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Construction & HomeplumbingUnited States only

Rainwater Harvesting Calculator

Rainwater harvest from a roof, with storage sized for the dry spell. A millimetre of rain on a square metre is exactly one litre, which makes the base calculation simple; the runoff coefficient is what varies.

Also called: rainwater collection calculator, roof water harvesting.

m²
mm
litres
Annual harvest
86,400

86,400 litres a year from 120 m² at 900 mm, after a runoff coefficient of 0.8 for a tile roof. That covers 43.84% of a 197,100 litre demand. A 45 day dry spell needs 24,300 litres of storage. Divert about half a litre per square metre of roof at the start of each event to discard debris.

Runoff coefficient
0.8
Annual household demand
197,100
Share of demand covered
43.84
Storage for the dry spell
24,300
Average monthly harvest
7,200
First flush to divert
60
On first flush
Divert about half a litre per square metre of roof at the start of each event to discard debris.
How this is calculated
Method and background

How this is calculated

One millimetre of rainfall on one square metre is one litre by definition, so the harvest is area times rainfall times a coefficient for what the surface actually delivers. Metal sheet runs at about 0.9 and a green roof at 0.3, with most of the loss to evaporation and wetting. Storage is sized on the dry spell rather than the annual total, because a system that harvests plenty across the year and runs dry in May has not solved the problem. First flush diversion discards the initial runoff carrying roof debris.

catchment area times rainfall depth times a runoff coefficient for the surface
A
Roof area
R
Rainfall
C
Runoff coefficient

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a 120 m² tiled roof at 900 mm

Catchment area
120 m²
Annual rainfall
900 mm
Roof surface
Tile or concrete
People in the household
4
Daily use per person
135 litres
Longest dry spell
45

Annual harvest86,400

120 x 900 x 0.8; 540 litres a day x 45

Open this example

a metal roof delivers more

Catchment area
120 m²
Annual rainfall
900 mm
Roof surface
Metal sheet
People in the household
4
Daily use per person
135 litres
Longest dry spell
45

Annual harvest97,200

boundary: the highest coefficient

Open this example

Method and limits

What it assumes

  • Rainfall spread through the year outside the stated dry spell.

What it deliberately does not model

  • Seasonal distribution matters more than the annual total, and a monsoon climate needs much larger storage.
  • Potable use requires treatment, which this does not address.
  • Local regulation on rainwater collection varies and sometimes prohibits it.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

How much water does a millimetre of rain give?
One litre per square metre, exactly. That is what a millimetre of depth over a square metre is.
How big should the tank be?
Enough for the longest dry spell, not a share of the annual harvest. A system sized on the annual total runs empty in the dry season.