Rainwater Harvesting Calculator

One clean identity sits at the heart of this: a millimetre of rain on a square metre is a litre of water. Everything else follows. A shed roof in a wet month makes more than a butt can hold, which is why the butt overflows in November and is empty in July — the problem is never the rainfall, it is the storage.

Roof area in square metres

The footprint it covers, not the sloping surface.

Annual rainfall in millimetres

Around 600 in the driest parts of England, 1,500 in the wettest.

Roof surface

Filter and gutter loss

A fraction, 0 to 0.5. A tenth is typical.

Butt or tank capacity in litres

A standard water butt is around 200.

Litres used a day

A watering can is about 9 litres.

24480 litres a year

With a 400 litre butt you would actually use 4800 of it — 65.8% of what you wanted — and 19680 litres would overflow.

Month	Rain	Collected	Used	Overflow	Short
Jan	84mm	2570.4	400	2170.4	220
Feb	60mm	1836	400	1436	160
Mar	64mm	1958.4	400	1558.4	220
Apr	52mm	1591.2	400	1191.2	200
May	52mm	1591.2	400	1191.2	220
Jun	52mm	1591.2	400	1191.2	200
Jul	56mm	1713.6	400	1313.6	220
Aug	64mm	1958.4	400	1558.4	220
Sep	60mm	1836	400	1436	200
Oct	88mm	2692.8	400	2292.8	220
Nov	88mm	2692.8	400	2292.8	200
Dec	80mm	2448	400	2048	220

✓ A millimetre on a square metre is a litre — 40m² of roof at 800mm, times 0.765 for the surface and the filter, is 24480 litres a year
✓ The months add up to the year — 12 months collecting 24480 litres against an annual 24480
✓ The monthly rainfall adds up to the annual figure — 800mm across the year, spread on a typical British pattern
✓ The butt never goes below empty or above full — the level runs between 0 and 0 litres of a 400 litre butt
✓ Every litre collected is used, overflows, or is still there — 4800 used, 19680 overflowed, 0 left at the end of December
✓ The demand met is what was supplied over what was wanted — 65.8% of 7300 litres wanted over the year

What this generator does

Multiplies the roof's footprint by the rainfall and the surface's runoff coefficient, spreads it over the months, and runs the butt's level through the year against your daily draw.

How to use this tool

Type the roof's footprint in square metres, not its sloping area.
Put in the annual rainfall where you are.
Choose the roof surface and how much the gutters and filter lose.
Set the butt size and what you draw off a day, and read the year.

Understanding the controls

Roof area: The footprint the roof covers, in square metres — the area seen from above. A steeper roof has more surface but catches no more rain.
Annual rainfall: Millimetres a year where you are. It ranges from around 600 in the driest parts of England to well over 1,500 in the wettest parts of Britain.
Roof surface: What the roof is made of, which decides how much of the rain runs off rather than soaking in or evaporating. A green roof keeps most of it.
Filter and gutter loss: What is lost before the water reaches the butt: the first flush, leaks, and what the filter rejects. A tenth is a reasonable default.
Butt or tank capacity: How much you can store. This is usually the binding constraint, not the rainfall — a bigger butt collects more of the same rain.
Litres used a day: What you draw off. A watering can holds about nine litres, so a summer evening round the garden is easily forty.

Common use cases

Sizing a water butt against a roof and a garden
Seeing how much rain a greenhouse or shed roof actually yields
Working out how much of a summer's watering a butt could cover
Deciding whether a second butt is worth linking in
Explaining why a full butt in winter runs dry in a dry spell

How this generator works

The yield is the roof's footprint times the rainfall times the runoff coefficient, resting on the identity that a millimetre over a square metre is exactly one litre. The year is spread across the months on a typical British pattern, wet at both ends and drier in spring, and the butt is then run forward month by month: filled by what falls, drawn down by what you use, overflowing when it is full and running short when it is empty. Four things are checked afterwards — the identity itself, that the months add back up to the year, that the butt's level never goes below empty or above full, and that every litre collected is either used, overflowed, or still sitting in the butt at the end of December.

Randomness and fairness

Nothing is random. The same roof and rainfall always give the same year.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

The monthly split is a typical British pattern, not your local one — a wet autumn and a dry spring shift the answer considerably.
Real rain arrives in storms rather than evenly through a month, so a butt overflows more than a monthly model shows.
Runoff coefficients are typical figures and vary with pitch, surface age and how clean the gutters are.
Collected rainwater is fine for the garden; drinking it is a different question entirely and not one this answers.

Privacy and your data

The year is simulated in your browser. Nothing about your roof or your garden is transmitted or stored.