Convert rise and run to roof pitch (x in 12), degrees and percent slope, get the rafter factor, and turn your home’s footprint into roof area and roofing squares.
How to use the roof pitch calculator
- Measure the rise over a known run. The simplest way is to hold a level horizontally against the roof or a rafter, mark 12 in along it, and measure straight down to the roof surface.
- Enter the rise and run, or switch to enter a pitch or an angle you already know.
- Optionally enter the length and width of the house including overhangs to get the sloped roof area and the number of squares.
Roof pitch formulas
Pitch (x in 12) = rise ÷ run × 12 Angle = arctan(pitch ÷ 12) Slope (%) = pitch ÷ 12 × 100 Rafter factor = √(1 + (pitch ÷ 12)²) Roof area = footprint area × rafter factor Squares = roof area ÷ 100
The rafter factor turns horizontal distance into sloped length. It also turns a plan area into roof area, and that works for gable and hip
roofs alike as long as every plane has the same pitch. A roofing square is 100 ft² of roof surface.
Worked example: a 6 in 12 roof
- Pitch: 6 ÷ 12 × 12 = 6:12.
- Angle: arctan(6 ÷ 12) = 26.57°. Slope: 50%.
- Rafter factor: √(1 + 0.5²) = 1.118.
- A 1,500 ft² footprint including overhangs has 1,677 ft² of roof, or 16.77 squares.
Roof pitch chart
| Pitch | Angle | Slope | Rafter factor |
|---|---|---|---|
| 1:12 | 4.76° | 8.3% | 1.003 |
| 2:12 | 9.46° | 16.7% | 1.014 |
| 3:12 | 14.04° | 25% | 1.031 |
| 4:12 | 18.43° | 33.3% | 1.054 |
| 5:12 | 22.62° | 41.7% | 1.083 |
| 6:12 | 26.57° | 50% | 1.118 |
| 7:12 | 30.26° | 58.3% | 1.158 |
| 8:12 | 33.69° | 66.7% | 1.202 |
| 9:12 | 36.87° | 75% | 1.25 |
| 10:12 | 39.81° | 83.3% | 1.302 |
| 12:12 | 45° | 100% | 1.414 |
Pitch and asphalt shingles
Under IRC R905.2.2, as summarized by the City of Hopkins, Minnesota, asphalt shingles may only be used on roof slopes of 2:12 or greater,
and roofs from 2:12 up to 4:12 need double underlayment. Flatter roofs need a low-slope roofing system. The calculator flags which case
your pitch falls in. Shingle manufacturers’ instructions and local amendments can add requirements.
Tips for measuring
- Measure from inside when you can. A rafter in the attic gives the same pitch as the roof surface without climbing up.
- Use a longer run for accuracy. Measuring the rise over 24 in and halving it reduces the error from a small misreading.
- Check each plane. Additions and dormers often have a different pitch from the main roof; calculate their areas separately.
- Next step: the rafter length calculator uses the pitch to size common rafters.
Questions
What is a 6/12 roof pitch in degrees?
26.57°. A 6:12 roof rises 6 in for every 12 in of horizontal run.
How do I calculate roof pitch?
Divide the rise by the run and multiply by 12. A 4 in rise over 8 in of run is 4 ÷ 8 × 12 = 6, a 6:12 pitch.
What is the minimum pitch for shingles?
2:12 under the IRC, with double underlayment required from 2:12 to 4:12.
How many squares is my roof?
Multiply the footprint (including overhangs) by the rafter factor and divide by 100. A 1,500 ft² footprint at 6:12 is 16.8 squares, before waste.
Sources
- Asphalt Shingles (R905.2), City of Hopkins, Minnesota.
Used for: Asphalt shingles only on 2:12 or steeper; double underlayment from 2:12 to 4:12. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Exact inch and foot definitions; cubic foot, cubic yard and gallon conversions.