Roof Pitch Calculator
Convert between rise-over-run pitch (X-in-12), degrees, and slope percentage. Includes rafter-length multiplier for figuring out material lengths.
Last updated: September 2026
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About this calculator
Roof pitch is expressed three ways: rise-in-12 (US carpentry — inches of vertical rise per 12 inches of horizontal run), degrees (trigonometric angle), and percent (rise / run × 100, common in Europe and highway grades).
Conversions: angle = arctan(rise / 12) = arctan(percent / 100). A 6-in-12 pitch is arctan(6/12) = 26.57° = 50%. Common carpentry pitches: 4-in-12 (18.4°), 6-in-12 (26.6°), 8-in-12 (33.7°), 12-in-12 (45°).
Rafter length (from ridge to eave, hypotenuse) is run × √(1 + (rise/run)²), or equivalently run × sec(angle). A 6-in-12 pitch on a 12 ft run has a rafter of 12 × √(1 + 0.25) = 12 × 1.118 = 13.42 ft. This multiplier (1.118 for 6-in-12, 1.202 for 8-in-12, 1.414 for 12-in-12) is the "rafter length factor" — carpentry tables list it for every common pitch.
Building code implications: IRC requires low-slope (< 2:12) roofing materials rated for the reduced water flow. Steeper roofs shed water faster but cost more materials per floor-plan square foot. Snow-load code varies by pitch too — pitched roofs shed snow better than flat.
How to use
Example — a 6-in-12 pitch. Angle = arctan(6/12) = 26.57°. Slope % = 6/12 × 100 = 50%. Rafter length factor = √(1 + (6/12)²) = √1.25 = 1.118. Example — a 22° angle. Rise = 12 × tan(22°) = 4.85 in, so about a 4.85-in-12 pitch. Percent = tan(22°) × 100 = 40.4%. Useful when architectural drawings give the pitch as a degree measure but the framer works in rise-in-12.
Frequently asked questions
What is the most common residential roof pitch?
In the US, 6-in-12 (26.6°) is the most common. It sheds water and snow adequately, allows walking with reasonable care, and works with standard asphalt shingles. 4-in-12 is common in warmer climates; 8-in-12 or steeper in heavy-snow regions.
What is the minimum pitch for asphalt shingles?
Asphalt shingles are rated for 2-in-12 minimum with a double-layer underlayment (and 4-in-12 or steeper with single underlayment). Below 2-in-12, use a membrane roof (TPO, EPDM, modified bitumen) — shingles will leak by capillary action and back-water on a slope that shallow.
How do I compute rafter length?
Rafter length = horizontal run × √(1 + (rise/run)²). For a 6-in-12 pitch on a 12 ft run, rafter = 12 × √1.25 = 13.42 ft. Add 12–16 in for eave overhang plus the tail cut. Use a framing square or a calculator like this one to avoid arithmetic errors.
Do steeper roofs cost more?
Yes. Material area scales with the rafter-length factor, so a 12-in-12 roof uses 1.414× the shingles of a flat area for the same floor plan. Labor is also slower due to safety requirements. Total roofing cost on a 12-in-12 is roughly 1.5–2× a 4-in-12 for the same house.