How the gutter size calculator works
The sizing method
Flow (gpm) = roof area × intensity ÷ 96.23 — the standard conversion from square-feet-inches-per-hour to gallons. Compare against gutter capacity: 5-inch K-style handles ~6.7 gpm, 6-inch K-style ~11.2; half-round carries ~40% less at the same nominal size. The calculator applies your pitch factor and reads the verdict.
Why intensity, not volume
A gutter never needs to carry annual rainfall — only the worst five minutes of it. NOAA's Atlas publishes those peak rates by location, and they vary 2× across the US. Sizing to annual averages is why undersized gutters overflow exactly when it matters most.
Upsizing economics
Six-inch gutters cost 10–20% more than five-inch and double the capacity — cheap insurance where valleys concentrate two slopes or trees add debris dams. Oversizing has no failure mode except aesthetics; undersizing has fascia rot and foundation splash.