Use the plan area, not the sloped area
This is the single most common error in rainwater sizing, and it inflates the answer by 15 to 40% depending on the pitch.
Rain falls vertically. A pitched roof intercepts exactly the rainfall that would have landed on its footprint — the shadow it casts straight down — regardless of how much actual surface the tiles cover. A steeply pitched roof has far more surface area than footprint, but it does not catch any more rain.
So measure the outline of the building, not the slope. The calculator asks for plan dimensions for this reason. Where a roof overhangs the walls, use the outline of the gutters rather than the walls themselves.
| Roof surface | Runoff coefficient | Why |
|---|---|---|
| Metal or corrugated | 0.90 | Smooth, almost no absorption |
| Tile or slate | 0.85 | Slight absorption and evaporation |
| Asphalt or felt | 0.80 | Rougher, holds a little |
| Green roof | 0.35 | Most is absorbed by the planting |
Size the butt for the dry spell
The annual total tells you what a roof could collect over a year. It does not tell you what butt to buy, because storage is limited by the gap between rain events rather than by the yearly figure.
A shed roof in a wet climate might yield thousands of litres a year, but if it rains every fortnight then a butt larger than about a fortnight's use simply sits full and overflows. Conversely, in a climate with a genuinely dry summer, the useful size is set by how long you need to bridge, and that can mean far more storage than the annual figure would suggest.
The working rule this calculator uses is about three weeks of typical summer use. That covers the ordinary dry spell without buying tanks that spend ten months full. If your summers are reliably dry for six weeks or more, size for that instead — and connect a second butt in series rather than replacing the first, since linked butts are cheaper per litre than one large tank.
What rainwater is and is not good for
Rainwater is better than tap water for most garden use, and genuinely important for some of it. It is soft, slightly acidic and free of the chlorine and dissolved lime in mains water. Ericaceous plants — blueberries, rhododendrons, camellias — decline over years watered with hard tap water because the lime slowly raises the pH of their compost. For those, rainwater is not a preference but a requirement.
It is not sterile. Water stored in a butt through a warm summer grows algae and bacteria, and the first flush off a roof carries whatever was on the roof — bird droppings, moss, dust. That is fine for established ornamentals and fruit bushes. It is a poor idea for seedlings, where damping-off disease is a real risk, and for anything that will be eaten raw shortly before harvest.
Two practical measures make a large difference. Keep the butt covered and out of direct sun to slow algae. And fit a first-flush diverter, which sends the initial dirty runoff to waste before filling the butt — it costs little and improves the water noticeably.
Mains top-up is worth thinking about too. A butt that runs dry in August when you most need it has failed at its job, and the calculator's "weeks of use covered" row is there to show you how close that is.
Common questions
How much rainwater can I collect from my roof?
Multiply the roof plan area in m² by annual rainfall in mm by about 0.85 for tile. A 6 × 3 m roof with 800 mm of rain yields roughly 12,000 litres a year.
Do I use the sloped roof area or the footprint?
The footprint. Rain falls vertically, so a pitched roof catches exactly the rainfall of its plan area no matter how steep it is. Using the slope overestimates by 15 to 40%.
What size water butt should I get?
Enough for about three weeks of typical summer use. A larger butt only helps if there is enough roof and rain to refill it, otherwise it sits full and overflows.