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Rispri LabsENGINEERING A CIRCULAR FUTURE

Free tool · Water conservation

Rainwater Harvesting Potential Calculator

Rainwater harvesting potential is roof area (m²) × annual rainfall (mm) × runoff coefficient × collection efficiency. One millimetre of rain on one square metre gives one litre, so a 100 m² RCC roof in a city with 650 mm of rain can yield about 44,000 litres a year.

Your inputs

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

Measure the plan (top view) area, not the sloped surface.

mm

From IMD normals for your district. Jaipur about 600 mm, Agra about 700 mm, Mumbai about 2,200 mm.

Runoff coefficient: share of rain that actually runs off the surface.

%

Allows for first flush, splash and gutter overflow. 75 to 90% is typical.

L/day

For example 4 people × 135 L per person per day.

How to use this calculator

  1. Enter the plan area of your roof or paved catchment, in m² or square feet.
  2. Enter the average annual rainfall for your location in millimetres.
  3. Pick the surface type to set the runoff coefficient.
  4. Adjust collection efficiency if you know your losses, then read the yearly, monthly and single storm volumes.

How is rainwater harvesting potential calculated?

Harvest (L) = A × R × C × η

  • A: catchment area in m²
  • R: rainfall in mm (1 mm on 1 m² = 1 litre)
  • C: runoff coefficient of the surface
  • η: collection efficiency (first flush, splash, overflow losses)

Worked example

With roof or catchment area 100 m², average annual rainfall 650 mm, catchment surface: RCC or metal sheet roof (0.85), collection efficiency 80 %, daily water use to compare against 540 L/day:

Rainwater you can harvest per year: 44,200 litres

The calculator also gives annual harvest 44.2 m³ (kL); monthly average 3,683 L; from one 50 mm rain day 3,400 L.

Assumptions and limits

  • Uses long term average rainfall, so dry years will yield less.
  • Runoff coefficients are typical values from CGWB and CPWD guidance; old or porous roofs run off less.
  • Does not size the tank for the dry season; that needs monthly rainfall and demand.

Frequently asked questions

How much rainwater can a 1000 sq ft roof collect?

A 1000 sq ft (92.9 m²) RCC roof collects about 79 litres for every millimetre of rain at a 0.85 runoff coefficient. With 700 mm of annual rain that is roughly 55,000 litres a year before losses, or about 44,000 litres at 80% collection efficiency.

What runoff coefficient should I use for a roof?

Use 0.8 to 0.95 for RCC, metal or glazed roofs, 0.7 to 0.8 for tiled roofs, 0.6 to 0.85 for paved areas and 0.1 to 0.2 for lawns or bare soil.

Is rainwater harvesting mandatory in India?

Many states and city building bye-laws make rooftop rainwater harvesting mandatory for new buildings above a set plot size. Delhi, for example, requires it for plots of 100 m² and above. Check the rules of your local development authority.

Should I store rainwater or recharge groundwater?

Store it if you can use it within weeks, for example for flushing, gardening or washing. Where storage is not practical, route it through a filter into a recharge pit or recharge borewell to raise the local water table.

References

  • CGWB, Manual on Artificial Recharge of Ground Water (2007)
  • CPWD, Rain Water Harvesting and Conservation Manual (2002)
  • IMD district rainfall normals

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