How does the Stormwater Runoff Calculator work?

It uses the Rational Method, the standard way to size plot drainage. Peak runoff equals the runoff coefficient times the rainfall intensity times the catchment area, divided by 3600. Because 1 mm/hr of rain over 1 square metre yields 1 litre per hour, the result comes out in litres per second, and is also shown in cubic metres per hour and as the volume shed by a one-hour storm.

What inputs do I need and what values should I use?

You need three values. Catchment area in square metres is the plan area draining to your point of interest, roof plus paving plus open ground. The runoff coefficient C is the fraction of rain that runs off, typically around 0.90 for roofs, 0.85 for paving, 0.50 for gravel and 0.15 for lawn, area-weighted for a mixed plot. Rainfall intensity in mm per hour is your design storm value, commonly 50 to 90 mm/hr in Indian cities.

How accurate is the result and what should I verify?

The Rational Method gives a reliable indicative peak flow for concept planning and gutter, downpipe and drain sizing, but it is only as good as your inputs. Pick the rainfall intensity from your city IDF curve for the required return period, not a round number, and confirm C for your actual surface mix. Verify the design against NBC 2016 Part 9, local bye-laws and a qualified consultant before procurement.

Interactive Calculator · 2026

Stormwater Runoff Calculator

Estimate the peak stormwater runoff from a plot by the Rational Method. Enter the catchment area, the runoff coefficient of the surface and the design rainfall intensity — get the peak flow in L/s and m³/hr and the volume produced by a one-hour storm.

Peak runoff (at 50 mm/hr)0.00 L/s≈ 0.00 m³/hr · a one-hour storm sheds 8.5 m³

Peak runoff (L/s) rises linearly with rainfall intensity

Your catchment

Catchment area (m²)

Plan area draining to the point of interest — roof plus paving plus open ground.

Runoff coefficient (C)0.85

Fraction of rain that runs off. Typical: roof ≈ 0.90, paved ≈ 0.85, gravel ≈ 0.50, lawn ≈ 0.15. Use an area-weighted value for a mixed plot.

Rainfall intensity (mm/hr)50 mm/hr

Design rainfall intensity for the return period set by your local bye-laws — Indian cities commonly design storm drains for 50–90 mm/hr.

Peak runoff

0.00 L/s

Peak runoff

0.00 m³/hr

1-hr storm volume

0.00 m³

Runoff across rainfall intensities

For your area and C, peak runoff scales in direct proportion to rainfall intensity.

At your design intensity of 50 mm/hr the plot sheds 2.36 L/s — the number that sizes gutters, downpipes, catch pits and the outfall drain. Halving or doubling the intensity does the same to the runoff.

Pick the intensity from your city's IDF curve for the required return period, not a round number.

How this is calculated

Rational Method — 1 mm/hr of rain over 1 m² produces 1 L/hr of runoff, so peak flow Q (L/s) = (C × intensity × area) ÷ 3600 = (0.85 × 50 × 200) ÷ 3600 = 2.36 L/s.
In m³/hr = Q (L/s) × 3.6 = 2.36 × 3.6 = 8.5 m³/hr.
One-hour storm volume = flow × 1 hr = 8.5 m³.

Indicative sizing for concept planning. Use your city's design rainfall intensity for the return period required by local bye-laws; C is a surface property, so verify it for your actual mix of surfaces. Confirm with a qualified consultant / NBC 2016 Part 9 and the local drainage authority before procurement.

Understand the number you just got

Stormwater Drainage in IndiaHow to plan, size and detail a plot's stormwater drainage.Rainwater Harvesting Guide (India)Turn that runoff into a resource — recharge pits, storage and reuse.

Frequently asked questions

How does the Stormwater Runoff Calculator work?: It uses the Rational Method, the standard way to size plot drainage. Peak runoff equals the runoff coefficient times the rainfall intensity times the catchment area, divided by 3600. Because 1 mm/hr of rain over 1 square metre yields 1 litre per hour, the result comes out in litres per second, and is also shown in cubic metres per hour and as the volume shed by a one-hour storm.
What inputs do I need and what values should I use?: You need three values. Catchment area in square metres is the plan area draining to your point of interest, roof plus paving plus open ground. The runoff coefficient C is the fraction of rain that runs off, typically around 0.90 for roofs, 0.85 for paving, 0.50 for gravel and 0.15 for lawn, area-weighted for a mixed plot. Rainfall intensity in mm per hour is your design storm value, commonly 50 to 90 mm/hr in Indian cities.
How accurate is the result and what should I verify?: The Rational Method gives a reliable indicative peak flow for concept planning and gutter, downpipe and drain sizing, but it is only as good as your inputs. Pick the rainfall intensity from your city IDF curve for the required return period, not a round number, and confirm C for your actual surface mix. Verify the design against NBC 2016 Part 9, local bye-laws and a qualified consultant before procurement.

Related Guides — Deep-dive reading

Stormwater & Surface Drainage in India: Getting Rain Off Your Roof, Paving and Plot

A homeowner's guide to stormwater drainage — sizing roof outlets and downpipes for monsoon rainfall, surface channels and catch basins, grading the plot away from the house, and where the water finally goes, kept firmly separate from your foul drains.

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

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

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