns start being interesting around 1,000 gallons.
▸Can I drink harvested rainwater?
Not without treatment designed for potable use — filtration plus disinfection, and testing. Roof surfaces shed metals, and the tank is a warm dark container. Irrigation and toilet flushing need none of that, which is why almost all residential systems start there.
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