Rainwater harvesting:
how many litres your roof can collect

One millimetre, one square metre, one litre

The whole subject rests on a neat identity. Picture one square metre of flat roof under 1 mm of rain. That is a layer 1 mm deep over an area of 1 m², which is 0.001 m³, or exactly 1 litre. So rainfall in millimetres and roof area in square metres multiply straight into litres with no conversion factor at all.

This is why rainwater estimates are so easy to do on the back of an envelope, and why the answer is mostly limited by the quality of your rainfall data and your assumption about how much of the rain really reaches the tank.

The collection formula

Measure roof area as the plan footprint under the roof, not the sloping surface: rain falls vertically, so the horizontal projection is what catches it. The runoff factor covers losses to evaporation, splash, overflow and the first flush that you divert away to wash the dirt off. Values around 0.8 to 0.9 are often quoted for tiled or sheet roofs, and lower values for rough or porous surfaces. Use the figure your local guidance recommends.

Worked example: a 100 m² roof after 25 mm of rain

So a single 25 mm shower delivers 2,500 litres onto the roof and a well-run system keeps about 2,000 of them. A tank with a capacity of 2,000 L would just catch it; a 1,000 L tank would overflow halfway through.

Scaling up to a season or a year

Annual rainfall figures are published by weather departments and are quoted in mm per year. Plug that in as the rainfall term. If a place receives 800 mm in a year, the same 100 m² roof at 0.8 would collect 800 × 100 × 0.8 = 64,000 litres over the year, which is about 175 L a day on average.

Averages hide the fact that most of it falls in a few weeks. A tank large enough to hold 64,000 litres is rarely practical, so a smaller tank captures the early rain, fills repeatedly and overflows into a recharge pit. That is why storage size is chosen from monthly or storm-level figures, not annual totals.

Sizing storage sensibly

1. 1Work out your daily demand for the water you plan to replace, for example 150 L a day for flushing and gardening.
2. 2Decide how many dry days you want to cover, such as 10 days in a short gap between showers, giving 1,500 L.
3. 3Compare that with the volume from one design storm, here 2,000 L.
4. 4Choose a tank that covers the smaller of the two, then plan an overflow to a soakaway or recharge well for the rest.

Overbuilding storage is the usual mistake. A huge tank that never fills gives no benefit, while a small tank with a good overflow path still catches most of the useful water.

Why measured rainfall beats rule-of-thumb figures

A monthly rainfall table for your town is worth the few minutes it takes to find. Annual totals can look generous and hide the fact that most months are dry. The same roof that collects 64,000 litres in a year at 800 mm might collect nothing for five months, and a lot in two. Using monthly figures, you can simulate the tank: add each month's catch, subtract your demand, cap the total at the tank size and see how many months run dry.

A simple single-storm number is useful for different reasons. Designers often take the heaviest rain in an hour that occurs once a year or so and use it to size gutters and downpipes. A 50 mm per hour shower on 100 m² of roof is 5,000 litres an hour, which is 83 litres a minute. Gutters and pipes need to carry that without overflowing, or the harvesting yield will fall below what the volume formula predicts.

Recharge pits and what to do with the overflow

When the tank is full, extra water should go somewhere useful rather than down the drain. In many parts of India the answer is a recharge pit or a recharge well that lets the water percolate into the ground and top up the aquifer. A trench or pit filled with graded stones and sand, with a filter layer on top, can take the overflow from several storms.

Sizing is empirical: the pit has to accept the peak inflow while the soil takes its time absorbing it. Sandy soils drain fast and need a smaller pit, while clay needs much more storage. Local groundwater boards publish design norms, and many municipalities require a recharge feature for plots above a certain size. The harvested volume you worked out above is the number you hand to the designer, along with your soil type and space.

As a sanity check, convert your answer into days of use. If a household needs 400 litres a day for non-drinking purposes, 2,000 litres covers five days. Annual potential of 64,000 litres sounds large, but against a demand of 400 litres a day (146,000 litres a year) it supplies about 44 %. Knowing that fraction keeps expectations realistic and helps you decide whether the equipment is worth the cost.

Roof area also changes if you use more than one roof. Add the plan areas of the main house, the garage and any shed whose gutters drain to the same tank, but keep each runoff factor separate if the materials differ.

What to check before you build

• Water from roofs with bitumen coatings, certain paints or bird droppings needs treatment and may not be suitable for drinking.
• Fit a first-flush diverter so the dirtiest few millimetres are discarded.
• Rules and incentives vary by city and state, so ask the local municipal body about mandatory harvesting requirements.
• Gutters sized for your peak rainfall intensity matter: an undersized gutter loses water over the sides.

This calculation tells you volume, not quality or legality, so treat it as the first step of a design rather than the final answer.