Roof Pitch Calculator
Roof pitch — the ratio of vertical rise to horizontal run — is the most basic spec on any roofing job. Enter rise and run and the tool instantly returns the US-style X-in-12 notation (e.g. 6:12), the slope angle in degrees, the slope as a percentage, and the rafter-length multiplier so you can estimate material, choose between shingles and membrane, or check a drawing.
Enter positive numbers — run (horizontal) must be greater than 0
Pitch (X-in-12)
6:12
—
Slope angle
26.57°
Slope %
50.00%
Rafter length multiplier
1.118
Formula
pitch (X-in-12) = rise / run × 12 | angle = arctan(rise / run) | slope % = rise / run × 100 | rafter = √(rise² + run²)
- · IRC (International Residential Code) R905 splits roofs roughly as: < 2:12 is "low-slope" and requires membrane roofing; 2:12 to < 4:12 needs double-layer underlayment for asphalt shingles; 4:12 to 9:12 is the standard residential band; > 9:12 is "steep-slope" with additional fasteners and steep-slope underlayment.
- · Hong Kong "Small House" (丁屋) and village roofs often sit around 4:12 to 6:12 — steep enough to shed rain and resist wind uplift. Commercial and high-rise rooftops are typically flat (< 0.5:12) with bituminous or PVC membrane.
- · A symmetric gable roof has rise measured from the eaves up to the ridge, and run measured horizontally from the outer wall to the point directly below the ridge — i.e. half of the total span.
- · Rafter multiplier = √(rise² + run²) / run = sec(angle). Multiply your run by this number to get actual rafter length — for a 6:12 pitch the factor is ≈ 1.118.
- · The multiplier ignores eave overhang and ridge thickness; add manufacturing tolerance and overhang to the cut length when ordering lumber.
- · Sources: IRC 2021 R905, NRCA Roofing Manual, Hong Kong Buildings Department B(C)R Cap. 123B Reg. 39.
Frequently asked
What is a 6:12 pitch in degrees?
arctan(6 / 12) = arctan(0.5) ≈ 26.57°, with a slope of 50 %. 6:12 sits inside the conventional band (4:12 to 9:12) — the most common pitch for asphalt-shingle homes and a frequent default in DIY framing examples.
Why is roof pitch written as “X-in-12” instead of just degrees?
Because carpenters and roofers lay out cuts with a framing square (a 24″ × 16″ steel L-square). X-in-12 maps directly to the inch markings on the tool, so no calculator is needed at the cut. European and Hong Kong drawings tend to use degrees or slope-% instead — the three notations are interchangeable: 12:12 = 45° = 100 % slope.
What units do I use for rise and run? Do they need converting?
They just need to share the same unit — the ratio cancels units out. So 1.5 m / 3 m gives the exact same angle, slope and X-in-12 as 5 ft / 10 ft. The unit dropdown is only used when echoing back the rafter length.
I am building a Small House (丁屋) in Hong Kong with asphalt shingles — what pitch should I pick?
For asphalt shingles aim for at least 4:12 (≈ 18.4°). Anything below that needs the double-underlayment treatment in IRC R905 to be code-compliant — and Hong Kong's summer rainfall makes shallower pitches risky. If you want quick run-off without piling weight onto the purlins, 4:12 to 6:12 (18.4°–26.6°) is the most common range; once you go past 7:12 (30°+) you start paying for fall-arrest scaffolding and slower install times.
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