Rainwater Harvesting Calculator

Size a rainwater harvesting system from roof area and rainfall.

About Rainwater Harvesting Calculator

One millimetre of rain on one square metre yields one litre, so harvest scales directly with roof area and rainfall. Tank size is determined by how long the system must bridge without rain, not by the annual total. Oversizing the tank wastes capital that never fills, while undersizing means overflow during storms, so the dry period is the parameter that matters.

Formula

Rainwater Harvesting

Harvest = roof area × rainfall × runoff coefficient; one millimetre on one square metre is one litre

runoff coefficient: Losses from evaporation, first flush and gutter overflow
tank sizing: Set by the longest dry period rather than by annual volume

Worked example

With the default values below, Rainwater Harvesting Calculator returns Annual Harvest Potential of 57,375. Change any input above and every figure here is recalculated in your browser.

Inputs used

Roof catchment areaminimum 0 m2: 90 m2
Annual rainfallminimum 0 mm: 750 mm
Runoff coefficientaccepts 10–100 %: 85 %
Daily water demandminimum 0 L: 120 L
Longest dry periodminimum 0 days: 21 days

Results

Annual Harvest Potential: 57,375 L
Recommended Tank Size: 2,520 L
Share of Demand Met: 100.00%
Annual Harvest Potential: 57.375 m3
Days of Supply From a Full Tank: 21.0 days

Step by step

1Values used
2Rainwater Harvesting
3Annual Harvest Potential= 57,375 L
4Recommended Tank Size= 2,520 L
5Share of Demand Met= 100.00
6Annual Harvest Potential= 57.375 m3
7Days of Supply From a Full Tank= 21.0 days

Assumptions and limits

The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or fees or charges into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Rainwater Harvesting calculated?

Harvest = roof area × rainfall × runoff coefficient; one millimetre on one square metre is one litre One millimetre of rain on one square metre yields one litre, so harvest scales directly with roof area and rainfall. Tank size is determined by how long the system must bridge without rain, not by the annual total.

Why does Rainwater Harvesting matter?

Oversizing the tank wastes capital that never fills, while undersizing means overflow during storms, so the dry period is the parameter that matters.

What values do I need to enter?

This calculator takes 5 inputs: Roof catchment area, Annual rainfall, Runoff coefficient, Daily water demand, Longest dry period. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.