How does the first flush diverter calculator work?

It estimates how much dirty first-flush water to divert before storage. Since 1 mm of rain over 1 m² of roof equals 1 litre, the flush volume is simply roof area multiplied by the first-flush depth you want to divert. It then works out how long a standpipe of your chosen diameter must be to hold that volume, using the pipe cross-section area.

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

You need the catchment roof plan area in square metres, the first-flush depth to divert in millimetres, and the standpipe diameter in millimetres. A depth of 0.5 to 2 mm is a common range depending on roof cleanliness and local dust; 110 mm is a typical standpipe diameter. Use the actual plan area of the roof feeding this one diverter, not the whole building.

How much first flush should I divert in India?

There is no single fixed figure; it depends on how dusty and dirty the roof gets. Many rainwater harvesting installers divert roughly the first 0.5 to 2 mm of rainfall, which washes off accumulated dust, leaves and bird droppings. Dustier locations and roofs cleaned less often need more diverted. Treat this as indicative and adjust after seeing how clean the stored water stays.

Interactive Calculator · 2026

First Flush Diverter Calculator

Size the first-flush diverter for a rainwater harvesting system. Enter the catchment roof area, the first-flush depth to divert and the standpipe diameter — get the dirty-water volume to divert in litres and the standpipe length to hold it.

First-flush volume to divert (at 1.5 mm)0.0 L≈ 0.0 L per 100 m² · a 15.78 m standpipe of 110 mm holds it

First-flush volume by roof area (at 1.5 mm diverted)

Your catchment

Roof area (m²)

Plan area of the catchment feeding this diverter.

Standpipe diameter (mm)

The vertical diverter pipe that fills first — often 110 mm.

First-flush depth to divert1.5 mm

The first millimetres of rain that wash roof dust, bird droppings and debris off the catchment — 0.5–2 mm is a common range.

First-flush volume

0.0 L

Per 100 m² roof

0.0 L

Standpipe length

0.00 m

First-flush volume by roof area

Bigger catchments wash off proportionally more dirty water at the same divert depth.

At 1.5 mm of divert depth your 100 m² roof sheds 150 L of dirty first flush. A vertical standpipe of 110 mm holds 9.5 L per metre, so it needs to be about 15.78 m tall (or use a wider chamber) before it seals and clean water flows on to storage.

Fit a slow-release valve or drip so the diverter empties between showers and is ready for the next one.

How this is calculated

First-flush volume = roof area × divert depth (1 mm over 1 m² = 1 L) = 100 × 1.5 = 150 L.
Pipe volume per metre = π × (dia ÷ 1000)² ÷ 4 × 1000 = π × (110 ÷ 1000)² ÷ 4 × 1000 = 9.5 L/m.
Standpipe length = flush volume ÷ pipe volume per metre = 150 ÷ 9.5 = 15.78 m.

First flush carries roof dust and bird droppings — divert it before storage. 0.5–2 mm is a common range; verify against your roof cleanliness and local dust load. Indicative sizing for concept planning — confirm with a qualified consultant / manufacturer data / NBC 2016 Part 9 before procurement.

Understand the number you just got

Rainwater Filtration in IndiaFirst flush, filters and screens that keep harvested water clean.Rooftop Rainwater Harvesting in IndiaCatchment, conveyance, first flush, storage and recharge — the full system.

Frequently asked questions

How does the first flush diverter calculator work?: It estimates how much dirty first-flush water to divert before storage. Since 1 mm of rain over 1 m² of roof equals 1 litre, the flush volume is simply roof area multiplied by the first-flush depth you want to divert. It then works out how long a standpipe of your chosen diameter must be to hold that volume, using the pipe cross-section area.
What inputs do I need and what values should I use?: You need the catchment roof plan area in square metres, the first-flush depth to divert in millimetres, and the standpipe diameter in millimetres. A depth of 0.5 to 2 mm is a common range depending on roof cleanliness and local dust; 110 mm is a typical standpipe diameter. Use the actual plan area of the roof feeding this one diverter, not the whole building.
How much first flush should I divert in India?: There is no single fixed figure; it depends on how dusty and dirty the roof gets. Many rainwater harvesting installers divert roughly the first 0.5 to 2 mm of rainfall, which washes off accumulated dust, leaves and bird droppings. Dustier locations and roofs cleaned less often need more diverted. Treat this as indicative and adjust after seeing how clean the stored water stays.

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

Rainwater Filtration & First-Flush in India: Cleaning Rain Before You Store or Recharge It

A homeowner's guide to the step that makes or breaks a rainwater harvesting system — diverting the dirty first flush, screening leaves, desilting, and choosing the right filter before the water reaches your tank or recharge pit.

Plumbing

Rainwater Harvesting Guide for India: Catchment, Storage, Recharge & Bye-laws

The section pillar for rainwater harvesting — why it matters for water security and groundwater, the full catchment-to-store-or-recharge chain, the two paths of using rain versus recharging aquifers, indicative yield and sizing, first-flush and filtration, maintenance, and the mandatory-RWH reality in Indian cities.

Plumbing

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