How to use the roof pitch calculator
- Choose what you know: rise and run, the pitch (x in 12) or the angle in degrees.
- Enter the numbers. Rise and run can be in any unit as long as you choose it in the unit menu.
- To get a rafter length, enter the run — the horizontal distance from the outside of the wall to the centre of the ridge.
Roof pitch formulas
Pitch (x in 12) = Rise ÷ Run × 12 Angle (°) = arctan(Rise ÷ Run) Slope (%) = Rise ÷ Run × 100 Pitch factor = √(1 + (Rise ÷ Run)²) Rafter length = Run × Pitch factor
Worked example
A gable roof rises 4 ft over a run of 12 ft (a 24 ft wide building):
- Pitch = 4 ÷ 12 × 12 = 4/12
- Angle = arctan(0.333) = 18.43°
- Pitch factor = √(1 + 0.333²) = 1.0541
- Rafter length = 12 × 1.0541 = 12.65 ft, plus the overhang
Roof pitch to angle chart
| Pitch | Angle | Slope % | Pitch factor |
|---|---|---|---|
| 2/12 | 9.46° | 16.7% | 1.0138 |
| 3/12 | 14.04° | 25.0% | 1.0308 |
| 4/12 | 18.43° | 33.3% | 1.0541 |
| 5/12 | 22.62° | 41.7% | 1.0833 |
| 6/12 | 26.57° | 50.0% | 1.1180 |
| 7/12 | 30.26° | 58.3% | 1.1577 |
| 8/12 | 33.69° | 66.7% | 1.2019 |
| 9/12 | 36.87° | 75.0% | 1.2500 |
| 10/12 | 39.81° | 83.3% | 1.3017 |
| 12/12 | 45.00° | 100% | 1.4142 |
Using the pitch factor for roof area
Measure the house footprint including overhangs (the plan area) and multiply by the pitch factor to get the sloped roof area you will cover. A 30 × 40 ft footprint (1,200 ft²) with a 6/12 roof has about 1,200 × 1.118 = 1,342 ft² of roof — about 13.4 squares of shingles before waste. Add 10–15% for cuts, ridges and hips.
Common pitches and what they suit
- Under 2/12: flat or low-slope roofs — membrane, EPDM or modified bitumen.
- 2/12 to 4/12: low slope — shingles allowed with double underlayment.
- 4/12 to 9/12: the most common range for homes; walkable with care.
- Over 9/12: steep — great for shedding snow, but roofers need harnesses and roof jacks, which raises labour cost.
Three ways to measure an existing roof
- From the attic: place a 12-inch level against the underside of a rafter, keep it level and measure vertically from the 12-inch mark up to the rafter.
- From the gable end: measure the total height from the top of the wall plate to the ridge (rise) and half the building width (run). Enter both in “Rise & run”.
- With a phone: most phones have a level or angle app. Lay the phone on a rafter or the roof surface, read the angle and use “Angle (degrees)” to convert it to pitch.
Measurements on the roof surface include the thickness of shingles and battens, so they can read slightly higher than the rafter itself. For framing, measure the rafters.