Roof Pitch Calculator
Calculate roof pitch, slope percentage, angle and rafter factor.
To calculate roof pitch, divide vertical rise by horizontal run. This calculator converts verified rise and run into normalized pitch per 12, slope percentage, angle in degrees and rafter-length factor. Switching units converts both measurements.
Watch: How to Use the Roof Pitch Calculator
Watch this short tutorial to learn how to enter verified rise and run measurements and read the roof-pitch ratio, angle, and rafter factor.
Enter vertical rise and horizontal run in the same unit. Changing units converts both measurements and preserves the roof slope.
Your Estimate
Normalized Pitch
6/12
Slope
50%
Angle
26.565051°
Rafter Length Factor
1.118034 ft per ft of run
Pitch = rise ÷ run × 12; slope percentage = rise ÷ run × 100. Results display up to six decimals. Multiply the unrounded rafter factor by a separately known horizontal run for geometric slope length before overhangs and framing details; this is not a rafter cutting length or structural design.
Results Actions
Use measurements from plans or a safely accessible location. Zero rise represents flat geometry only, not approval of a roof assembly. Blank or zero run values cannot define a pitch.
Compare the roof pitch chart, worked examples and printable reference
How to Use the Roof Pitch Calculator
- 1Use verified vertical rise and horizontal run measurements from plans or a safely accessible location.
- 2Enter both dimensions in inches or both in centimeters. Any positive horizontal run works; it need not be 12.
- 3Read normalized pitch per 12, slope percentage, angle and rafter factor. A blank or zero run shows an error instead of an estimate.
How This Calculator Works
- 1Normalized rise per 12 = rise ÷ horizontal run × 12; display that result over 12.
- 2Slope percentage = rise ÷ run × 100.
- 3Angle in degrees = arctangent(rise ÷ run) × 180 ÷ π.
- 4Rafter factor = √(1 + (rise ÷ run)²).
Rise and Run Example
A 9 in rise over 18 in horizontal run gives 6/12 pitch, 50% slope, 26.565051° and a 1.118034 rafter factor. Switching to Metric converts the inputs to 22.86 cm and 45.72 cm while keeping those results. A separately measured 10 ft horizontal run has about 11.18 ft of geometric slope length before overhangs and framing details.
Accuracy & Assumptions
- Results are estimates only.
- Actual pitch and rafter length can vary with roof framing and structural details.
- Verify measurements for each roof plane using plans or safe access; do not assume every plane has the same pitch.
- Verify all rise and run measurements on site before ordering trusses or materials.
- Check local building codes and manufacturer specifications for roofing at your pitch.
This calculator provides an estimate for planning purposes only. Material needs vary by site conditions, waste and supplier specifications — always confirm final quantities with your contractor or supplier before purchasing.