Rainfall Calculator

Calculate how much water a roof or catchment area collects. Enter the catchment area and rainfall depth to get total volume in gallons, liters, and cubic feet — plus the collectible amount after runoff losses.

Quick Facts

Rule of thumb
1 inch of rain on 1,000 sq ft = ~623 gallons
Exact constant: 0.6233 gallons per sq ft per inch (144 cubic inches / 231 cubic inches per gallon). In metric: 1 sq m under 1 mm of rain = exactly 1 liter.

Your Results

Calculated
Total volume
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Gallons collected
Total volume
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Liters collected
Total volume
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Cubic feet collected
Collectible volume
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Gallons after efficiency loss

Ready

Enter your catchment area and rainfall depth, then press Calculate.

Understanding Rainfall Volume

This tool estimates how much water falls on — and can be collected from — a roof, driveway, garden bed, or any flat catchment area during a rain event. Give it the catchment's area and the rainfall depth (the number of inches or millimeters that fell), and it multiplies them together, converting units consistently, to report the total volume of water involved.

The core formula

Volume is simply area times depth, once both are expressed in compatible units:

  • US units: gallons = area (sq ft) × depth (inches) × 0.6233. The constant comes from 1 sq ft × 1 inch = 144 cubic inches, and 1 gallon = 231 cubic inches, so 144 ÷ 231 = 0.6233 gallons.
  • Metric units: liters = area (sq m) × depth (mm). This needs no rounding constant: 1 sq m × 1 mm of depth = 0.001 cubic meters, and 1 cubic meter = 1,000 liters, so the two units cancel to an exact 1-to-1 ratio.
  • Cubic feet: cubic feet = area (sq ft) × depth (inches) ÷ 12, since 12 inches make a foot.

If your area is in square meters or acres, or your depth is in millimeters, the calculator converts everything to square feet and inches (or square meters and millimeters) first, using standard factors: 1 square meter = 10.7639 square feet, 1 acre = 43,560 square feet, and 1 inch = 25.4 millimeters.

Collection efficiency

Not every drop that falls on a roof reaches a storage tank. Some evaporates, splashes past the gutters during heavy bursts, or is intentionally diverted by a first-flush device that keeps the initial dirty runoff out of storage. Rainwater harvesting guides typically apply a collection efficiency of 75-90% for a pitched roof with gutters — this calculator defaults to 85%. Set it to 100% to see the theoretical maximum before any losses.

Worked example

A 1,500 sq ft roof catches 2 inches of rain: 1,500 × 2 × 0.6233 ≈ 1,870 gallons of total rainfall. At 85% collection efficiency, roughly 1,590 gallons would actually reach a storage tank. In metric terms, a 140 sq m roof under 50 mm of rain collects 140 × 50 = 7,000 liters before efficiency losses.

Common reference points

  • 1 inch on 1,000 sq ft ≈ 623 gallons ≈ 2,360 liters.
  • 1 mm on 100 sq m = exactly 100 liters.
  • 1 inch on a 2,000 sq ft roof ≈ 1,247 gallons of total rainfall, before any collection losses.

Frequently Asked Questions

How do I calculate rainwater harvesting volume?
Multiply the catchment area by the rainfall depth, converting units so they agree, then apply a unit-conversion constant. In US units: gallons = area in square feet × depth in inches × 0.6233. In metric units: liters = area in square meters × depth in millimeters, since 1 square meter of area under 1 millimeter of rain holds exactly 1 liter.
How many gallons does 1 inch of rain produce on 1,000 square feet?
About 623 gallons. The exact constant is 0.6233 gallons per square foot per inch of rain (144 cubic inches per square-foot-inch, divided by 231 cubic inches per gallon), so 1,000 sq ft × 1 inch × 0.6233 = 623.3 gallons.
What collection efficiency should I use for a roof catchment?
Most rainwater harvesting guides use 75-90% for a pitched roof with gutters, and this calculator defaults to 85%. The loss accounts for evaporation, splash-out, gutter overflow during heavy bursts, and the first-flush water diverted to keep debris out of storage. Flat roofs or well-sealed metal roofs can run toward the higher end; rough or debris-prone surfaces should use a lower figure.
How do I convert square meters and millimeters to liters?
The conversion is exact and needs no rounding constant: 1 square meter of catchment under 1 millimeter of rainfall depth yields exactly 1 liter of water, because 1 m² × 0.001 m = 0.001 m³, and 1 m³ equals 1,000 liters. So liters collected = area in square meters × depth in millimeters.

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