Convert a roof's pitch between rise and run, degrees, a ratio such as 6 in 12 and a percentage. Enter the run too and the calculator gives the rise and the rafter length.
Results are estimates for planning. Check the figures before you make a financial or health decision.
How the Roof pitch calculator works
Pitch is the relationship between the rise (vertical height) and the run (horizontal distance). The calculator turns every input into the slope t = rise ÷ run, then derives the rest:
- Angle = arctan(t), in degrees
- Pitch as x in 12 = 12 × t
- Slope as a percentage = 100 × t
- Rafter length = run ÷ cos(angle), which equals the square root of rise² + run²
If you start from an angle, a ratio or a percentage, it uses the run you enter to find the rise. For an even-sided roof, the run is half the span. An optional horizontal overhang is added to the run to give a rafter length including the overhang.
The rafter length is measured along the slope from the wall plate line to the ridge line. It does not allow for the ridge board, birdsmouth cuts or the rafter thickness, and the angle must be between 0 and 90 degrees.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| You know | Rise and run |
| Measurements in | Metres |
| Rise (vertical height) | 2.5 |
| Run (horizontal distance) | 5 |
| Overhang, measured horizontally (optional) | 0 |
Roof pitch: 26.6°
| Angle | 26.57° |
|---|---|
| Pitch as x in 12 | 6 in 12 |
| Slope | 50% |
| Pitch as 1 in N | 1 in 2 |
| Rise over the run | 2.5 m |
| Rafter length (eaves to ridge line) | 5.59 m |
Tips and common mistakes
Measure the run horizontally, not along the slope, and the rise vertically. A level and tape are fine for a rough check. Make sure all measurements use the same unit. Different roof coverings have a minimum pitch, so check the manufacturer’s guidance and Building Regulations before you choose tiles or slates. Add extra length for the ridge and eaves detail when you order timber, and ask a structural engineer or builder to confirm sizes.
Frequently asked questions
What is a 6 in 12 pitch in degrees?
It is 6 units of rise for every 12 of run, a slope of 0.5, which is an angle of about 26.6 degrees or 50 per cent.
What is the difference between pitch and slope?
They describe the same steepness in different ways. Pitch is often given as an angle or an x in 12 ratio, while slope can also be expressed as a percentage.
How do I find the rafter length?
Divide the run by the cosine of the angle, or use Pythagoras with the rise and run. The calculator does this and can add an overhang.