Rainwater Barrel Sizing Calculator

Work out rainwater barrel sizing instantly with clear inputs, formula shown and shareable results.

About Rainwater Barrel Sizing Calculator

One millimetre of rain on one square metre is one litre, so collection is roof area times rainfall in millimetres times the collection efficiency. The litres-per-millimetre figure is the useful planning number: a 40 square metre roof yields 34 litres from every millimetre, so a single shower fills a small barrel.

Formula

Water collected

litres = roof area m2 x rainfall mm x collection efficiency

Worked example

With the default values below, Rainwater Barrel Sizing Calculator returns Water collected of 2,040.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Roof catchment areaminimum 0.1 m2: 40 m2
Rainfall in the periodminimum 0.1 mm: 60 mm
Collection efficiencyaccepts 1–100 %: 85 %
Barrel size you haveminimum 1 L: 200 L

Results

Water collected: 2,040.0 L
Overflow beyond your barrel: 1,840.0 L
Barrels of that size needed: 11
Litres collected per millimetre of rain: 34.00 L/mm

Step by step

1Values used
2Water collected
3Water collected= 2,040.0 L
4Overflow beyond your barrel= 1,840.0 L
5Barrels of that size needed= 11
6Litres collected per millimetre of rain= 34.00 L/mm

Assumptions and limits

The result is derived only from the 4 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 2 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 the Rainwater Barrel Sizing figure worked out?

litres = roof area m2 x rainfall mm x collection efficiency. One millimetre of rain on one square metre is one litre, so collection is roof area times rainfall in millimetres times the collection efficiency.

Why does this matter?

The litres-per-millimetre figure is the useful planning number: a 40 square metre roof yields 34 litres from every millimetre, so a single shower fills a small barrel.

What do I need to enter?

Enter roof catchment area, rainfall in the period, collection efficiency, barrel size you have. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.