Roof Pitch Calculator
Convert between roof pitch, angle and rise over run, and get the rafter length.
Last reviewed September 19, 2026
Result
13.4164 ft
These are exact geometry, not estimates, so nothing is added.
- Angle
- 26.565 °
- Pitch, rise per 12
- 6
- Slope factor
- 1.118034
https://calcgallery.com/roofing/roof-pitch-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Choose whether you know the pitch, the angle, or a rise and run you measured.
- Enter it. A pitch of 6 in 12 means the roof rises 6 in for every 12 in of horizontal run.
- Enter the run from the wall to the ridge if you want a rafter length.
- Read the angle, the pitch and the slope factor.
How it is calculated
Roof pitch is a right triangle, so everything comes from the tangent:
angle = arctan(rise ÷ run)
pitch = rise per 12 of run
slope factor = √(rise² + run²) ÷ run
rafter = run × slope factorThe slope factor is the useful one. It is the ratio between the sloped surface and its flat footprint, so multiplying a plan area by it gives the real roof area. For a 6 in 12 pitch that factor is 1.118034, which means the roof is about 12 per cent bigger than the building looks from above.
Pitch expressed as “x in 12” is just the rise in inches for every foot of run. It is a ratio, not an angle, which is why 12 in 12 is 45 degrees rather than anything to do with 12.
Practical tips
Measure inside if you can. A rafter in the loft is safe, dry and straight. Measuring a finished roof from a ladder is neither safe nor accurate, because the covering hides the actual line of the structure.
Check more than one rafter. Older roofs sag, and the pitch at the gable end is often not the pitch in the middle. If you are ordering material, use the shallowest reading you get, since that is the one that produces the largest area.
Pitch decides what you can put on the roof. Every covering has a minimum pitch, and going below it voids the warranty and usually leaks. Check the pitch before choosing the material, not after.
Rafter length is not board length. The number here is wall to ridge. You need extra for the overhang, and plumb and seat cuts at either end eat more. Buy the next length up.
Remember the pitch applies to both sides. A gable roof has two slopes and the footprint covers both. The slope factor handles that: it converts the whole footprint to the whole roof area.
With your numbers
- Pitch = 6 in 12, which is 26.565 °
- Slope factor = √(1 + slope²) = 1.118034
- Rafter = 12 ft × that factor = 13.4164 ft
Quick reference
These are exact geometry, not estimates, so nothing is added.
| Job | Rafter length |
|---|---|
| 3 in 12, low slope | 12.3693 ft |
| 4 in 12 | 12.6491 ft |
| 6 in 12, the common one | 13.4164 ft |
| 8 in 12 | 14.4222 ft |
| 12 in 12, a 45 degree roof | 16.9706 ft |
| 30 degrees, 4 m run | 4.6188 m |
| 35 degrees, 4 m run | 4.8831 m |
| 45 degrees, 4 m run | 5.6569 m |
Common questions
- What angle is a 6 in 12 pitch?
- 26.57 degrees. The pitch is a tangent, so the angle is the arctangent of 6 divided by 12. A 4 in 12 roof is 18.43 degrees and a 12 in 12 roof is exactly 45.
- What is a slope factor and what is it for?
- It converts a flat footprint into sloped roof area. A 6 in 12 roof has a factor of 1.118, so 1000 ft² of footprint is 1118 ft² of roof surface. Every roofing material estimate depends on it, and forgetting it is the most common way to under-order.
- Is pitch the same as slope or grade?
- Not quite. Roof pitch in the imperial trades is rise per 12 in of run. Slope and grade are usually a percentage or an angle. A 6 in 12 pitch is a 50 per cent grade, which surprises people who expect 6 in 12 to mean 50 degrees. It is 26.57.
- How do I measure the pitch of an existing roof?
- Hold a level against the underside of a rafter in the loft, mark 12 in along it from where it touches, and measure straight down from that mark to the rafter. That measurement is the rise per 12, which is the pitch. A smartphone level app against a rafter works too.
- What counts as a low pitch?
- Below about 4 in 12, or roughly 18 degrees, most shingle manufacturers require extra underlayment, and below about 2 in 12 asphalt shingles are not suitable at all. Low pitches shed water slowly and are far less forgiving of poor detailing.
- Does the rafter length include the overhang?
- No. The rafter length here is the diagonal from the outside of the wall to the ridge. Add whatever overhang you want at the eaves, and subtract half the ridge board thickness at the top.