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How to build a recharge pit: size, depth and design

One recharge pit of 2 × 2 × 2.5 m with a 150 mm recharge bore takes the runoff from about 280 m² of concrete roof in a heavy downpour. Most houses on plots up to 300 sq yd need one pit; larger roofs need one pit for every 280 m² or so. Here is how the size is worked out and how the pit is built.

By the TerraSetu engineering team · Last checked

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Sizing: how many pits

Pits are sized for one heavy storm, not the whole year. We design for 25 mm of rain in an hour. A concrete roof sheds about 85% of the rain that falls on it, so 100 m² of roof sends 100 × 0.025 × 0.85 = 2.1 m³ of water in that hour.

A 2 × 2 × 2.5 m pit filled with boulders and gravel holds about 40% of its volume as water, which is 4 m³, and a working recharge bore drains about 2 m³ an hour. So one pit handles about 6 m³ in the design hour, enough for roughly 280 m² of roof.

Use the recharge pit calculator to do this for your roof and district.

PlotTypical roofPitsWater recharged a year at 650 mm rain
100 sq yd70 m²1about 39,000 litres
200 sq yd130 m²1about 72,000 litres
500 sq yd330 m²2about 1.8 lakh litres
1 acre campus2,000 m² of roofs8about 11 lakh litres

Litres a year = roof area × rainfall × 0.85. Check your district's rainfall on the rainfall pages.

How the pit is built

  1. Pick the spot

    At least 3 m from foundations and 15 m from any septic tank or soak pit, where downpipes can reach it by gravity.

  2. Dig and line

    Dig 2 × 2 m to 2.5 m deep. Line the sides with honeycomb brickwork (gaps between bricks) so water can seep out sideways and the walls do not collapse.

  3. Drill the recharge bore

    Drill a 150 mm bore from the pit floor to the water-bearing layer and case it with slotted PVC pipe wrapped in mesh. Depth depends on your water table: 60 ft where water is shallow, about 120 ft in most of NCR, 200 ft where it is deep.

  4. Fill the media

    From the bottom: boulders, then 20 to 40 mm gravel, then coarse sand on top, each about a third of the depth. Leave 0.3 to 0.5 m empty at the top to hold water.

  5. Filter the inflow

    Run roof water through a filter chamber or first-flush diverter first, so leaves and the dirty first rain do not choke the sand.

  6. Cover and overflow

    An RCC slab cover with a manhole, and an overflow pipe to the storm drain for the rare storm larger than the design.

What it costs

A single pit with a 120 ft bore, filter chamber and piping costs about ₹1.1 lakh to ₹1.7 lakh with GST in NCR. See the price for your district on the rainwater harvesting cost pages and for different bore depths on our borewell recharge page.

Mistakes we see on site

  • No recharge bore: a pit alone in clay or hard soil fills up and stays full.
  • Yard and road water with no silt trap: the sand chokes in one season.
  • Pit next to a septic tank or drain, which can carry sewage into the groundwater.
  • No cleaning: the top sand layer needs scraping every year before the monsoon.

Check the numbers for your area

Frequently asked questions

What is the standard size of a recharge pit?

A common size for houses and small buildings is 2 × 2 m and 2.5 m deep with a 150 mm recharge bore. Smaller pits (1 to 1.5 m across) without a bore are used only where the soil is sandy and the water table shallow.

How deep should the recharge bore be?

Deep enough to reach a permeable, water-bearing layer, usually 60 to 200 ft. Check how deep the water is in your area on the water table pages.

Is a recharge pit mandatory?

In many states it is, for plots above a set size. In Haryana it applies where the roof is 100 m² or more. See the rules in Gurugram and every state's rules.

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