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Construction & home

Rainwater Harvest Calculator

Estimate collectible roof runoff from rainfall and collection efficiency.

Editable waste and coverage assumptionsCheck supplier specifications

Project measurements

Runs on your device

MATERIAL ESTIMATE

Your result

Let’s calculate.

Use the Calculate button to see your result.

Review the method below for assumptions and conventions.

How to use this tool

  1. Enter area to cover, rainfall, collection efficiency.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

The method, explained

Liters = roof plan area × rainfall(mm) × efficiency/100.

A WORKED EXAMPLE

Using area to cover = 20 m², rainfall = 20 mm, collection efficiency = 80 %, the result is 320 L. Change these example inputs to match your task; use the method above to check each step.

What to keep in mind

Does not assess water quality or storage overflow. A material-quantity estimate, not a structural design or building-code check. Confirm site measurements, supplier coverage and waste before ordering.

Common questions

Which inputs does this calculation need?

Estimate collectible roof runoff from rainfall and collection efficiency. Enter area to cover in m², rainfall in mm, collection efficiency in %. The filled example is editable and is not a saved personal record.

How should I interpret the result?

Does not assess water quality or storage overflow. A material-quantity estimate, not a structural design or building-code check. Confirm site measurements, supplier coverage and waste before ordering.

Methodology maintained by ClarityKit. How these tools are built and checked.