What is a runoff coefficient?

The runoff coefficient (Cv) is the fraction of rainfall that actually flows into your collection system. Metal or tile roofs score 0.85–0.95; gravel or flat roofs 0.70–0.80; green roofs 0.30–0.50. Losses come from evaporation, splash, and first-flush diversion.

How do I find my local rainfall data?

National meteorological services publish monthly and annual averages for most locations. Search for "[your city] average annual rainfall" or check resources like World Bank Climate Knowledge Portal.

Should I include a first-flush discount?

Yes. The first 1–2 mm of rain washes dirt and bird droppings off the roof. Many designers discard this volume. You can simulate this by subtracting 1–2 mm from your rainfall input before calculating.

How accurate is this calculator?

It gives a theoretical maximum based on the standard formula V = A × R × Cv. Real-world yield depends on gutter efficiency, overflow losses, and seasonal distribution. Treat results as planning estimates, not guarantees.

What tank size do I need?

A common rule of thumb is to size the tank to hold 1–2 months of expected collection. Divide your annual harvest volume by 12 for a monthly figure, then multiply by 1–2 to get a starting tank capacity.

Can I calculate collection for an annual rainfall total?

Yes. Enter the full annual rainfall figure (e.g. 600 mm) in the rainfall field. The result will be the total annual harvestable volume, which you can then divide by 12 for a monthly average.

Rainwater Harvesting Calculator

Name: Rainwater Harvesting Calculator
Availability: InStock
Author: Nham Vu

Calculate how much rainwater your roof can collect based on catchment area, rainfall, and runoff coefficient.

Catchment Inputs

Catchment Area

Horizontal projection of your roof or hard surface.

Rainfall Amount

Single event, monthly, or annual total.

Runoff Coefficient (Cv) 0 – 1

First-Flush Discount (optional)

mm to discard per event

Typical value: 1–2 mm. Set 0 to skip.

Harvest Estimate

Fill in the inputs and click Calculate.

Runoff Coefficient Reference

Surface Type	Typical Cv Range	Notes
Metal / corrugated iron roof	0.85 – 0.95	Most efficient common material
Glazed tile / slate roof	0.80 – 0.90	Good runoff, some absorption at joints
Concrete / asphalt flat roof	0.70 – 0.85	Ponding losses on flat surfaces
Asphalt shingle / bitumen felt	0.70 – 0.80	Slight absorption in porous material
Green / vegetated roof	0.25 – 0.50	Substrate retains significant water
Gravel or paved courtyard	0.50 – 0.70	Depends on permeability and slope

Summary

Calculate how much rainwater your roof can collect based on catchment area, rainfall, and runoff coefficient.

How it works

Enter your roof or catchment area in square meters or square feet.
Input the rainfall amount — a single event depth (mm or inches) or a monthly/annual total.
Select or enter a runoff coefficient (0–1) representing how much rain actually runs off — roofs are typically 0.75–0.95.
The calculator multiplies Area × Rainfall × Runoff Coefficient to get the harvestable volume.
Results are displayed in liters, US gallons, and cubic meters for easy tank sizing.

Use cases

Size a rainwater storage tank for a residential property.
Estimate seasonal water supply for irrigation on a smallholding or farm.
Compare collection potential across different roof materials and pitches.
Plan off-grid water independence for a cabin or rural property.
Assess whether a single rainfall event can top up a storage cistern.
Support grant applications by documenting expected water savings.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu