Gutter sizing debates run on folklore until one number arrives: the 5-minute rainfall intensity — the heaviest five minutes a typical year delivers, in inches per hour. It is the number the trade sizes to, and it explains every overflow complaint a correctly-hung gutter ever received.
Why five minutes
A gutter's job is peak flow, not volume. The heaviest cloudburst concentrates an inch of rain into minutes; the annual total is irrelevant to a channel that drains continuously. Hydraulic design therefore keys on the maximum sustained intensity — NOAA's Atlas 14 publishes it by location, and the map runs from ~4.5 in/hr in the arid interior to 9+ on the Gulf Coast.
The conversion
| Roof area | 5 in/hr | 7.5 in/hr | 9 in/hr |
|---|---|---|---|
| 600 sq ft | 3.6 gpm — 5-in K fine | 5.4 — 5-in at limit | 6.5 — 6-in |
| 900 sq ft | 5.4 — 5-in at limit | 8.1 — 6-in | 9.7 — 6-in |
| 1,400 sq ft | 8.4 — 6-in | 12.6 — 6-in + split | 15.1 — split |
(K-style capacities: 5-inch ≈ 6.7 gpm, 6-inch ≈ 11.2 gpm at standard slope.) The table shows the design pattern: area and intensity trade against each other, and the Gulf Coast needs 6-inch where the Plains get by on 5.
What the model ignores — deliberately
- Valleys: two slopes converge and concentrate — splash guards and local upsizing are the field fix.
- Debris: guards and maintenance matter more than capacity; a dammed gutter fails at any size.
- Climate change: Atlas updates keep raising intensity numbers — when re-roofing, size for the current atlas, not the gutter that worked in 1995.
The practical procedure
- Measure the plan-view roof area draining to each run (footprint, not slope area).
- Look up your 5-minute intensity (NOAA Atlas 14, or the calculator's presets).
- Run the size calculator, apply the pitch factor, and choose 5 or 6.
- Count downspouts with the downspout tool — capacity per outlet finishes the design.
Storms are the design condition; everything else is maintenance. Size to the storm and the average takes care of itself.