Rainfall Volume Calculator

Rainfall volume is the total amount of water that falls onto a surface area during a storm or time period. Hydrologists use this to plan drainage, reservoirs, and flood mitigation. Home owners use it to size rain barrels, gutters, and garden irrigation.

Rainfall Volume formula: Volume = Rainfall Depth × Catchment Area

Runoff Volume formula (using Runoff Coefficient): Runoff Volume = C × Rainfall Depth × Area

What each variable means:

• Rainfall Depth: measured in millimeters (mm) or inches; represents the depth of water that would accumulate on a perfectly flat, impermeable surface
• Catchment Area: the horizontal surface area collecting the rain (e.g., roof area, watershed, field)
• C: Runoff Coefficient, fraction of rainfall that becomes runoff (0 = all absorbed, 1 = all runs off)
• Volume: typically expressed in liters or gallons

Unit conversion: 1 mm of rain on 1 m² = 1 liter of water 1 inch of rain on 1 ft² = 0.6234 gallons

Worked example: A house has a roof area of 150 m². A storm delivers 25 mm of rainfall. What volume of water hits the roof?

Volume = 25 mm × 150 m² = 3,750 liters = 3.75 cubic meters

With a runoff coefficient of C = 0.90 (typical for a metal roof): Runoff = 0.90 × 3,750 = 3,375 liters — enough to fill over 11 bathtubs.

Runoff Coefficient reference values (C):

• Flat rooftop (metal/tile): 0.85–0.95
• Asphalt pavement: 0.70–0.95
• Lawns (sandy soil): 0.05–0.35
• Lawns (clay soil): 0.13–0.45
• Forest / woodland: 0.05–0.25
• Urban commercial district: 0.70–0.95

Practical uses:

• Sizing rain barrels (typical barrel = 200–400 liters)
• Designing stormwater detention ponds
• Estimating irrigation needs
• Flood risk modeling for low-lying areas

A 100-year storm in most US cities delivers 3–5 inches (75–125 mm) of rain in 24 hours. Sizing drainage for this event is standard engineering practice.