Runs in your browser · nothing is uploaded
Roof Pitch Calculator
Enter a rise and run, an angle or an X:12 pitch to get every other form — plus the rafter length from the run or span, with a labelled diagram.
Geometry only. Rafter length here is the measured slope line — it doesn't size timber, allow for the ridge board or birdsmouth cuts, or check building-code requirements.
The formulas
- Angleangle = atan(rise ÷ run)
- Pitch and slopeX:12 = rise ÷ run × 12 · slope % = rise ÷ run × 100
- Rafter length√(rise² + run²)
Worked example
A roof rises 6 for every 12 across: 6 ÷ 12 = 0.5, so it's a 6:12 pitch, 50% slope and atan(0.5) = 26.565°. On a 24 ft span the run is 12 ft, the rise 6 ft, and the rafter √(6² + 12²) = 13.42 ft to the wall line, before any overhang.
Frequently asked questions
How do I calculate roof pitch?+
Measure the rise (vertical height) over a horizontal run, then divide rise by run. Multiply by 12 for the X:12 pitch, by 100 for percent slope, and take the arctangent for the angle. A 6-inch rise over 12 inches of run is 6:12, a 50% slope and 26.57°.
What's the difference between run and span?+
The span is the full width of the building, wall to wall. On a symmetrical gable roof each side covers half of it, so the run — from the outside of the wall to the centre of the ridge — is half the span.
Is the rafter length the length of timber to buy?+
No. It's the geometric slope length from the wall line to the ridge centre (plus any overhang you enter). Cutting length also depends on the ridge board, birdsmouth cut and the timber's depth, and structural sizing is a job for building rules or an engineer.
How do I measure pitch from inside the loft?+
Hold a level horizontally against the underside of a rafter, mark 12 inches (or 300 mm) along it, and measure straight up from that mark to the rafter. That vertical distance over 12 is the pitch.
What pitch is a “low slope” roof?+
Roughly anything below 3:12 (about 14°) is usually called low slope, and many roofing materials have minimum pitches — check the manufacturer's instructions for the covering you plan to use.