Rainwater Harvesting Calculator
Find how much rainwater your roof collects in a year, a month or one storm. Advanced adds a month-by-month balance against your household use, the tank size you need, days of supply and the mains water you save.
Use the footprint seen from above, not the sloped surface: the rain falls straight down, so a steep roof catches no more than a flat one of the same footprint.
Your weather service or national met office publishes average rainfall for your town. Enter the figure for where you live.
Enter your own monthly rainfall and rain days (from your weather service). The numbers filled in at first are only an example.
A first-flush diverter sends the first, dirtiest water away at the start of each rain. Put 0 if you don't have one.
Count only the uses rainwater will cover, such as toilets, laundry and garden. The balance needs a yearly or monthly figure, not one storm.
The rain from your heaviest usual storm: how much one storm brings in, and whether your tank can hold it all.
Month-by-month balance
| Month | Rain | Water in | You use | From tank | Mains | Overflow | Tank at end |
|---|
How your rainwater is worked out
Roof area × rainfall gives the water that lands; the roof type and first flush take their share.
- 1
Roof area
10 m × 10 m= 100 m²
- 2
Rain that lands
100 m² × 800 mm1 mm on 1 m² = 1 L= 80,000 L
- 3
Runoff from the roof
80,000 L × 0.9= 72,000 L
- 4
Less first flush
72,000 − 50 × 60= 69,000 L a year
Read more: the formula, a worked example, runoff values and tank sizing
Rain falls straight down, so the water that lands on a roof is its footprint area times the depth of rain. One millimetre of rain on one square metre is exactly one litre (1 m² × 0.001 m = 0.001 m³). Not all of it reaches the tank: some splashes off, soaks in or evaporates (the runoff coefficient), the first-flush diverter sends some away, and a filter may lose a little more.
The formula
roof area = length × width (× share that drains to the tank)
water that lands (litres) = area in m² × rain in mm
water that lands (US gallons) = area in ft² × rain in inches × 0.6234
water collected = water that lands × runoff coefficient × filter efficiency
− first flush × rain days
a year from a month = a month × 12
tank month by month: tank + water in − water used, never below 0 or above the tank size
days a full tank lasts = tank size ÷ water used a day
money saved = rainwater used ÷ (1,000 L or 1,000 gal) × price
Worked example
A tile roof 10 m × 10 m = 100 m², with 800 mm of rain a year. Water that lands: 100 × 800 = 80,000 L. Times the runoff coefficient: 80,000 × 0.9 = 72,000 L. A 50 L first flush on each of 60 rain days loses 50 × 60 = 3,000 L, so you collect 72,000 − 3,000 = 69,000 L a year. That is 69 m³, 18,227.87 US gallons or 15,177.88 UK gallons. Every 1 mm of rain on this roof gives 100 × 0.9 = 90 L.
In gallons and inches
1 inch of rain on 1 ft² is 144 cubic inches. A US gallon is 231 cubic inches, so that is 144 ÷ 231 = 0.6234 gallons. A 1,000 ft² roof catches 1,000 × 0.6234 = 623.38 US gallons for every inch of rain, or 561.04 gallons with a 0.9 runoff coefficient.
Typical runoff coefficients
| Roof | Typical value |
|---|---|
| Metal sheet | 0.95 |
| Tile or concrete | 0.9 |
| Asphalt shingle | 0.85 |
| Gravel or flat roof | 0.7 |
| Green (planted) roof | 0.5 |
These are rough typical values, not measurements. Old, rough or dirty roofs, light showers and very hot weather lose more; smooth roofs in heavy rain lose less. Change the coefficient if you know a better figure.
Sizing the tank month by month
In Advanced, enter the rain and rain days for each month and how much rainwater you use. The calculator keeps a running balance: each month the tank gets the water collected and gives what you use. It runs the year twice so the dry months at the start of the year can draw on the wet months before them, and shows the second, settled year.
Example: the example months (10, 10, 15, 20, 40, 120, 200, 180, 120, 50, 20 and 15 mm, with 1, 1, 1, 2, 3, 9, 14, 13, 9, 4, 2 and 1 rain days) add up to 800 mm and 60 rain days, so the same roof still collects 69,000 L a year. 4 people using 50 L a day each need 200 × 365 = 73,000 L, more than the roof gives. June to September collect 4,350 + 11,100 + 9,350 + 4,350 = 29,150 L more than you use, so a 29,150 L tank catches every drop. You then use all 69,000 L, buy 4,000 L from the mains, cover 94.5% of your use, and a full tank lasts 29,150 ÷ 200 = 145.75 days with no rain. At 2 per m³, 69 m³ saves 138 a year.
If your roof gives more than you use, the suggestion is instead the smallest tank that never runs dry: the largest shortfall that builds up over the dry months. With 150 L a day in the example, October to May fall 21,000 L short, so a 21,000 L tank never runs dry and 14,250 L a year overflows.
UK rule of thumb: many UK suppliers size a home tank at about 5% of the yearly rainwater collected or the yearly use, whichever is smaller (about 18 days' worth). The calculator shows this figure too, as a quick check.
Tips
- Keep gutters and the first-flush diverter clean; a blocked diverter or gutter loses far more than the coefficient.
- Rain days count each day of rain as one first flush. Short showers on a dry, dusty roof may give little or nothing once the first flush is taken.
- Untreated rainwater suits toilets, laundry, gardens and washing cars. Drinking it needs proper filtering and treatment. Rules on harvesting and using rainwater differ by country and city.
Common questions
How much rainwater can I collect from my roof?
1 mm of rain on 1 m² of roof is 1 litre. Multiply the roof area by the rainfall, then by the runoff coefficient for your roof. A 100 m² tile roof (0.9) with 800 mm of rain a year collects 100 × 800 × 0.9 = 72,000 litres before any first flush is taken away.
How many gallons do I get per inch of rain?
1 inch of rain on 1 square foot is 144 cubic inches, which is 144 ÷ 231 = 0.6234 US gallons. A 1,000 ft² roof catches 623.38 gallons per inch of rain, or 561.04 gallons once a runoff coefficient of 0.9 is applied.
What is a runoff coefficient?
It is the share of the rain that actually reaches your gutters. Some splashes off, soaks in or evaporates. Typical values are about 0.95 for metal, 0.9 for tile or concrete, 0.85 for asphalt shingles, 0.7 for gravel or flat roofs and 0.5 for green roofs. These are rough typical values; change them if you know better for your roof.
What size rainwater tank do I need?
In Advanced, enter the rain for each month and your daily use. The calculator keeps a running balance month by month. If your roof gives more than you use, it finds the smallest tank that never runs dry. If it gives less, it finds the smallest tank that never overflows, so every drop gets used. With 4 people using 50 L a day each and the example months, that is 29,150 L.
What is a first-flush diverter?
It is a pipe or chamber that catches the first, dirtiest water off the roof at the start of each rain and sends it away, so dust, leaves and bird droppings stay out of the tank. That water is lost, so the calculator takes it away once for each rain day: 50 L × 60 rain days = 3,000 L a year.
Can I drink harvested rainwater?
Not without proper filtering and treatment. Most homes use it for toilets, laundry, gardens and washing cars. Rules on using and storing rainwater differ by country and city, so check what applies where you live.
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