How Much Water Is an Inch of Rain?
An inch of rain on an acre is about 27,154 gallons. The same inch on a roof, a lawn or a square mile, with the arithmetic shown.
Published September 29, 2026 · Raindex
The short answer
One inch of rain falling on one acre is about 27,154 gallons of water, weighing about 113 tons (USGS). Everything else scales from that.
An inch on the areas people actually have
An acre is 43,560 square feet, so one inch is about 0.623 gallons per square foot. From there:
- 1,000 sq ft roof: about 623 gallons per inch of rain.
- 2,000 sq ft roof: about 1,250 gallons.
- Quarter-acre lot: about 6,800 gallons.
- 160-acre field: about 4.3 million gallons.
- One square mile (640 acres): about 17.4 million gallons.
Why the same inch does different things
The volume is fixed; what happens to it is not. An inch that falls gently over a day has time to soak in. The same inch in twenty minutes, especially on crusted or already saturated ground, sends far more of it off as runoff. So two storms with identical totals can leave a field or a basement in very different states, which is why Raindex shows hour-by-hour detail for any day, not only the total.
Snow is different again: Raindex reports frozen precipitation as liquid equivalent, the water you would have if it melted, and labels it, rather than guessing a snow depth.
Turning your own totals into volume
Multiply the inches of rain by the area in square feet and by 0.623 to get gallons. For a field, multiply inches by acres by 27,154. The lookup gives you the last 24 hours for any ZIP code to plug in; the Raindex app keeps the day-by-day totals for each place you save, and Pro exports them as CSV for your own spreadsheet.
See the last 24 hours for any ZIP code with the free rainfall lookup, or keep a labelled, day-by-day record for the places you care about in Raindex.
Common questions
How many gallons is an inch of rain per acre?
About 27,154 gallons, weighing about 113 tons, according to the USGS.
How many gallons does a 1,000 square foot roof collect from an inch of rain?
About 623 gallons before any losses: 1,000 × 0.623.
Is an inch of rain a lot?
It depends on how fast it falls and where. In most of the eastern US an inch in a day is a solid soaking rain; in the desert Southwest it can be a large share of a month's normal. Each city page shows that city's monthly normals for comparison.
Rain totals by city
More guides
- Why Your Rain Gauge and a Rainfall App DisagreeRadar rainfall and a backyard gauge measure different things in different places. Here is where the gap comes from, and how to compare them fairly.
- How NOAA's MRMS Rainfall Estimates WorkWhat the Multi-Radar Multi-Sensor system is, how it turns radar echoes and gauge reports into hourly rainfall on a 1 km grid, and what its passes and flags mean.
- Gauge-Adjusted, Radar Only, Provisional: What Rainfall Labels MeanEvery Raindex reading carries a label saying how the number was made. What gauge-adjusted, radar only, provisional and no reading mean, and why there is no accuracy percentage.