Rainfall Calculator
Calculate total water volume from rainfall over an area, or convert between rainfall rate units. See also Snow Day Calculator and Humidity Calculator.
How Rainfall Is Measured
Rainfall is measured as the depth of water that would accumulate on a flat surface if none of it ran off, evaporated, or soaked into the ground. In the United States, rainfall is typically reported in inches; most other countries use millimeters. A rain gauge collects precipitation over a set period, and the depth of collected water is measured. One inch of rain over one square foot equals approximately 0.623 gallons of water. Over larger areas, even small amounts of rainfall represent enormous volumes of water.
Rainfall Volume Formula
Volume (cubic feet) = (Rainfall in inches / 12) x Area (sq ft)
Volume (gallons) = Volume (cubic feet) x 7.48052
Volume (liters) = Volume (gallons) x 3.78541
Weight (lbs) = Volume (gallons) x 8.34
Conversion: 1 inch = 25.4 mm
Example
1 inch of rain on 1,000 sq ft:
Volume = (1/12) x 1000 = 83.33 cubic feet
Gallons = 83.33 x 7.48 = 623.3 gallons
That is 623 gallons (5,197 lbs) of water!
Rain Gauge Types
- Standard (graduated cylinder) — Simple tube that collects rain; read manually. Most common for home use.
- Tipping bucket — Automated gauge that tips when a set amount (usually 0.01 in) collects. Used in weather stations.
- Weighing gauge — Measures weight of collected precipitation. Can measure all precipitation types including snow.
- Optical rain gauge — Uses infrared light to detect and measure falling drops. No collection needed.
- Radar estimation — Weather radar estimates rainfall over large areas by measuring reflectivity of precipitation.
Rainfall Intensity Scale
| Category | Rate (in/hr) | Rate (mm/hr) | Description |
|---|---|---|---|
| Light | < 0.1 | < 2.5 | Drizzle, barely wets surfaces |
| Moderate | 0.1 - 0.3 | 2.5 - 7.6 | Steady rain, puddles form slowly |
| Heavy | 0.3 - 2.0 | 7.6 - 50 | Rapid puddle formation, reduced visibility |
| Violent | > 2.0 | > 50 | Flash flood risk, very poor visibility |
Average Annual Rainfall by City
| City | Annual Rainfall | Rainy Days/Year |
|---|---|---|
| Seattle, WA | 37.5 in (953 mm) | 152 |
| Miami, FL | 61.9 in (1,572 mm) | 135 |
| New York, NY | 49.9 in (1,268 mm) | 122 |
| Phoenix, AZ | 8.0 in (203 mm) | 36 |
| Chicago, IL | 36.9 in (937 mm) | 125 |
| Houston, TX | 49.8 in (1,264 mm) | 104 |
| Denver, CO | 15.6 in (396 mm) | 88 |
| London, UK | 23.8 in (601 mm) | 156 |
Frequently Asked Questions
How much water is in 1 inch of rain?
One inch of rain on 1,000 square feet produces about 623 gallons of water, weighing approximately 5,197 pounds. On an acre (43,560 sq ft), one inch of rain equals about 27,154 gallons or over 113 tons of water.
What is considered heavy rainfall?
The NWS classifies rainfall as heavy when it exceeds 0.3 inches per hour (7.6 mm/hr). At this rate, puddles form rapidly, storm drains may overflow, and flash flooding becomes possible in low-lying areas.
How do I calculate rainwater harvesting potential?
Multiply your roof area (sq ft) by the annual rainfall (inches), divide by 12 to get cubic feet, then multiply by 7.48 for gallons. Apply a 75-90% efficiency factor for losses. A 2,000 sq ft roof in a 40 in/year area can collect about 37,000-45,000 gallons annually.
Why does 1 mm of rain not seem like much?
While 1 mm sounds small, it represents 1 liter of water per square meter. Over a typical house roof (150 sq meters), 1 mm of rain produces 150 liters (about 40 gallons). Over a city, even light rain involves millions of gallons.
How accurate are rain gauges?
Standard rain gauges are accurate to about 0.01 inches when properly placed. Wind can cause under-measurement (up to 10-20% in windy conditions). Gauges should be placed in open areas away from buildings and trees, with the opening level and at least 2 feet above ground.
What is the difference between rainfall and precipitation?
Precipitation includes all forms of water falling from the atmosphere: rain, snow, sleet, hail, and freezing rain. Rainfall specifically refers to liquid water. Snow is typically converted to rainfall equivalent at a 10:1 ratio (10 inches of snow equals about 1 inch of rain).