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Rispri LabsENGINEERING A CIRCULAR FUTURE

Free tool · Water conservation

First Flush Volume Calculator (Rainwater Harvesting)

First flush volume = roof area (m²) × first flush depth (mm). Diverting the first 0.5 to 2 mm washes dust, leaves and droppings off the roof. A 100 m² roof at 1 mm needs a 100 litre diverter, which is about 5.7 m of 150 mm pipe.

Your inputs

Results update as you type. Example values are filled in.

mm

0.5 for clean metal roofs, 1 typical, 2 for dusty, urban or industrial areas.

mm

Common: 75, 100 or 150 mm (3, 4, 6 inch).

How to use this calculator

  1. Enter the roof area that drains to the downpipe.
  2. Pick a first flush depth: more for dusty sites.
  3. Enter the diverter pipe size to get its length.

How big should a first flush diverter be?

V (L) = A (m²) × d (mm)

Pipe length (m) = V ÷ 1,000 ÷ (π × D² ÷ 4)

The Texas Water Development Board recommends at least 1 to 2 gallons per 100 sq ft (about 0.4 to 0.8 mm); dusty Indian rooftops often need 1 to 2 mm.

Worked example

With roof area 100 m², first flush depth 1 mm, diverter pipe internal diameter 150 mm:

First flush volume: 100 litres

The calculator also gives diverter pipe length 5.66 m; number of 3 m pipe lengths 2 nos; volume in us gallons 26.4 gal.

Assumptions and limits

  • Assumes the whole roof drains to one diverter. Split the area if there are several downpipes.
  • Fit a slow drip outlet or cleanout at the bottom so the diverter empties between rains.

Frequently asked questions

What is a first flush diverter?

It is a pipe or chamber that fills with the first, dirtiest rain off the roof. Once it is full, cleaner water flows on to the storage tank or recharge pit.

How much first flush do I need for 1000 sq ft?

A 1000 sq ft (92.9 m²) roof needs about 46 litres at 0.5 mm, 93 litres at 1 mm and 186 litres at 2 mm.

Can I skip the first flush if I have a filter?

A first flush diverter protects the filter. Without it, the filter clogs quickly and needs cleaning after almost every storm.

References

  • Texas Water Development Board, The Texas Manual on Rainwater Harvesting (2005)
  • CGWB, Manual on Artificial Recharge of Ground Water (2007)

Built and maintained by the Rispri Labs engineering team. Last updated .