Rainwater Harvesting at Home and How to Size the Tank

You want to stop paying for treated drinking water to flush toilets and water the garden, but every tank you look at seems either too small to matter or too big to fit. Getting rainwater harvesting right comes down to one calculation most suppliers skip: how much water your roof can actually collect, and how much of it you will really use.

I have specified rainwater systems on terraced houses, schools and a timber community hall, and the mistakes are almost always the same. People buy the biggest tank they can afford, fit it, and find it either sits full all winter or runs dry in the first hot fortnight. A little arithmetic before you order saves money and a lot of digging.

How much water a roof collects

The basic formula is simple. Multiply the roof plan area by the annual rainfall and a runoff coefficient that accounts for losses.

Litres per year = roof area (m2) x rainfall (mm) x runoff coefficient x filter efficiency

The roof plan area is the footprint seen from above, not the sloped surface. A pitched roof over a 10 by 8 metre house has a plan area of 80 m2 whatever its pitch. Typical runoff coefficients:

Filter efficiency is usually taken as 0.9, because the first-flush device and leaf filter divert some water. So a house with 80 m2 of concrete tile roof in a town with 650 mm of annual rain collects roughly 80 x 650 x 0.8 x 0.9 = 37,440 litres a year. That is a useful amount, about 100 litres a day on average, though it arrives unevenly.

Sizing a rainwater harvesting tank for real demand

Collection is only half the picture. The other half is how much you will draw. A common rule of thumb in European guidance is to size the tank at around 5 percent of either the annual yield or the annual demand, whichever is smaller. That covers about 18 days of use, which bridges most dry spells in a temperate climate.

Work out demand from what you plan to connect:

• Toilet flushing: a modern dual flush toilet uses about 4 to 6 litres per flush. At five flushes per person per day, a family of four uses around 25,000 litres a year.
• Washing machine: roughly 50 litres per cycle. At four loads a week, that is about 10,000 litres a year.
• Garden watering: highly seasonal. A 30 m2 vegetable bed can easily take 300 litres a week in summer and none in winter.

For our example house, demand for toilets and laundry is about 35,000 litres a year, close to the yield of 37,440. Five percent of the smaller figure is around 1,750 litres. In practice I would round up to a 2,000 to 3,000 litre tank. Not 10,000.

Garden-only systems are different

If you only water the garden, the tank has to carry spring rain into summer, when it barely rains and you need the most water. Here, bigger does help, but only up to a point. A 1,000 litre tank fed from a 40 m2 garage roof refills in one wet week and empties in two dry ones. Many households are better served by two or three 300 litre water butts on different downpipes than by one large buried tank.

Why bigger tanks are often the wrong choice

Here is where I push back on what tank sellers tell you. Bigger is sold as safer. It is not. A tank that rarely empties holds water for months, which encourages sediment and stale smells, and it never overflows enough to skim off floating debris. A properly sized tank overflows several times a year, which keeps the water fresher.

There is also the embodied cost. A buried 10,000 litre concrete or plastic tank means a large excavation, a crane or digger, gravel bedding and a lot of material. We have written about cutting embodied carbon in buildings early, and an oversized tank is a textbook case of carbon spent for no return. A modest tank that you fill and drain regularly does more good than a giant one you rarely touch.

Components that make a system work

A good system is more than a tank. In order of the water's path:

1. Gutters and downpipes. Check falls of around 1:350 to 1:600 and clear them twice a year.
2. Leaf guard and first-flush diverter. The first-flush device sends the dirtiest first 20 to 50 litres of each storm to drain.
3. Filter. A fine mesh filter, typically 0.2 to 1 mm, stops grit before the tank.
4. Calmed inlet. A pipe that turns upward at the bottom of the tank, so incoming water does not stir sediment.
5. Floating extraction. The pump draws water from just below the surface, where it is cleanest.
6. Overflow with an anti-backflow device. Connected to a soakaway or the surface water drain.
7. Pump and control unit. With mains backup that tops up automatically when the tank runs low, and an air gap so rainwater can never flow back into the drinking supply.

Every pipe and tap carrying rainwater inside the house should be labelled and coloured differently from drinking water. Plumbers in most countries will insist on it, and building control will check.

Costs and payback

A pair of water butts costs about 100 to 200 in local currency and pays back in a season. A whole-house system with a 3,000 litre buried tank, pump and plumbing usually costs between 3,000 and 6,000 installed on a new build, and noticeably more as a retrofit because of the extra pipework. At typical water and sewer charges, the money payback can take 15 years or longer.

That sounds poor, and on money alone it often is. The stronger case is resilience during hosepipe bans and reducing peak stormwater load on drains, which matters for flood-prone streets. For a new house, the cheapest move is to plan the dual pipework in from the start, alongside the measures in our guide to how better insulation lowers a building's running costs, even if the tank comes later.

Frequently asked questions

How big should a rainwater tank be for a house?

For toilets and laundry in a typical family house, a tank of 2,000 to 5,000 litres is usually enough. Size it at about 5 percent of the smaller of your annual roof yield or annual demand. Bigger tanks cost more and keep water stagnant for longer.

Can harvested rainwater be used for drinking?

Not without proper treatment. Most domestic systems are designed for toilets, laundry and gardens only. Drinking use needs extra filtration, UV disinfection and regular testing, and is regulated differently in most countries.

Is rainwater harvesting worth it in a dry climate?

It can be, but the tank has to be larger to carry rain from the wet season into long dry months. In very dry regions, reducing demand with low flush toilets and drought tolerant planting often saves more water than a tank collects.