How much rain a roof actually catches
Why the footprint matters more than the slope, what the runoff coefficient and filter losses take away, how the tank sizing works, and where the numbers come from so you can check them yourself.
Footprint, not slope area
Rain falls straight down. A pitched roof has more surface area than a flat one, but it catches exactly the same amount of rain because the rain only sees the area the roof covers when viewed from above. That area is the footprint.
A 45-degree roof has about 41 per cent more surface than its footprint. Using the slope length instead of the plan length overstates the catchment by that same 41 per cent, which is a large error. It means buying a tank that is too big for the water that will actually arrive.
Measure the footprint by standing at ground level and looking straight up, or use the dimensions on the house plan. The two fields are Roof length and Roof width, both in metres, and both from the plan view. If your roof is not rectangular, break it into rectangles, calculate each one, and add the yields.
Annual rainfall and where to find it
Yearly rainfall is given in millimetres. One millimetre of rain on one square metre of ground is exactly one litre of water. That identity makes the whole calculation easy to check in your head. A hundred square metres of roof under 800 millimetres of rain is 80,000 litres before any losses.
The default is 800 millimetres, which is close to the English average. But the UK ranges from about 600 in parts of East Anglia to over 1,600 in western Scotland. Your national weather service publishes the figure by region or postcode. Use the local number, not the national average, because the difference matters.
Roof surface and the runoff coefficient
Not all the rain that lands on a roof reaches the gutter. Some evaporates, some splashes off the edges, and some soaks in. The fraction that actually runs off is called the runoff coefficient.
Roof surface offers six choices. Metal sheet is the best at 0.95, meaning 95 per cent of the rain reaches the gutter. Clay or concrete tile gives back 0.85. Slate is 0.90. Flat felt or EPDM is 0.85. Gravel-ballasted flat roofs drop to 0.70 because the gravel holds water. And a green roof soaks up most of the rain, returning only about 0.35.
The choice makes a real difference. The same 80 square metre roof under 800 millimetres of rain yields 60,800 litres through metal sheet at 0.95 but only 22,400 through a green roof at 0.35. Pick the surface that matches what is actually up there.
Filter efficiency and the first flush
Between the gutter and the tank sits a filter. It catches leaves, moss and debris. A first-flush diverter sends the dirtiest water -- the first few litres off the roof after a dry spell -- straight to the drain. Together they lose some of the yield.
Filter efficiency defaults to 90 per cent, meaning ten per cent of the runoff is lost to the filter and the first flush. A well-maintained leaf screen with a proper first-flush diverter sits around this figure. A simpler setup with just a mesh screen might lose only five per cent, but it lets more dirt through to the tank.
The calculator multiplies the gross yield by the runoff coefficient and then by the filter efficiency to give the net yield -- the water that actually ends up in the tank over the course of a year.
How the tank suggestion works
A tank sized to hold a full year of rain would be enormous and would spend most of the summer empty. The useful size is set by how fast you use the water, not how much falls in a year.
If you fill in Daily use in litres, the calculator suggests a tank that holds roughly 18 days of that demand. It also caps the suggestion at about five per cent of the annual yield. Whichever rule gives the smaller tank wins. For most garden watering that lands somewhere between 200 and 1,000 litres.
Without a Daily use figure the calculator can only go by the yield, so it suggests a tank at the five per cent cap. Giving it a demand figure always produces a better answer.
Daily use and what the roof can cover
Daily use is the amount of water you use from the tank on an average day, in litres. For garden watering this varies wildly by season. A rough estimate is ten litres per square metre of garden per week in the growing season, which comes to about one and a half litres per square metre per day.
With a demand figure the calculator also tells you what share of your annual use the roof can supply. In a wet region with a large roof and modest demand, it might cover everything. In a dry region with a small shed roof, it might cover a quarter.
For household uses beyond the garden -- washing machine and WC flushing -- demand is steadier. A washing machine uses about 50 litres a load. A toilet flush is six to nine litres. Adding these to the daily figure gives a more accurate picture of how much the tank has to deliver.
A worked example for a typical house
A semi-detached house has a roof footprint of 10 metres by 8 metres. The roof is concrete tile. The area is in south-east England with about 600 millimetres of rain a year. The filter efficiency is 90 per cent.
Gross yield: 80 square metres times 600 millimetres is 48,000 litres. After the tile coefficient of 0.85 it drops to 40,800. After the filter at 90 per cent it drops again to 36,720 litres a year. That is about 100 litres a day on average, though of course it arrives in bursts rather than steadily.
The owner waters a small garden and runs a washing machine from the tank. Daily use is about 60 litres. The 18-day rule suggests a tank of 1,080 litres. The 5 per cent cap gives 1,836. The smaller wins, so a 1,000-litre tank is the practical choice. It will fill from a single heavy shower and empty over a dry fortnight.
Overflow is normal
A sensibly sized tank will overflow in winter. That is expected and fine. The rain keeps falling whether you need the water or not, and a tank small enough to be useful will be full long before spring.
What matters is where the overflow goes. Pipe it to a soakaway, a drain, or a planted area. Do not let it pool against the house wall or near a foundation, because standing water against masonry causes damp problems and can undermine footings over time. Every tank needs an overflow pipe, and it should be at least the same diameter as the inlet so the tank can never fill faster than it can spill.
Rainwater is not for drinking
Harvested rainwater is fine for the garden, for the washing machine, and for flushing the toilet. Washing machines actually benefit because rainwater is naturally soft, so it needs less detergent and leaves no limescale.
But it is not potable. It must never be plumbed into the drinking water supply. In most countries the law requires that rainwater and mains water run on completely separate pipes with no possible cross-connection. A clearly marked circuit and a non-return valve at the mains backup point are the minimum. This is a legal requirement, not a suggestion.
Mains water cost and payback
Mains water price is optional, given per cubic metre. A cubic metre is a thousand litres. If you fill this in, the calculator shows how much money the harvested water is worth per year at your local rate.
The figure is the gross saving, not the payback period. It does not include the cost of the tank, the plumbing, the pump if you need one, or the electricity to run it. But it does tell you the annual benefit, and from that you can work out how many years the system takes to pay for itself.
In areas with expensive mains water and high rainfall, the payback can be surprisingly short. In areas with cheap water and low rainfall, the saving may be too small to justify anything more than a basic water butt off a downpipe.
Everything stays on your own machine
The yield, the tank sizing, and the cost sums all run in your browser. Your roof dimensions, your rainfall figure, and your water price are not sent to any server. There is no account, no login, and no data stored anywhere outside your device. Disconnect from the internet and the calculator still works, because it never needed a connection in the first place.