Rainwater Harvest Volume Calculator

How this is worked out

1 mm of rain falling over 1 m² is exactly 1 litre of water (since 1 mm = 0.001 m, and 0.001 m × 1 m² = 0.001 m³ = 1 litre) — so the theoretical maximum is simply area × rainfall. In practice, not all of that reaches a water butt: some evaporates, splashes out, or is lost to the gutter and first-flush diverter, which is what the runoff coefficient accounts for — a smooth, steep roof captures a high fraction (around 0.8–0.9), while a flatter or more absorbent surface captures noticeably less.

Frequently asked questions

What's the "catchment area" for a sloped roof?

Its horizontal (plan) footprint, not the sloped surface area — rain falls vertically, so what matters is the ground area the roof shadows, which is normally what's given in a building's floor plan or satellite measurement.

How do I use this for planning a water butt or tank?

Run it for a typical rainfall event (or your area's average monthly rainfall) to estimate how quickly a tank of a given size would fill, bearing in mind some of that capacity will already be used between rain events.

Related tools: Concrete Calculator, Gravel, Mulch & Soil Calculator, Roofing Calculator.

Source / Last verified: 1mm rainfall over 1m² = 1 litre follows directly from the metric definitions of the millimetre and litre, not a further source. Runoff coefficient ranges are commonly-cited reference figures used in rainwater-harvesting sizing guides (general civil/environmental engineering practice) — read/checked 1 October 2026; treat them as planning estimates, not guarantees for a specific roof.