Case 1
- Roof plan area
- 50m²
- Rainfall
- 20mm
- Runoff coefficient
- 0.85
850L
Open with these valuesRainwater Harvesting Calculator
850L
Result: 850 LOne millimetre of rain on one square metre is exactly one litre — that identity is why area in m² times rainfall in mm gives litres with no conversion factor at all. The runoff coefficient then takes off what splashes, evaporates or goes to the first flush: about 0.85 for a hard roof.
The numbers at a glance
Held fixed: Roof plan area 50.0 m², Rainfall 20.0 mm.
| Runoff coefficient | Result (L) |
|---|---|
| 0.65 | 650 |
| 0.70 | 700 |
| 0.75 | 750 |
| 0.80 | 800 |
| 0.85Your value | 850 |
| 0.90 | 900 |
| 0.95 | 950 |
| 1.00 | 1,000 |
Worked examples
Case 2
- Roof plan area
- 100m²
- Rainfall
- 10mm
- Runoff coefficient
- 1
1,000L
Open with these valuesCase 3
- Roof plan area
- 200m²
- Rainfall
- 35mm
- Runoff coefficient
- 0.75
5,250L
Open with these valuesHow it's calculated
litres = area (m²) × rainfall (mm) × runoff
- StepTake the plan area of the roof — the footprint seen from above, not the slope.
- StepEnter the rainfall in millimetres: one shower, or the annual total for a year.
- StepSet the runoff coefficient, around 0.85 for a hard roof and less for a green one.
- ResultArea × rainfall already gives litres; the coefficient takes off the losses.
What this number means
The runoff coefficient is the part people drop
Area times rainfall is the rain that lands on the roof, not the water that reaches the tank. A 50 m² roof in 20 mm of rain catches 1000 litres, and at 0.85 only 850 of them arrive. Leaving the coefficient at 1 quietly overstates every result.
One millimetre on one square metre is a litre
1 mm is 0.001 m, and 0.001 m across 1 m² is 0.001 m³, which is a litre by definition. That identity is why the formula carries no conversion factor at all.
Plan area, not the sloped area
Take the footprint the roof covers, seen from above. Rainfall is measured as a vertical depth, so the pitch does not change how much water lands on the roof.
A year of rain is not a year of supply
Put the local annual rainfall into the rainfall field and a 50 m² roof at 0.85 gathers about 34000 litres. A full tank overflows, so in practice the storage you have limits what you actually keep.
Commonly misread
I entered 85 for an 85 percent runoff coefficient.
The field takes a share between 0.1 and 1, so 85 percent is 0.85. Enter the fraction rather than the percentage.
I measured the sloping surface of the roof.
The effective collection surface is the area the roof covers, seen from above. Rain falls as a vertical depth, so the pitch adds nothing to the catch.
My roof is planted, so 0.85 works there as well.
A green roof holds back far more water and sits nearer 0.3, while a sealed surface approaches 1.0. Both of those edge figures are rough guides and do not come from the source behind this calculator.
34000 litres a year means 34000 litres in my tank.
That is what the roof delivers over the year, not what you can store. A full cistern overflows, and the tank is usually the real limit.
Reference table
| Area, rainfall, runoff | Rain on the roof | Collected (L) |
|---|---|---|
| 1, 1, 1 | 1 L | 1 |
| 25.5, 12.3, 0.9 | 313.65 L | 282 |
| 50, 20, 0.85 | 1000 L | 850 |
| 100, 10, 1 | 1000 L | 1000 |
| 200, 35, 0.75 | 7000 L | 5250 |
Questions
How much rainwater can I collect?
Multiply the roof's plan area in square metres by the rainfall in millimetres and by the runoff coefficient. A 50 m² roof in 20 mm of rain at 0.85 collects 850 litres.
Why is 1 mm of rain on 1 m² exactly 1 litre?
Because 1 mm is 0.001 m, and 0.001 m across 1 m² is 0.001 m³, which is one litre by definition. It is a unit identity rather than an approximation, and it is the reason the formula carries no conversion factor.
What runoff coefficient should I use?
It is the share of rain that actually reaches the tank after splashing, evaporation and the first flush. Installers assume 75 to 90 percent for a hard roof, so 0.85 is a fair default; a sealed surface approaches 1.0 and a green roof is nearer 0.3.
Plan area or sloped area?
The plan area — the footprint the roof covers, seen from above. Rainfall is measured as a vertical depth, so the pitch of the roof does not change how much water lands on it.
How do I estimate a whole year?
Put your local annual rainfall into the rainfall field instead of a single shower. A 50 m² roof under 800 mm a year at 0.85 gathers about 34000 litres — though a full tank overflows, so storage usually limits what you actually keep.
Sources and last check
Information, not professional advice.