Research
How much rain your roof sheds, in a year and in a hard storm
In a normal year, a house with a 2,000-square-foot roof footprint sheds about 70,300 gallons of rain in Cumming, 62,900 in Atlanta, 58,500 in Macon, 56,400 in Cincinnati and 48,900 in South Bend, based on NOAA's 1991-2020 climate normals.
By Top Rated Gutters · Published
Gutters are sized for the worst few minutes of a storm, but they carry water all year. This study puts both in gallons for a house with a 2,000-square-foot roof footprint in the five areas we serve, using NOAA's official figures.
Key numbers
70,300 gallons
Rain a 2,000 sq ft roof sheds in a normal year in Cumming, the wettest of the five (56.37 inches a year).
Source: NOAA NCEI 1991-2020 normals, Cumming 2N station
48,900 gallons
The same roof in South Bend (39.23 inches a year).
Source: NOAA NCEI 1991-2020 normals, South Bend airport station
3,030 gallons
Rain on that roof in one hour of a 10-year storm in Macon (2.43 inches), the most of the five.
Source: NOAA Atlas 14, 10-year 1-hour depth at the Macon airport station
169 gallons a minute
Peak flow off that roof in the heaviest 5 minutes of a 10-year storm in Macon (8.1 inches an hour).
Source: NOAA Atlas 14, 10-year 5-minute depth; our calculation
1,247 gallons
What every inch of rain puts through the gutters of a 2,000 sq ft roof.
Source: 1 inch on 1 sq ft = 144 cubic inches; 231 cubic inches = 1 US gallon
Rain a roof sheds in a normal year
The table behind the chart
For a 2,000 sq ft roof footprint. Annual figures are NOAA's 1991-2020 normals; storm figures are NOAA Atlas 14 estimates for a 10-year storm. Every inch of rain on this roof is 1,247 gallons.
| Area (weather station) | Rain a year, inches | Gallons a year | Days with 1 in. or more | 10-year storm, 1 hour, inches | Gallons in that hour | 10-year storm, peak 5 minutes, in./hour | Peak gallons a minute |
|---|---|---|---|---|---|---|---|
| Cumming, GACumming 2N (NOAA cooperative station) | 56.37 | 70,300 | 17.5 | 2.11 | 2,630 | 8.0 | 166 |
| Atlanta, GAAtlanta Hartsfield-Jackson International Airport | 50.43 | 62,900 | 14.9 | 2.18 | 2,720 | 7.7 | 161 |
| Macon, GAMacon Middle Georgia Regional Airport | 46.91 | 58,500 | 14.0 | 2.43 | 3,030 | 8.1 | 169 |
| Cincinnati, OHCincinnati/Northern Kentucky International Airport (CVG), Hebron, KY | 45.26 | 56,400 | 10.8 | 1.99 | 2,480 | 7.1 | 148 |
| South Bend, INSouth Bend International Airport | 39.23 | 48,900 | 7.5 | 2.01 | 2,510 | 7.2 | 149 |
Want the figure for your own roof? Our gutter size calculator and measuring guide walk through it. Download the table (CSV).
What this means for your gutters
- Size for the storm, not the year. A year's total doesn't size a gutter; the heaviest minutes of a storm do. At the peak of a 10-year storm a 2,000 sq ft roof in Macon sheds about 169 gallons a minute, against 148 in Cincinnati, about 14% more at the peak.
- Whether a 5-inch gutter is enough depends on your roof, not just your city. What decides it is how much roof drains to each downspout, how long each run is and how steep the roof is. The trade's sizing reference is the SMACNA Architectural Sheet Metal Manual, which works from a 5-minute rainfall rate like the one in the table below. Our gutter size calculator gives a planning estimate; we measure the roof, the runs and the outlets on site before recommending a size.
- Big rain days are routine here. Cumming averages 17.5 days a year with 1 inch of rain or more, and South Bend 7.5. Each of those days puts at least 1,247 gallons through the gutters of a 2,000 sq ft roof, so a clogged gutter or a downspout that empties beside the foundation gets tested many times a year.
- Up north, part of the water arrives as snow. NOAA counts snow as the water it melts into, so the Cincinnati and South Bend totals include snowmelt that reaches the gutters days or weeks later. When meltwater refreezes at the eaves, the cause is heat escaping from the attic; gutters and guards neither cause nor cure ice dams.
Method
Annual rain is each station's 1991-2020 annual precipitation normal from NOAA's National Centers for Environmental Information. Snow is included as its melted water.
Storm figures are NOAA Atlas 14 point precipitation frequency estimates (partial duration series) at each station's coordinates: the 10-year, 1-hour depth and the 10-year, 5-minute depth, which we multiply by 12 to give a rate in inches an hour. A 10-year storm has about a 1-in-10 chance of happening in any given year.
Gallons = roof footprint in square feet x inches of rain x 144 / 231 (cubic inches in a square foot inch, and in a US gallon), about 0.623 gallons per square foot per inch. Annual totals are rounded to the nearest 100 gallons, the 1-hour storm to the nearest 10.
Footprint is the area a roof covers seen from above. Rain lands on that area whatever the pitch, so a steep roof sheds the same water as a shallow one over the same footprint; wind-driven rain on steep slopes is a sizing question the table does not cover.
Sources
- NOAA NCEI, U.S. Climate Normals 1991-2020 (annual/seasonal station files). Period: 1991-2020.
- NOAA National Weather Service, Atlas 14 Precipitation Frequency Data Server. Period: Volume 9 Version 2 (Southeastern States); Volume 2 Version 3 (Ohio River Basin).
Limits
- These are station figures. Rain varies by several inches a year across a metro area, and the station is the spot where it was measured, not your street.
- The Cincinnati figures come from the official Cincinnati weather station at CVG airport in Hebron, Kentucky. Northwest Georgia and Marietta use the Atlanta airport station; Northeast Georgia uses the NOAA cooperative station in Cumming, whose rain normal NOAA grades representative (19 years of data, gaps filled from nearby stations). The other four rain normals are graded standard.
- Every drop that lands is counted. Some evaporates or soaks into the roof, so treat the gallons as what lands on the roof, not a measured flow.
- Atlas 14 values are estimates with confidence bands NOAA publishes alongside them. The 1-hour storm in Macon is 22% deeper than in Cincinnati; the table shows each place's value.
- This is not a gutter sizing for any house. It does not account for valleys, roof planes or where downspouts drain.
Cite this study
Top Rated Gutters (2026). How much rain your roof sheds, in a year and in a hard storm. Published 12 Nov 2026. https://topratedgutters.com/research/roof-runoff-gallons-by-city/
The table is free to download as a CSV file and free to reuse with credit to Top Rated Gutters and a link to this page (CC BY 4.0). The underlying public data stay under their publishers’ own terms.
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