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. To reproduce that above, enter 25 mm and 150 sq m, pick the metal roof surface, and leave the efficiency box empty.

One thing that catches people out: use the roof’s footprint, not the sloped surface area. A pitched roof collects rain falling vertically, so it catches exactly what its outline on the ground catches. Adding for the pitch overstates the harvest every time.

Runoff Coefficient reference values (C):

• Flat rooftop (metal/tile): 0.85–0.95
• Asphalt shingle roof: 0.80–0.90
• Asphalt pavement: 0.70–0.95
• Gravel or unpaved drive: 0.15–0.35
• 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

Pick the surface above and the calculator uses the middle of that band as C. The separate efficiency box is a different loss and the two multiply. C is what leaves the surface; efficiency is what survives the gutters, the first-flush diverter and the overflow before it reaches your tank. A metal roof at C = 0.90 feeding a system that loses 10% in transit collects 0.90 × 0.90 = 81% of the rain that landed on it.

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.