Roof Pitch Calculator
Find a roof's pitch in degrees, as a rise in 12 and as a percentage, from its rise and run, plus the rafter length.
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- Roof pitch
- 29.9°
- Rise in 12 (US style)
- 6.9 in 12
- As a percentage
- 57.5%
- Rafter length (no overhang)
- 4.61 m
What it does
Enter a roof's rise and run to get its pitch three ways: in degrees (used in the UK and Europe), as a rise in 12 (used in the US, such as 6/12) and as a percentage. It also gives the rafter length from the wall plate to the ridge.
How to use it
- Rise
The vertical height from the top of the wall plate to the ridge.
- Run
The horizontal distance from the outside of the wall to the centre line of the ridge. For a simple pitched roof, that's half the span.
Common uses
- Checking whether roof tiles or slates suit the pitch.
- Ordering rafters and working out a loft conversion.
- Working out concrete for a base. Try the Concrete Calculator
The formula
Pitch = tan⁻¹(rise ÷ run)Rise in 12 = rise ÷ run × 12Rafter = √(rise² + run²)Where:
- tan⁻¹
- inverse tangent (arctan)
Rafter length is to the ridge centre line, before any overhang or ridge board allowance.
Worked example
With a rise of 6 ft and a run of 12 ft:
- Pitch: tan⁻¹(0.5) = 26.6°
- Rise in 12: 0.5 × 12 = 6 in 12
- Rafter: √(6² + 12²) = 13.42 ft
Questions people ask
What does a 6/12 roof pitch mean?
The roof rises 6 units for every 12 units across, which is 26.6 degrees or a 50% slope. US roofers describe pitch this way; in the UK it's usually given in degrees.
What's a typical roof pitch?
Many UK houses have pitches between 30 and 45 degrees. Low-pitch roofs need tiles or coverings rated for the angle, so check the manufacturer's minimum pitch before choosing.
Is the run the full width of the building?
No. For a roof that slopes both ways from a central ridge, the run is half the span: from the outside of one wall to the middle.
Last checked 25 October 2026