Rainfall
A rain gauge reports water depth at its location. If one inch falls evenly over a rectangular garden bed, the geometric volume is the bed's area multiplied by that one-inch depth.
Rainfall depth converted to garden volume
How many gallons is 1 inch of rain on a garden bed?
One inch describes a depth spread evenly across the bed. The number of gallons depends on the bed's surface area.
Quick answer
0.623 gallons
One inch of rain over one square foot equals approximately 0.623 U.S. gallons. Over 100 square feet, that is about 62.3 gallons.
What “1 inch” means
It is a depth across the whole surface—not one inch in a bucket.
Irrigation
The same geometry applies when irrigation is expressed as an equivalent depth. Delivering the calculated number of gallons does not prove that a hose, sprinkler, soaker hose, or drip system spread it evenly.
Worked example
A 4 × 8 foot raised bed
4 ft × 8 ft = 32 sq ft
32 sq ft × 0.623 gal = 19.9 U.S. gal
Therefore, one inch of rain falling evenly on a 4 × 8 foot bed represents approximately 19.9 gallons of water.
4 × 4 ft bedabout 10.0 gal
4 × 8 ft bedabout 19.9 gal
4 × 12 ft bedabout 29.9 gal
10 × 10 ft gardenabout 62.3 gal
The formula
Calculate any rectangular bed.
Gallons = length (ft) × width (ft) × rain depth (in) × 0.623
The factor 0.623 converts one inch of water over one square foot into U.S. gallons. Results are geometric volumes; they do not automatically become watering recommendations.
Open the Rainfall Gallons CalculatorImportant boundary
Falling water is not the same as stored, usable soil water.
The calculation represents
- Rainfall or irrigation depth
- Measured surface area
- Total geometric water volume
It does not prove
- Even distribution across the bed
- How much ran off or drained away
- How much infiltrated or stayed in the root zone
- How much water a particular crop needs
Slope, soil condition, crusting, cracks, compaction, mulch, existing moisture, rainfall intensity, and irrigation uniformity can affect where water goes after it reaches the surface. Check the soil itself before treating a geometric volume as plant-available water.
Continue with a measurement
Use the calculator that matches the question.
Rain depth → volume
Convert rain-gauge depth over a measured bed or garden into litres and U.S. gallons.
Calculate rainfall volumeWater volume → depth
Convert a known volume applied over an area into geometric millimetres and inches.
Calculate applied water depthFlow → runtime or volume
Use measured whole-zone flow to calculate delivery volume or the runtime for a selected target.
Use measured water flowCheck the root zone
Use several locations and preserve the method, depth, and conditions behind a moisture observation.
Check soil moisture before wateringReviewed sources
Where the conversion and examples come from
University of Minnesota Extension: reports that one inch over 100 square feet is about 62 gallons and gives about 20 gallons for a 4 × 8 foot raised bed.
Watering the vegetable garden — UMN Extension
Gardening in hot weather — UMN Extension
U.S. Geological Survey: reports approximately 27,154 gallons for one inch of rain over one acre, which is consistent with approximately 0.623 gallon per square foot.
Rain and Precipitation — USGS Water Science School
Sources reviewed August 29, 2026. TerraEvidence shows the arithmetic and its boundary; it does not turn a general example into a location-, crop-, soil-, or weather-specific watering schedule.