Home / Roofing
Roof Pitch Calculator
Convert a roof pitch between rise per 12, degrees, percent, and 1:n ratio, and turn a plan area into the real roof area, or a run into a rafter length.
Roof Pitch Calculator: A pitch of 6 in 12 rises 6 units for every 12 across: a slope of 0.5, 50%, or 26.57°. The slope factor, the square root of 1 plus the slope squared, is how much longer a sloped line is than its flat projection, so it turns plan area into roof area and run into rafter length. Runs 100% locally in your browser with zero server file uploads.
- Category
- Home & family tools
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
The slope factor turns flat measurements into sloped ones: plan area times the factor is the roof area, for any roof whose faces all share one pitch, including hips. Add overhangs to the plan, and extra for waste, before ordering.
Common pitches
3 in 12 is 14.04°, 4 in 12 is 18.43°, 6 in 12 is 26.57°, 8 in 12 is 33.69°, 10 in 12 is 39.81°, and 12 in 12 is 45°. Their slope factors are 1.031, 1.054, 1.118, 1.202, 1.302, and 1.414.
For other right-angle sums, the Pythagorean theorem calculator solves any side; for a staircase, the stair calculator.
A worked example
A house 10 m by 8 m with a 6 in 12 roof and 0.5 m overhangs has a plan of 11 × 9 = 99 m². Multiplying by 1.118 gives about 110.7 m² of roof, before waste.
How to use it
- Enter the pitch and choose how it is given: rise per 12 of run, degrees, percent, or a 1:n ratio.
- Read the other forms and the slope factor.
- Optionally enter the plan area or the run to the ridge to get the roof area or the rafter length.
Privacy & limitations
Everything is worked out in your browser.
Related tools
Frequently asked questions
Is a 4/12 pitch steep?
No: 4 in 12 is about 18.4°, a low to moderate slope. 12 in 12 is 45°.
Does the roof area work for hip roofs?
Yes, as long as every face has the same pitch: plan area times the slope factor gives the total, whatever the shape. Measure the plan including the overhangs.
Which pitch is right for my roofing material?
Each material has its own minimum, set by its maker; check the installation guide before building.
Free tool · runs in your browser · no account required