Roof Pitch Calculator
Convert between rise and run, the X-in-12 pitch roofers quote, a slope percentage and an angle. The slope factor is what turns a building footprint into a roof area.
Switching units keeps your measurements. Nothing is reset.
Measure the slope
How far it goes up.
How far it goes across. Usually 12 in.
Your result appears here
Enter the rise and the run to work out the pitch.
How to use this calculator
- 1
Measure the run: a horizontal distance along the roof, traditionally 12 inches, which is why pitch is quoted in twelfths.
- 2
Measure the rise over that same run, vertically. A level and a tape against the underside of the rafters is the usual way.
- 3
Read the pitch as X in 12, and the angle if you need it for a cut or for a tool setting.
- 4
Take the slope factor across to the roofing calculator to turn your building footprint into roof area.
A worked example
A rise of 6 inches over a run of 12 inches:
Rise divided by run
6 in ÷ 12 in0.5
As an angle
0.526.57°
You will need
6 in 12
This example is produced by running the calculator above, so it always matches what the tool does.
The formula
The pitch is the rise divided by the run, expressed per 12 units of run. The slope percentage is the same ratio as a percentage. The angle is the arctangent of rise over run. The slope factor is the rafter length divided by the run, which is the square root of rise squared plus run squared, all over the run. Because everything is a ratio, the units you measure in do not matter as long as both are the same.
Measuring a pitch
Measure inside if you can
A level held against a rafter in the loft gives a reading without anyone going on the roof. Mark 12 inches along the level, then measure straight up to the rafter.
The slope factor is the useful output
A 6 in 12 roof has a slope factor of about 1.118, so its surface is roughly 12% larger than the ground it covers. That is the number that turns a plan area into a material quantity.
Pitch is not a safety assessment
Knowing the angle tells you nothing about whether the roof is safe to walk on. Anything steep enough to matter is a job for someone with the right equipment.
Common questions
- What does a 6 in 12 pitch mean?
- The roof rises 6 inches for every 12 inches it runs horizontally. That is a 50% slope and an angle of about 26.6 degrees.
- How do I convert roof pitch to degrees?
- Take the arctangent of the rise divided by the run. A 4 in 12 pitch is about 18.4 degrees, 6 in 12 is about 26.6 degrees, and 12 in 12 is exactly 45 degrees.
- What is the slope factor for?
- It converts the flat area a roof covers into the actual sloped surface you have to cover with material. Multiply the building footprint by it to get the roof area for a simple gable.