Roof pitch calculator
Convert a roof pitch between degrees, percent and rise over run, then work out rafter length and roof area from the same slope.
Slope in percent
- Slope in percent
- 70%
- Pitch in degrees
- 35°
- Pitch as x/12 (US plans)
- 8.4/12
- Rise to run
- 1 : 1.43
- Slope factor
- 1.2208Rafter length per metre of run
Rafter length and roof area
Half the span for a simple gable roof.
The roof seen from above, including the eaves overhang.
Rafter length and roof area
- Rafter length before overhang
- 4.88 m
- Roof surface area
- 73.2 m²
Estimate only. Check the product label and the building regulations where you live before you buy or build.
Quick answer
Roof pitch is the angle between the roof slope and the horizontal, and its tangent is the rise divided by the run. A 35° roof rises tan(35°) = 0.70 m for every metre of run, which is a 70% slope. Multiply the run by the slope factor √(1 + (rise ÷ run)²), 1.2208 at 35°, to get the rafter length: a 4 m run needs a 4.88 m rafter.
How to use the roof pitch calculator
- Choose how you know the pitch: in degrees from drawings or a digital angle finder, as a percent, as a rise and run you measured yourself, or as x/12 from American plans.
- Type the value. The pitch in degrees, the percent slope, the rise-to-run ratio and the slope factor update as you type.
- For rafter length, enter the horizontal run from the outside of the wall to the centre of the ridge. For roof area, enter the plan area of the roof seen from above, eaves overhang included.
How do you work out roof pitch in degrees?
Take the inverse tangent of the rise divided by the run. Degrees, percent and rise over run all describe the same ratio: how far the roof climbs for each metre it covers horizontally. A roof that rises 2.8 m over a 4 m run has a ratio of 0.7, a slope of 70% and a pitch of 34.99°, which builders call 35°.
The slope factor is the length of the sloped side of a right-angled triangle whose run is 1. Multiply any horizontal measurement by it to get the matching measurement along the slope, which is how you get rafter length from the run and roof area from the plan area.
pitch (degrees) = arctan(rise ÷ run)slope % = rise ÷ run × 100rise per metre of run = tan(pitch) × 1,000 mmslope factor = √(1 + (rise ÷ run)²)rafter length = run × slope factorroof area = plan area × slope factorRoof pitch chart: degrees to percent and rise per metre
Every value below comes from the formulas above, rounded to one decimal for the percent, the nearest millimetre for the rise and four decimals for the slope factor.
| Pitch | Slope | Rise per metre of run | Slope factor |
|---|---|---|---|
| 12.5° | 22.2% | 222 mm | 1.0243 |
| 15° | 26.8% | 268 mm | 1.0353 |
| 17.5° | 31.5% | 315 mm | 1.0485 |
| 20° | 36.4% | 364 mm | 1.0642 |
| 22.5° | 41.4% | 414 mm | 1.0824 |
| 25° | 46.6% | 466 mm | 1.1034 |
| 27.5° | 52.1% | 521 mm | 1.1274 |
| 30° | 57.7% | 577 mm | 1.1547 |
| 35° | 70.0% | 700 mm | 1.2208 |
| 40° | 83.9% | 839 mm | 1.3054 |
| 45° | 100.0% | 1,000 mm | 1.4142 |
| 50° | 119.2% | 1,192 mm | 1.5557 |
| 55° | 142.8% | 1,428 mm | 1.7434 |
| 60° | 173.2% | 1,732 mm | 2.0000 |
Worked examples
Rafters and roof area for a 35° gable roof
A house is 8 m wide between the outside faces of the walls, with a simple gable roof pitched at 35°. The run of each rafter is half the width, 4 m. The slope factor for 35° is 1.2208, so each rafter is 4 × 1.2208 = 4.88 m before you add the eaves overhang.
Seen from above, the roof covers 60 m² including the overhang. The sloped surface is 60 × 1.2208 = 73.2 m². Order tiles, battens and membrane for the sloped area, not the plan area, and add the waste allowance your supplier or the tile maker recommends.
Finding the pitch in the loft with a spirit level
Hold a spirit level horizontally with one end touching the underside of a rafter. At a point 600 mm along the level, measure straight up to the rafter. If that measurement is 420 mm, enter a rise of 420 and a run of 600: the pitch is 34.99°, a 70% slope, so the roof is pitched at 35°.
What is the minimum pitch for roof tiles?
The minimum pitch depends on the tile, and the manufacturer states it in degrees on the product data. Marley rates its concrete plain tile for pitches from 35° to 90°. One of its interlocking concrete tiles goes down to 22.5° with a smooth finish and a 100 mm headlap, 25° with a smooth finish and a 75 mm headlap, or 30° with a granular finish and a 75 mm headlap.
So the same roof can be fine for one tile and too shallow for another. Check the data sheet for the exact tile before you buy, and if you are re-roofing with a different tile, measure the existing pitch first. Building rules differ between England, Wales, Scotland and Northern Ireland; for a new roof or any structural change, your building control body and a qualified roofer or engineer take priority over this calculator.
Questions
What is a 35 degree roof pitch as a ratio?
A 35° roof rises about 700 mm for every metre of run, a rise-to-run ratio of 1 : 1.43 and a slope of 70%. The tangent of 35° is 0.7002, which gives all three figures. The slope factor is 1.2208, so each metre of horizontal run needs 1.22 m of rafter, and the roof surface is about 22% larger than its plan area.
How do I convert rise and run to degrees?
Divide the rise by the run and take the inverse tangent of the result. A rise of 1.5 m over a run of 3 m gives 1.5 ÷ 3 = 0.5, and arctan(0.5) = 26.57°. On most phone calculators the key is labelled tan⁻¹. To go the other way, multiply the run by the tangent of the pitch: a 30° roof over a 3 m run rises 3 × 0.5774 = 1.73 m.
How much larger is the roof than the plan area?
Multiply the plan area by the slope factor for your pitch. A 22.5° roof is 8.2% larger than its plan area (factor 1.0824), a 30° roof is 15.5% larger (1.1547), a 35° roof is 22.1% larger (1.2208) and a 45° roof is 41.4% larger (1.4142). If every slope has the same pitch, this works for hipped roofs too, because each face is tilted by the same amount.
What is a 6 in 12 roof pitch in degrees?
A 6 in 12 pitch, written 6/12 on American plans, is 26.57°. It means 6 inches of rise for every 12 inches of run, so rise ÷ run is 0.5, a 50% slope. Choose x/12 (US) in the calculator to convert any American pitch to degrees. For example, 4/12 is 18.43° and 12/12 is 45°.
What is a 35° roof pitch in x/12?
A 35° roof is an 8.4/12 pitch in the American notation: it rises 8.4 inches for every 12 inches of run, because tan(35°) × 12 = 8.40. You will see x/12 on US plans and in US roofing guides, while British drawings and tile data sheets give the pitch in degrees. Choose x/12 (US) in the calculator to turn an American figure into degrees.
Sources
- Marley, Plain Concrete Roof Tiles (checked 19 October 2026)Concrete plain tile: minimum pitch 35°, maximum pitch 90°.
- Marley, Ludlow Major Concrete Roof Tile (checked 19 October 2026)Interlocking tile: minimum pitch 22.5° smooth with 100 mm headlap, 25° smooth with 75 mm headlap, 30° granular with 75 mm headlap; maximum 90°.
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