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Rainwater Harvesting Requirement Calculator

Work out rainwater harvesting requirement instantly with clear inputs, formula shown and shareable results.

About Rainwater Harvesting Requirement Calculator

Harvestable volume is catchment area times rainfall depth times a runoff coefficient that accounts for evaporation, first flush and losses: about 0.85 for a hard roof. Storage is sized on a few days of intense monsoon rather than the annual total, because a tank large enough to hold a year's rain would be absurd.

Formulas

Harvestable volume

m³ = roof area m² x rainfall mm x runoff coefficient / 1000

Storage

Tank m³ = peak daily yield x storage days, peak daily taken as annual yield over 60 rain days

Worked example

With the default values below, Rainwater Harvesting Requirement Calculator returns Annual harvestable volume of 612.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Catchment roof areaminimum 0.1 m²
800 m²
Annual rainfallminimum 1 mm
900 mm
Runoff coefficientaccepts 0.1–1
0.85
Storage as days of peak rainfallminimum 0.5 days
3 days

Results

Annual harvestable volume
612.00 m³
Annual volume in litres
612,000 L
Suggested storage tank size
30.60 m³
Average monthly yield
51.00 m³

Step by step

  1. 1Values used
  2. 2Harvestable volume
  3. 3Storage
  4. 4Annual harvestable volume= 612.00 m³
  5. 5Annual volume in litres= 612,000 L
  6. 6Suggested storage tank size= 30.60 m³
  7. 7Average monthly yield= 51.00 m³

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Rainwater harvesting requirements, first flush arrangements and recharge design are set by local regulations.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Rainwater harvesting requirements, first flush arrangements and recharge design are set by local regulations.

Frequently Asked Questions

How is the Rainwater Harvesting Requirement figure worked out?

m³ = roof area m² x rainfall mm x runoff coefficient / 1000. Harvestable volume is catchment area times rainfall depth times a runoff coefficient that accounts for evaporation, first flush and losses: about 0.85 for a hard roof.

Should the tank be sized on demand instead?

For a supply-led system yes. Where the aim is recharge rather than storage, the sizing question becomes percolation capacity of the pit rather than tank volume.

What values do I need to enter?

This calculator takes 4 inputs: Catchment roof area, Annual rainfall, Runoff coefficient, Storage as days of peak rainfall. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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