Rainfall Volume Calculator

When rain falls on a roof, yard, or field, knowing how much water you can actually collect depends on the size of the surface and how efficiently it channels runoff. Enter your catchment area, rainfall depth, and runoff coefficient into the Rainfall Volume Calculator to find the total rainwater volume in gallons. Secondary outputs include volume in cubic feet and liters, plus your collection efficiency percentage.

sq ft

Total area where rain is collected (roof, yard, etc.)

inches

Amount of rainfall measured

Efficiency factor (0.8 for roof, 0.3 for grass, 0.9 for concrete)

Results

Total Rainwater Volume

--

Volume (Cubic Feet)

--

Volume (Liters)

--

Collection Efficiency

--

More Chemistry Tools

Frequently Asked Questions

How do I calculate rainfall volume?

Rainfall volume is calculated using the formula: Volume = Catchment Area × Rainfall Depth × Runoff Coefficient. This gives you the total amount of water collected from rain falling on a given area.

What is a runoff coefficient?

The runoff coefficient represents the efficiency of water collection from different surfaces. Smooth surfaces like roofs have coefficients around 0.8-0.9, while grass and soil have lower coefficients around 0.3-0.5.

How much water can I collect from my roof?

For every inch of rainfall, you can collect approximately 0.623 gallons per square foot of roof area. A 1,000 sq ft roof can collect about 623 gallons from 1 inch of rain.

What factors affect rainwater collection?

Key factors include catchment area size, rainfall amount, surface material, roof slope, and collection system efficiency. Losses occur due to evaporation, absorption, and system inefficiencies.

How do meteorologists measure rainfall depth?

Meteorologists use rain gauges - cylindrical containers that collect rainwater. The depth is measured in inches or millimeters, representing how deep the water would be if it didn't run off.

Can I use this calculator for different surface types?

Yes, adjust the runoff coefficient based on your surface: concrete/asphalt (0.9), metal roof (0.85), tile roof (0.8), gravel (0.6), grass (0.3), or forest (0.2).

What's the difference between rainfall depth and volume?

Rainfall depth measures how deep water would be on a flat surface (inches/mm), while volume measures the total amount of water collected (gallons/liters) from a specific catchment area.