Rainfall Calculator

Convert rainfall depth and area to volume, estimate runoff, and explore unit conversions. Perfect for agriculture, stormwater management, and hydrology.

e.g., 0.3 for lawn
1 acre · 1 in 1 ha · 10 mm 1000 ft² · 0.5 in 1 km² · 25 mm 1 acre (ft²) · 1 in
Computing...
Rainfall Volume Results
1. Input summary
Area: 1 ac | Depth: 1 in | Runoff coefficient: 0.30
2. Total rainfall volume
102.8
cubic meters (m³)
102,800
liters (L)
27,160
US gallons
22,620
UK gallons
3,630
cubic feet (ft³)
3. Runoff volume (if C > 0)
30.8
cubic meters (m³)
30,840
liters (L)
8,148
US gallons
Infiltration / losses: 72.0 m³ (70%)
Total volume (m³) at current area
Current depth / volume

Understanding Rainfall Calculations

The fundamental relationship: Volume = Area × Depth. However, units must be consistent. This calculator converts everything to meters and cubic meters before converting to your preferred output.

Basic formula:

V (m³) = A (m²) × d (m)

Where 1 mm = 0.001 m, 1 cm = 0.01 m, 1 in = 0.0254 m.

Unit Conversions

  • Area: 1 acre = 4046.86 m², 1 ha = 10,000 m², 1 km² = 1e6 m², 1 ft² = 0.092903 m²
  • Volume: 1 m³ = 1000 L = 264.172 US gal = 219.969 UK gal = 35.3147 ft³

Runoff Coefficient (C)

The runoff coefficient represents the fraction of rainfall that becomes surface runoff. The rest is lost to infiltration, evaporation, or interception. Typical values:

  • Paved areas, roofs: 0.7–0.95
  • Lawns, sandy soil: 0.1–0.3
  • Clay soil, flat areas: 0.3–0.5
  • Forest: 0.1–0.2

Applications

  • Stormwater management: Sizing detention ponds, drainage systems.
  • Agriculture: Estimating irrigation needs, water availability.
  • Hydrology: Runoff prediction for flood forecasting.
  • Rainwater harvesting: Calculating potential collection from roofs.

Calculator features: Real‑time unit conversion, runoff estimation, interactive volume‑vs‑depth graph, and quick preset scenarios. Click the button to copy a summary of results.

Frequently Asked Questions

First determine the total area (using a planimeter, GIS, or approximate shapes), then enter it into the calculator. The volume calculation is independent of shape.

For a mix of lawn and impervious surfaces (driveway, roof), 0.3–0.5 is common. Use 0.3 for pervious lawn, 0.9 for roofs.

Yes, if you know the water equivalent of snow (typically 1 inch of rain ≈ 10 inches of snow), you can enter the water equivalent as depth.