Turn your roof's footprint into harvestable litres. One millimetre of rain on one square metre of roof yields one litre — apply your collection efficiency and local rainfall, and this calculator sizes the harvest per event and per year.
What Your Results Mean
Measure the roof's plan footprint: length along the eave times the plan width that drains to your tank. Use the footprint as seen from above, not the sloped rafter length — rainfall is measured horizontally, so a steeper roof does not catch more water per square metre of footprint. If only one side of the roof feeds the tank, use that side alone; if both sides feed it, the full footprint applies.
The one fact this calculator is built on: 1 mm of rain on 1 m² of roof delivers exactly 1 litre, before losses. A 72 m² footprint therefore receives 72 L per millimetre of rain in theory. Efficiency then trims that to what actually reaches the tank.
Collection efficiency covers everything between the cloud and the tank: a film of water that wets the roof and evaporates, gutter overshoot in intense downpours, and the first-flush water you deliberately divert to wash dust and bird droppings off the roof before storage begins. Roughly 75–90% is typical, and 80% is the common planning figure. Smooth metal roofs tend toward the top of that range and rough, porous tiles toward the bottom, but roof condition and gutter quality matter more than material.
The per-event figure answers "how much does one good storm give me?" — useful for seeing how fast a tank refills. The annual figure multiplies the same litres-per-mm by your annual average rainfall (the 600 mm default is a mid-range temperate figure; check your local weather service, because annual rainfall varies enormously by region, from a few hundred to several thousand millimetres).
Tank sizing is where most setups go wrong: the instinct is to size for the biggest possible storm, but a tank sized to maximum rainfall fills once and overflows for the rest of the year if demand is small. Match the tank to demand instead. Work out what you use the water for each day — garden watering is the classic load — multiply by the longest typical dry spell between refill events in your climate, and compare that against what one decent storm returns. The annual total tells you whether the roof can supply the demand at all; the dry-spell maths sets the tank size.
What the Results Mean
- Roof Footprint (m²): the catchment area feeding your tank, in plan view.
- Litres per mm: the workhorse figure — multiply by any rainfall depth to get litres harvested.
- Per Event: what one storm of the entered size delivers to the tank.
- Per Year: the annual harvest at your local average rainfall — the ceiling on what the roof supplies.
- Total: event and annual figures in one line for tank planning.
The Math Behind the Calculator
The calculation rests on the tidy equivalence of millimetres and litres: a 1 mm depth of water spread over 1 m² is 0.001 m³, which is exactly 1 litre. Efficiency scales it down to the harvested share.
footprint_m² = roof_length × roof_width
litres_per_mm = footprint_m² × (efficiency ÷ 100)
event_litres = litres_per_mm × event_rainfall_mm
annual_litres = litres_per_mm × annual_rainfall_mm
Because rainfall depth is measured with a horizontal rain gauge, the plan footprint is the correct catchment area — pitch does not enter the formula. The efficiency percentage is the single fudge factor: it absorbs wetting losses, overshoot, first-flush diversion and evaporation in one number. If your roof drains through long, leaky gutter runs, nudge it down; if it's a short, new, metal run into a well-sealed tank, nudge it up.
Worked Example: 12 × 6 m Roof, 25 mm Storm
A house roof measures 12 m along the eave with a 6 m plan width, and both sides drain toward the collection point. Collection efficiency is 80%, a recent storm delivered 25 mm, and the local annual average is 600 mm.
- Footprint: 12 × 6 = 72 m²
- Litres per mm: 72 × 0.80 = 57.6 L/mm
- One 25 mm storm: 57.6 × 25 = 1,440 L
- Annual harvest: 57.6 × 600 = 34,560 L ≈ 34.6 m³ per year
- Tank logic: at 200 L per day of garden watering, a 14-day dry spell needs 200 × 14 = 2,800 L — about two of those 25 mm storms refilled into storage. A tank in the 2,000–3,000 L class covers the dry spell without sitting three-quarters empty the rest of the year.
Notice the shape of the answer: the annual figure (34,560 L) is far larger than any sensible tank, while the useful size comes from demand between refills. That mismatch is normal — the roof overproduces on an annual basis in most temperate climates, and the tank is a buffer, not a reservoir for the year.
Reminder
Untreated roof runoff is not drinking water. Treatment and plumbing requirements vary by country and by intended use — garden, WC and washing applications typically need filtration and clearly identified pipework. Check local regulations before connecting stored rainwater to any household fitting.
Assumptions & Limitations
Every number above rests on these constants. If your roof differs, change the matching input — or read the linked guide for the full reasoning.
- 1 mm of rain on 1 m² = 1 L exactly; rainfall measured horizontally, so the plan footprint is the catchment
- 80% default efficiency is typical, not guaranteed — 75–90% is the common range
- Annual rainfall varies widely by region — the 600 mm default is a mid-range temperate figure, so replace it with your local average
- Snow, leaf-jammed gutters and prolonged droughts are not modelled
- Quantities only — this site deliberately does not price tanks, pumps or installation