How does the roof drain calculator work out how many downpipes I need?

It first finds the roof runoff in litres per second using runoff coefficient times rainfall intensity times roof area, divided by 3600. A roof is nearly impervious, so the coefficient is taken as about 1.0. It then divides that runoff by the capacity of your chosen downpipe size and rounds up, with a minimum of one downpipe, so heavier rain or a larger roof gives you more pipes.

What rainfall intensity and inputs should I enter for an Indian roof?

You enter three things: the plan area of the roof in square metres draining to these pipes, the downpipe size in millimetres, and the design rainfall intensity in mm per hour. Indian monsoon design values commonly sit around 75 to 150 mm per hour depending on the city, so use your local design storm rather than an average. When unsure, run the higher figure to stay on the safe side.

How accurate are the downpipe capacities, and what should I verify?

The per-pipe capacities here are indicative and depend on the outlet type, gutter design and how much blockage you allow for. Treat the result as concept-level sizing to plan roughly how many outlets and where. Before you procure, verify against manufacturer flow data and NBC 2016 Part 9, and add spare margin for leaves and cloudburst peaks rather than sizing to the exact number.

Interactive Calculator · 2026

Roof Drain Calculator

Work out how many rainwater downpipes a roof needs. Enter the roof area, the design rainfall intensity and the downpipe size — get the roof runoff in L/s, the number of downpipes required and the roof area each one drains.

Downpipes needed (100mm, at 75 mm/hr)0 nos0.00 L/s runoff · one pipe every 0 m² of roof

Downpipes needed at each pipe size — your selection in terracotta

Your roof & rainfall

Roof area (m²)

Plan area of roof draining to these downpipes.

Downpipe size (mm)

Indicative capacity per pipe (outlet-dependent).

Rainfall intensity75 mm/hr

Design storm intensity for your city — Indian monsoon design values commonly sit around 75–150 mm/hr.

Roof runoff

0.00 L/s

Downpipes needed

0 nos

Area per downpipe

0 m²

Downpipes needed by pipe size

Bigger pipes carry more, so fewer of them clear the same roof runoff.

Your 100 m² roof sheds about 2.08 L/s at 75 mm/hr. At the selected 100mm size (5.4 L/s each) that needs 1 downpipe — one every 100 m² of roof.

Spread downpipes evenly and keep a spare margin for leaves and cloudburst peaks.

How this is calculated

Roof runoff = (C × intensity × area) ÷ 3600, with C ≈ 1.0 for a roof = (1.0 × 75 × 100) ÷ 3600 = 2.08 L/s.
Downpipes needed = ⌈runoff ÷ pipe capacity⌉ = ⌈2.08 ÷ 5.4⌉ = 1 (minimum 1).
Area per downpipe = area ÷ downpipes = 100 ÷ 1 = 100 m².

Downpipe capacities here are indicative — they depend on the outlet type, gutter design and blockage allowance. Indicative sizing for concept planning: verify with manufacturer data and NBC 2016 Part 9, and add margin for leaves and monsoon peaks before procurement.

Understand the number you just got

Stormwater Drainage in IndiaHow roofs, plots and streets shed rain — design, codes and pitfalls.Rooftop Rainwater HarvestingTurn that roof runoff into recharge and storage instead of losing it.

Frequently asked questions

How does the roof drain calculator work out how many downpipes I need?: It first finds the roof runoff in litres per second using runoff coefficient times rainfall intensity times roof area, divided by 3600. A roof is nearly impervious, so the coefficient is taken as about 1.0. It then divides that runoff by the capacity of your chosen downpipe size and rounds up, with a minimum of one downpipe, so heavier rain or a larger roof gives you more pipes.
What rainfall intensity and inputs should I enter for an Indian roof?: You enter three things: the plan area of the roof in square metres draining to these pipes, the downpipe size in millimetres, and the design rainfall intensity in mm per hour. Indian monsoon design values commonly sit around 75 to 150 mm per hour depending on the city, so use your local design storm rather than an average. When unsure, run the higher figure to stay on the safe side.
How accurate are the downpipe capacities, and what should I verify?: The per-pipe capacities here are indicative and depend on the outlet type, gutter design and how much blockage you allow for. Treat the result as concept-level sizing to plan roughly how many outlets and where. Before you procure, verify against manufacturer flow data and NBC 2016 Part 9, and add spare margin for leaves and cloudburst peaks rather than sizing to the exact number.

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

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

Plumbing Ventilation Guide (India): Vent Systems, Stack, Loop & Wet Vents

A professional, India-first reference to drainage venting — why traps lose their seal, how vents equalise pressure, and how to size and route main, branch, loop and wet vents to code.

Plumbing

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