How to use this calculator
- 1Choose what you know: rise and run, rise and span, or the angle.
- 2Enter the measurements. A pitch measured with a level works too: switch the boxes to inches and enter the rise over a 12 in run.
- 3For the rafter length, use the full run (half the building width on a simple gable roof) and add the overhang if there is one.
- 4Read the pitch as X in 12, in degrees and as a percentage, and see where it sits in the table of common pitches.
The formula
- 01Pitch (X in 12) = rise ÷ run × 12
- 02Angle = arctan(rise ÷ run)
- 03Slope % = rise ÷ run × 100
- 04Pitch factor = √(1 + (rise ÷ run)²)
- 05Rafter length = (run + overhang) × pitch factor
Pitch, slope and angle are three ways of saying the same thing. In the US and Canada pitch is written as inches of rise per 12 inches of run, so a 6/12 roof rises 6 in for every foot. The UK and Europe usually give the angle in degrees, and a percentage is common on plans. A 6/12 roof is 26.6° or 50 %; a 12/12 roof is 45° or 100 %.
The run is measured level, from the outside of the wall to the centre line of the ridge — half the span on a simple gable roof. The pitch factor says how much longer the slope is than its run: a 6/12 rafter is 1.118 times its run. Multiply the run plus the overhang by the factor to get the rafter's line length. The same factor turns a roof's footprint into its real area.
The pitch decides which roof coverings you can use. NRCA calls roofs of 3/12 or flatter low-slope roofs; the US IRC allows asphalt shingles only from 2/12, with double underlayment up to 4/12; and OSHA treats roofs steeper than 4/12 as steep roofs that need fall protection.
Worked example
A roof that rises 6 ft over a run of 12 ft has a pitch of 6 in 12 — 26.6°, or a 50% slope.
With an 18 in overhang, each rafter is 15.09 ft long along the slope (pitch factor 1.118), before cuts.
Tips before you buy
- To measure pitch from the loft, hold a level against the underside of a rafter, mark 12 in along the level and measure straight up to the rafter: that is the rise per 12.
- Phone apps and angle finders give degrees. Choose "Angle in degrees" to turn them into X in 12.
- Check the minimum pitch of the covering you want before you buy — shingles, tiles, slate and metal sheets each have their own.
- The roofing calculator uses the pitch to turn a roof's footprint into its real area, squares and shingle bundles.
Frequently asked questions
How do I calculate roof pitch?
Divide the rise by the run and multiply by 12 to get inches of rise per foot: a roof that rises 4 ft over a 12 ft run is 4/12. For degrees, take the arctangent of rise ÷ run.
What is a 6/12 roof pitch in degrees?
26.57°. Other common pitches: 4/12 is 18.43°, 8/12 is 33.69°, 10/12 is 39.81° and 12/12 is 45°. The table under the results lists every pitch from 1/12 to 12/12.
What is the minimum pitch for shingles?
Under the US IRC, asphalt shingles need at least 2/12, with double underlayment between 2/12 and 4/12. Outside the US, and for tiles, slate or metal, follow the manufacturer's minimum pitch.
What counts as a low-slope or flat roof?
NRCA treats roofs of 3/12 (about 14°) or less as low-slope; they need membranes or other low-slope systems. Even a "flat" roof needs a small fall so the water drains off.
How do I convert roof pitch to a percentage?
Divide the rise by the run and multiply by 100. A 6/12 roof is 6 ÷ 12 × 100 = 50 %, and a 45° roof is 100 %.
How long are the rafters on my roof?
Add the level overhang to the run (half the building width on a simple gable) and multiply by the pitch factor. For a 24 ft wide house at 6/12 with an 18 in overhang: (12 + 1.5) × 1.118 = 15.09 ft, before cuts. Rafter sizes must come from span tables or an engineer.
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Sources
- US IRC R905.2.2: slope for asphalt shingles (ICC)
- NRCA / Professional Roofing: low-slope roof systems are 3-in-12 (14°) or less
- OSHA 29 CFR 1926.500: a steep roof has a slope greater than 4 in 12
Formulas last reviewed: September 30, 2026 ·How we check our formulas