Rainfall Volume Calculator

Calculate the total volume of rain collected over a given area.

Rainfall Depth (how much rain fell)

Depth Unit

Catchment Area

m²

Area Unit

⚠ Please enter valid positive numbers in all fields.

Results

Volume

—

Litres

Volume

—

Cubic Metres (m³)

Volume

—

US Gallons

Stormwater Runoff Calculator

Estimate surface runoff using the Rational Method (Q = CiA).

Rainfall Intensity (mm/hr)

mm/hr

Drainage Area (hectares)

Runoff Coefficient (C) – 0 to 1

0.50

Surface Type (sets typical C)

⚠ Please enter valid values in all required fields.

Runoff Results

Peak Runoff Rate (Q)

—

m³/s

Runoff Rate

—

Litres/second

Effective Rain

—

mm (depth × C)

Runoff Coefficient Reference (C values)

Surface Type	C Value Range	Typical C
Concrete / Asphalt	0.85 – 0.95	0.90
Rooftops	0.75 – 0.95	0.85
Gravel Roads	0.55 – 0.75	0.65
Cultivated Land	0.45 – 0.65	0.55
Lawns / Grass	0.25 – 0.50	0.35
Woodland	0.10 – 0.35	0.22

Rainfall Unit Converter

Convert rainfall measurements between mm, cm, inches, and metres instantly.

Rainfall Value

Convert From

⚠ Please enter a valid number.

Conversion Results

Rainwater Harvesting Calculator

Estimate how much rainwater you can collect from your roof or surface.

Annual Rainfall (mm)

Roof / Collection Area (m²)

m²

Collection Efficiency (typical 70–90%)

80%

Roof / Surface Type

⚠ Please enter valid values in all fields.

Harvest Potential

Annual Yield

—

Litres/year

Monthly Average

—

Litres/month

Daily Average

—

Litres/day

Annual Yield

—

m³/year

How to Use the Rainfall Calculator

This free online rainfall calculator provides four calculation modes to help farmers, engineers, students, and homeowners work with rainfall data accurately.

1. Volume Calculator

Enter the rainfall depth (in mm, cm, or inches) and catchment area (m², ha, km², ft², or acres) to find total water volume in litres, cubic metres, and gallons.

2. Runoff Calculator (Rational Method)

The Rational Method formula Q = C × i × A estimates peak stormwater runoff. Widely used in civil engineering and drainage design.

3. Unit Converter

Instantly convert between millimetres, centimetres, inches, metres, and feet.

4. Rainwater Harvesting

Calculate annual collection yield from a roof or surface — in litres per year, month, and day.

Rainfall Measurement Formula

Volume (L) = Depth (mm) × Area (m²) — 1 mm of rain over 1 m² = exactly 1 litre.

Frequently Asked Questions

How do I calculate rainfall volume?

Multiply the rainfall depth (in mm) by the area (in m²). 1 mm of rain over 1 m² = 1 litre. For example, 25 mm over 500 m² = 12,500 litres.

What is a runoff coefficient?

A number from 0 to 1 representing how much rainfall becomes surface runoff. 0.9 = concrete (most runs off); 0.1 = sandy soil (most absorbs).

How many litres is 1 mm of rainfall per m²?

Exactly 1 litre. 100 mm over 1 hectare (10,000 m²) = 1,000,000 litres (1,000 m³).

What efficiency for rainwater harvesting?

Most systems use 75–85%. Metal/tile roofs reach 85–90%. Use 80% as a safe planning estimate.

How to convert inches of rain to millimetres?

Multiply inches by 25.4. Example: 1 inch = 25.4 mm. Use the Unit Converter tab above for instant results.