How does the recharge pit size calculator work?

It sizes a rainwater recharge pit from three inputs. First it finds peak inflow as catchment area times rainfall intensity divided by 3600, giving litres per second. Then it multiplies that by your detention time in minutes to get the required storage volume in cubic metres. Finally it divides the volume by an assumed 2 metre effective depth to suggest a pit footprint in square metres.

What inputs and values should I use?

Enter the catchment area in square metres — the roof or paved plan area draining to the pit. Set the design rainfall intensity to your city's short-duration IDF value, often 50 to 100 mm per hour. Set detention time to how long the pit should hold peak inflow while it soaks away, typically 10 to 20 minutes. Defaults of 150 square metres, 50 mm per hour and 15 minutes are a reasonable starting point.

How accurate is the pit size and what should I verify?

Treat the result as an indicative concept size, not a final design. It assumes a simple 2 metre deep pit and does not model your soil percolation rate, water-table depth, filter media bed or desilting chamber, all of which change the real size. Confirm the design with a hydrogeologist and your local rainwater-harvesting rules, and check NBC 2016 Part 9 before you build or procure anything.

Interactive Calculator · 2026

Recharge Pit Size Calculator

Size a rainwater recharge pit for a roof or plot. Enter the catchment area, the design rainfall intensity and how long the pit should hold peak inflow — get the peak runoff in L/s, the required recharge volume in m³ and the pit footprint at 2 m depth.

Required recharge volume (for 15 min detention)0.00 m³peak inflow ≈ 0.00 L/s · footprint ≈ 0.00 m² at 2 m deep

Required recharge volume across detention times (5 → 40 min)

Your catchment

Catchment area (m²)

Roof or paved plot area draining to the pit (plan area).

Design rainfall intensity50 mm/hr

Peak short-duration rainfall the pit is designed for — use your city's IDF value (often 50–100 mm/hr).

Detention time15 min

How long the pit should temporarily hold the peak inflow while it percolates into the ground.

Peak inflow

0.00 L/s

Required volume

0.00 m³

Footprint at 2 m deep

0.00 m²

Required volume vs detention time

A longer detention time means a bigger pit — it stores more of the peak inflow before it soaks away.

At 50 mm/hr over 150 m² the catchment yields 2.08 L/s. Holding that for 15 min needs a pit of about 1.88 m³ — roughly 0.94 m² of plan area if you dig 2 m deep.

The bars show how the required volume grows as you let the pit hold the peak for longer.

How this is calculated

Peak inflow = area × intensity ÷ 3600 = 150 × 50 ÷ 3600 = 2.08 L/s.
Required volume = peak inflow × detention × 60 ÷ 1000 = 2.08 × 15 × 60 ÷ 1000 = 1.88 m³.
Footprint at 2 m deep = required volume ÷ 2 = 1.88 ÷ 2 = 0.94 m².

Indicative sizing for concept planning. The real design also depends on the soil percolation rate and the water-table depth, and needs a filter media bed plus a desilting chamber — verify with a hydrogeologist and your local rainwater-harvesting rules (and NBC 2016 Part 9) before procurement.

Understand the number you just got

Rainwater Recharge Pit — India GuideHow a recharge pit is built, sized and maintained for Indian conditions.Rainwater Harvesting Guide — IndiaThe full picture: catchment, conveyance, storage, recharge and rules.

Frequently asked questions

How does the recharge pit size calculator work?: It sizes a rainwater recharge pit from three inputs. First it finds peak inflow as catchment area times rainfall intensity divided by 3600, giving litres per second. Then it multiplies that by your detention time in minutes to get the required storage volume in cubic metres. Finally it divides the volume by an assumed 2 metre effective depth to suggest a pit footprint in square metres.
What inputs and values should I use?: Enter the catchment area in square metres — the roof or paved plan area draining to the pit. Set the design rainfall intensity to your city's short-duration IDF value, often 50 to 100 mm per hour. Set detention time to how long the pit should hold peak inflow while it soaks away, typically 10 to 20 minutes. Defaults of 150 square metres, 50 mm per hour and 15 minutes are a reasonable starting point.
How accurate is the pit size and what should I verify?: Treat the result as an indicative concept size, not a final design. It assumes a simple 2 metre deep pit and does not model your soil percolation rate, water-table depth, filter media bed or desilting chamber, all of which change the real size. Confirm the design with a hydrogeologist and your local rainwater-harvesting rules, and check NBC 2016 Part 9 before you build or procure anything.

Related Guides — Deep-dive reading

Rooftop Rainwater Harvesting in India: Catching Rain Off Your Roof for Storage or Recharge

A homeowner's guide to rooftop rainwater harvesting — using the roof as a catchment, sizing gutters and downpipes for your area and rainfall, first-flush and filtration, and where the water finally goes: a storage sump for use or a recharge structure for the ground.

Plumbing

Percolation Tanks & Ponds in India: Recharging Groundwater at Plot, Farm and Watershed Scale

A professional guide to percolation tanks — impounding monsoon runoff in a shallow surface basin so it slowly infiltrates and recharges the aquifer. How they work, where they suit, siting, indicative sizing, silt management and how they relate to check dams.

Plumbing

Rainwater Recharge Pit for Indian Homes: How It Works, Sizing, Filter Media, Distances and Maintenance

The soak-pit that puts rain back into the ground — how a rainwater recharge pit works, how to size it against your roof area and soil percolation, the boulder-gravel-sand filter media, the desilting chamber ahead of it, safe distances from foundation, borewell and septic, why it fails in clay, and how to keep it from choking.

Plumbing

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