Rainwater Harvesting Calculator
Estimate rainwater harvest from roof area and monthly rainfall, run a month-by-month tank balance, and size storage for a dry spell at your daily demand.
Monthly rainfall and tank balance
Replace the twelve rainfall figures with your own local averages in in. The tank balance carries over from month to month, spills whatever will not fit, and draws the monthly demand out of whatever is left.
| Month | Rain (in) | Harvest (gal) | Demand (gal) | In tank (gal) | Overflow (gal) | Shortfall (gal) |
|---|---|---|---|---|---|---|
| Jan | 2,585 | 1,860 | 640 | 725 | 0 | |
| Feb | 2,262 | 1,680 | 820 | 402 | 0 | |
| Mar | 2,908 | 1,860 | 640 | 1,228 | 0 | |
| Apr | 2,747 | 1,800 | 700 | 887 | 0 | |
| May | 3,151 | 1,860 | 640 | 1,351 | 0 | |
| Jun | 2,989 | 1,800 | 700 | 1,129 | 0 | |
| Jul | 3,312 | 1,860 | 640 | 1,512 | 0 | |
| Aug | 3,070 | 1,860 | 640 | 1,210 | 0 | |
| Sep | 2,828 | 1,800 | 700 | 968 | 0 | |
| Oct | 2,666 | 1,860 | 640 | 866 | 0 | |
| Nov | 2,747 | 1,800 | 700 | 887 | 0 | |
| Dec | 2,828 | 1,860 | 640 | 1,028 | 0 |
Harvest = roof footprint × rainfall depth × runoff coefficient, less the first-flush diversion. In US units a square foot of roof yields 0.6234 gallons per inch of rain (1 ft² × 1 in = 1/12 ft³, and a cubic foot is 7.48 gallons); in metric it is exactly 1 litre per square metre per millimetre. Slope does not matter — only the footprint the roof casts on the ground.
Monthly averages hide the real problem, which is the gap between storms. Size the tank on the longest dry spell you actually get, not on the annual total, and check local rules before committing: rainwater collection is regulated in some jurisdictions and potable use nearly always requires treatment. Estimates only.
What is the Rainwater Harvesting Calculator?
The ByteTools Rainwater Harvesting Calculator uses the exact catchment constant — 0. 6234 gallons per square foot per inch of rain, or exactly one litre per square metre per millimetre — applied to your roof footprint, runoff coefficient and first-flush loss.
- Exact catchment constants in both US and metric units
- Runoff coefficients for metal, shingle, concrete, gravel and green roofs
- Twelve-month tank balance with carry-over, overflow, shortfall and demand met
- Dry-spell tank sizing from daily demand and days of autonomy
- First-flush and filter loss applied before anything reaches the tank
- CSV export of the monthly table; all calculations run locally in your browser
How to use the Rainwater Harvesting Calculator
- 1
Pick US or metric units and enter the roof footprint — the plan area, since pitch does not change how much rain lands on it.
- 2
Choose the roof surface to set the runoff coefficient and enter your first-flush and filter loss percentage.
- 3
Set the daily demand the tank supplies, your tank size, and the dry spell in days you want to cover.
- 4
Replace the twelve monthly rainfall figures in the table with your own local averages.
- 5
Read the annual harvest, the percentage of demand met, the overflow and shortfall, and copy the monthly table as CSV.
About the Rainwater Harvesting Calculator
The ByteTools Rainwater Harvesting Calculator uses the exact catchment constant — 0.6234 gallons per square foot per inch of rain, or exactly one litre per square metre per millimetre — applied to your roof footprint, runoff coefficient and first-flush loss. That gives the yield per unit of rain, which is the number worth writing on the back of your hand.
From there it runs a twelve-month balance. Fill in your own local rainfall figures and it carries the tank level from month to month, spills whatever will not fit, draws your daily demand out of what is left, and reports overflow, shortfall and the percentage of demand actually met. Two passes are run so January opens with a settled level rather than an empty tank.
It also sizes a tank purely on the longest dry spell you want to ride out, which is usually a more honest constraint than the annual total. Everything is computed in your browser — no location is asked for and nothing is uploaded.
Frequently asked questions
How much rainwater can I collect from my roof?
About 0.62 gallons per square foot for every inch of rain, before losses. A 1,600 square foot roof in a one-inch storm at a 0.9 runoff coefficient yields roughly 900 gallons. In metric it is exactly one litre per square metre per millimetre, which makes the sum trivial.
Does roof pitch affect rainwater collection?
No. What matters is the footprint the roof casts on the ground, because that is the area the falling rain sees. A steeply pitched roof has more surface but collects exactly the same volume as a flat roof with the same plan area.
What is a runoff coefficient?
It is the fraction of the rain landing on the roof that actually reaches the gutter, after wetting the surface, evaporating and soaking into it. Smooth metal is around 0.95, asphalt shingle about 0.90, and a green roof can be 0.75 or lower.
What size rainwater tank do I need?
Size it on the gap between storms rather than the annual total. Multiply your daily demand by the longest dry spell you want to ride out — 60 gallons a day across 21 dry days is 1,260 gallons — then check the monthly balance to see how often it would actually run dry.
What is a first flush diverter?
It sends the first few gallons off the roof to waste, taking the accumulated dust, pollen and bird droppings with it before the clean water reaches the tank. Ten percent is a reasonable planning allowance and it is an editable field here.
Is rainwater harvesting legal where I live?
It depends entirely on your jurisdiction — some places encourage it with rebates, a few restrict or regulate it. Potable use almost always requires filtration and disinfection to a standard set by local health rules. Check before you build.
Related tools
Gutter Calculator
Total your gutter run from eave lengths, size 5 and 6 inch K-style or half-round against roof area and rainfall, and count downspouts, hangers, elbows and corners.
Water Bill Calculator
Price a metered water bill with tiered rate blocks, a sewer charge, fixed fee and tax, then see the effective cost per unit and what using less would save.
Sprinkler Run Time Calculator
Work out sprinkler precipitation rate from head spacing and flow, then get run minutes per zone with a cycle-and-soak schedule matched to your soil and slope.
Pool Volume Calculator
Calculate swimming pool volume in gallons and litres for rectangular, round, oval and kidney pools, plus fill time, pump turnover and chemical dose scaling.
Raised Bed Soil Calculator
Work out how much soil your raised beds need in cubic feet, cubic yards, litres and bags, then split it by mix ratio into compost, peat or coir and vermiculite.