Roof pitch calculator: x/12, degrees, percent and the pitch multiplier
This roof pitch calculator converts a roof’s steepness between every way people write it: rise per 12 (the US and Canadian x/12 pitch), degrees (used in the UK, Australia and on most metric drawings), percent slope, a rise : run ratio and millimetres of rise per metre. Enter the pitch in any one of them — or measure the rise and run, or the span and ridge height — and the rest appear at once.
It also gives the pitch multiplier (rafter factor) that turns a flat, plan measurement into the true length along the slope, the multiplier for regular hip and valley rafters, a scaled pitch triangle and a pitch chart from 1/12 to 24/12. Add a horizontal run to get the rafter length straight away.
How to use the Roof Pitch Calculator
- Choose how you know the pitch: x/12, degrees, percent, rise and run, or span and height.
- Type the value — “6”, “6/12”, “30” degrees, or two measurements in the same unit. Tap a preset for common pitches.
- Read the angle, percent, ratio, multipliers and roof category; the triangle is drawn to scale.
- Optionally enter a horizontal run (half the span on a gable) to get the slope length and the rise.
- Copy the conversions, or move on to the rafter calculator for cut lengths and birdsmouth sizes.
Roof Pitch Calculator features
Every pitch format
x/12, degrees, percent, rise : run, mm per metre and the old carpenter’s “¼ pitch” (rise ÷ span).
Pitch multiplier
√(1 + (rise/12)²) — multiply a plan length or area by it to get the length or area on the slope.
Hip and valley factor
Length of a regular hip or valley per unit of common run, and the hip’s own angle (rise in 16.97).
Measure it two ways
From rise and run in any unit, or from the building span and the ridge height above the walls.
Metric and imperial
Degrees and mm per metre for UK, Australian and Indian drawings; x/12 for US and Canadian ones.
Pitch chart
Angle, percent and both multipliers for 1/12 to 24/12, with your pitch highlighted.
Roof pitch formulas
To measure an existing roof, hold a level 12 in (or 1,000 mm) horizontally against the underside of a rafter or the roof surface and measure straight down to it: that drop is the rise per 12 (or per metre). From the ground, the span and the ridge height above the wall tops give the same answer: rise ÷ (span ÷ 2).
- Angle = arctan(rise ÷ run). A 6/12 roof is arctan(0.5) = 26.57°.
- Rise per 12 = 12 × tan(angle). 30° is 12 × 0.5774 = 6.93/12.
- Percent = rise ÷ run × 100. 6/12 is 50%; 12/12 is 100% and 45°.
- Pitch multiplier = √(1 + (rise ÷ 12)²) = 1 ÷ cos(angle). 6/12 gives 1.118, so a 12 ft run has a 13.42 ft slope.
- Hip / valley multiplier = √(2 + (rise ÷ 12)²) per unit of common run. A regular hip runs 16.97 in (12 × √2) for every 12 in of common run, so its angle is arctan(rise ÷ 16.97).
Common pitches and what they are used for
Minimum pitches for tiles and slates depend on the product and your exposure to wind-driven rain: in the UK the manufacturer’s data and BS 5534 govern, in Australia AS 2050 for tiles. Always check the product’s stated minimum.
- Flat to ¼/12 (2%): membrane roofs laid to falls so water drains; not truly level.
- Up to 3/12: “low-slope” in roofing trade terms — membranes and standing-seam metal. Asphalt shingles need at least 2/12, with double underlayment below 4/12 under the International Residential Code (R905.2.2).
- 4/12 to 9/12 (18°–37°): the conventional range for shingles and concrete tiles in North America; 22.5°–35° is typical for tiled roofs in the UK and Australia.
- Above 9/12: steep roofs — attic space and snow shedding, but roofers need roof jacks and harnesses.
Roof Pitch Calculator FAQ
What is a 6/12 roof pitch in degrees?
26.57°. The roof rises 6 in for every 12 in of horizontal run, so the angle is arctan(6 ÷ 12). Its slope is 50% and its pitch multiplier is 1.118.
How do I convert degrees to roof pitch?
Rise per 12 = 12 × tan(angle). 30° is 6.93/12, 35° is 8.40/12, 45° is 12/12 and 22.5° is 4.97/12. Choose Degrees in the roof pitch calculator and type the angle.
What is a pitch multiplier?
The number you multiply a horizontal distance by to get the length along the slope: √(1 + (rise/12)²). For 4/12 it is 1.054, for 8/12 1.202 and for 12/12 1.414. It also turns a footprint area into the roof surface area.
How do roofers in the UK give pitch?
Usually in degrees — a 35° tiled roof, a 22.5° extension roof. Metric drawings sometimes show a ratio such as 1 : 2 or a rise in mm per metre; this tool converts all of them.
How do I find the pitch without going on the roof?
Measure the span between the outside faces of the walls and the height of the ridge above the wall tops (from a loft or drawings), choose Span & height, and enter both. Rise per 12 = height ÷ (span ÷ 2) × 12.
Is 4/12 too flat for shingles?
No — shingles are allowed from 2/12, but from 2/12 up to 4/12 the IRC requires a double layer of underlayment. Many manufacturers recommend 4/12 or more; check their instructions.