How is rainwater harvesting yield calculated?

A simplified yield estimate is roof catchment area multiplied by rainfall depth, runoff coefficient and overall collection efficiency.

How much rainwater can a roof collect each year?

It depends on roof area, annual rainfall, runoff losses and collection efficiency. This calculator produces an estimate from those inputs.

Is annual rainfall enough to predict usable water?

Not necessarily. Timing matters because tank capacity can cause overflow during wet periods and empty periods during dry periods.

Water collection yield

Rainwater Harvesting Yield Calculator — Estimate Roof Water Collection

Convert rainfall into an estimated collection yield for a roof catchment. Use annual, monthly or custom-period rainfall, then keep runoff and collection losses visible so the result is transparent.

Harvesting inputs

Rainfall period

Roof catchment area (m²)

Rainfall depth for period (mm)

Runoff coefficient

Overall collection efficiency

Tank capacity (L)

Annual yield is not the same as annual usable supply. Tank capacity, demand timing, evaporation, overflow and dry periods can reduce the amount actually available for use.

Enter roof area and rainfall.

Estimated harvesting yield

Estimated collected water

—

—Theoretical volume

—Adjusted yield

—Tank equivalents

—Beyond one tank

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Rainwater harvesting formula

Theoretical runoff = roof area × rainfall depth
Adjusted yield = theoretical runoff × runoff coefficient × collection efficiency

With roof area in square metres and rainfall in millimetres, the theoretical volume is directly expressed in litres because 1 m² receiving 1 mm of rain equals 1 litre.

Why use two loss factors?

The runoff coefficient represents the fraction of rainfall expected to become runoff from the catchment. Collection efficiency can represent additional system losses such as screening, first-flush diversion, small conveyance losses and operational inefficiency.

Worked example

For a 150 m² roof receiving 1,200 mm of rainfall, a 0.90 runoff coefficient and 0.90 collection efficiency:

Theoretical = 150 × 1,200 = 180,000 L
Adjusted yield = 180,000 × 0.90 × 0.90 = 145,800 L

That is a potential collection estimate for the period. It does not mean 145,800 litres will be continuously available in a 5,000-litre tank.

Yield versus usable supply

A storage tank changes the problem from “how much rain falls?” to “how much can actually be retained when demand exists?”. Wet-season overflow can occur even when annual water demand is high, while dry-season shortages can occur even when annual rainfall totals are large.

For planning, test monthly rainfall and expected water use together when tank sizing matters.

Frequently Asked Questions

Does the calculator need annual rainfall?

No. Use any rainfall period you want.

Does it account for a tank?

It shows how many tank volumes the estimated yield represents, but it does not perform a month-by-month storage balance.

Can the result exceed tank capacity?

Yes. That simply means the period can generate more runoff than one tank can hold.

Is the runoff coefficient universal?

No. It is a modeling assumption and should match the catchment and intended method.