Last updated: 2026-06-16

Rainwater harvesting captures precipitation from a roof and directs it into storage for garden irrigation, toilet flushing, or other non-potable uses. The collectible volume depends on three quantities: the catchment area, the depth of rainfall, and how much of that rainfall survives losses on the way to the tank.

The area–depth–efficiency formula

The underlying relationship is simple: one millimetre of rain falling on one square metre of horizontal surface produces exactly one litre of water. In base units, a depth $R$ (metres) over an area $A$ (square metres) gives a raw volume of $A \times R$ cubic metres. Applying a collection efficiency $\eta$ (a fraction between 0 and 1) accounts for losses:

$V = A \times R \times \eta$

The efficiency term covers evaporation during the rain event, water retained by the roof surface, first-flush diversion (discarding the initial runoff that carries dust and bird droppings), and gutter overflow during intense bursts.

Why footprint area, not sloped area

Rain falls vertically. The volume intercepted depends on how much horizontal ground the roof covers — its plan-view footprint — not on the length of the actual roof surface. A steeply pitched roof and a flat roof with the same footprint intercept the same volume of rain. The footprint is normally visible on a site plan or can be measured with a mapping tool by tracing the outer walls of the building.

Typical collection efficiencies

A first-flush diverter — which discards the first few millimetres of runoff — is assumed to be in place at these figures. Without one, reduce the estimate by 5–10 percentage points. The calculator uses 85% as a default, which suits most residential metal or tile roofs with a diverter.

Worked example

A house has a roof footprint of 120 m². The local long-term average annual rainfall is 600 mm. With an 85% collection efficiency:

At that yield the household could expect roughly 5,100 L per month on average, though actual monthly totals vary with seasonal rainfall patterns.

For storage-tank sizing, see the Swimming Pool Volume Calculator to check how much volume a given tank shape holds.

Regulatory considerations

Some jurisdictions restrict or regulate rainwater collection, particularly in regions where surface-water rights are allocated to downstream users. Before installing a harvesting system, check applicable local rules on permitted uses, tank placement, and overflow discharge.