What this calculator does
One millimetre of rain on one square metre of roof is exactly one litre, which makes the arithmetic unusually clean. A 120 square metre roof in 25 mm of rain collects 2,550 litres once an 0.85 runoff coefficient is applied.
The area to use is the roof footprint seen from above, not the sloped surface area. Rain falls vertically, so a steep roof catches no more than a flat one of the same plan dimensions. Using the sloped area is the most common mistake here and overstates the yield by the slope factor.
The formula
The plan area is multiplied by the rainfall depth, giving litres directly because of the 1 mm on 1 m² equivalence. The runoff coefficient then accounts for what never reaches the tank: water that wets the roof and evaporates, splashes out of gutters, or overshoots during heavy falls. A well-guttered metal roof keeps about 90%; rougher surfaces keep less.
| Term | Meaning |
|---|---|
| Runoff coefficient | The fraction of rainfall that actually reaches the tank, typically 0.75 to 0.9. |
| Plan area | The roof footprint viewed from above, which is what catches rain. |
| First flush | Diverting the first few litres, which carry roof debris, away from the tank. |
| 1 mm = 1 L/m² | The identity that makes the calculation straightforward. |
The inputs explained
| Field | What to enter |
|---|---|
| Roof plan area (footprint, not sloped area) (m²) | Roof plan area in square metres, measured as the footprint rather than the sloped surface. |
| Rainfall (mm) | Rainfall depth in millimetres, for a single event or a whole month. |
| Runoff coefficient | Runoff coefficient for the roof type and guttering. |
| Tank size (0 to skip) (L) | Tank size, to see what share a given fall would fill. Set to 0 to skip. |
When to use it
Sizing a rainwater tank
Comparing typical monthly rainfall against demand shows what tank capacity is worth installing.
Estimating a single storm
A heavy fall on a large roof produces more water than most people expect, and can overflow a small tank.
Planning garden irrigation
Knowing what a roof yields per month sets how much of the garden can realistically be watered from it.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does each rainfall yield?
A range of rainfall depths on the same roof.
| Rainfall | Water collected | Share of the tank filled | Weight of that water |
|---|---|---|---|
| 5 mm | 510 litres | 10.2% | 0.51 tonnes |
| 25 mm | 2,550 litres | 51.0% | 2.55 tonnes |
| 50 mm | 5,100 litres | 1.02× the tank (overflows) | 5.10 tonnes |
| 100 mm | 10,200 litres | 2.04× the tank (overflows) | 10.20 tonnes |
Questions
Do I use the roof footprint or the sloped area?
The footprint, measured as though looking straight down. Rain falls vertically, so a 30 degree roof intercepts exactly the same rain as a flat roof covering the same ground. Using the sloped area overstates the yield by the slope factor, which on a steep roof is 15% or more.
What runoff coefficient should I use?
Around 0.85 for a typical pitched tile or metal roof with decent guttering. Use 0.9 for smooth well-guttered metal, and 0.6 to 0.75 for rough, porous or flat surfaces where water ponds and evaporates. The losses are mostly wetting, evaporation and gutter overshoot in heavy rain.
Why does 1 mm of rain give 1 litre per square metre?
Because a millimetre depth over a square metre is 0.001 × 1 m³, which is 0.001 cubic metres, and a cubic metre is 1,000 litres. The units work out exactly, which is why rainfall is reported in millimetres rather than as a volume.
Should I divert the first flush?
Generally yes. The first few litres off a roof carry dust, leaf litter and bird droppings, and a first-flush diverter sends them to waste instead of the tank. It costs a little yield but makes a substantial difference to water quality, particularly after a dry spell.
For sizing the tank itself, see the water tank volume calculator. For the roof geometry, see the roof pitch calculator.