Roof Pitch Calculator
Enter a roof's rise and run to get its pitch in inches per 12, its angle and its slope factor. A 6-in-12 roof rises 6 in per 12 in of run, sits at 26.6° and has a slope factor of 1.118, so its surface is about 11.8% larger than its footprint. Apply that with the Roof Area Calculator.
How it's calculated: Angle = arctan(rise ÷ run); pitch = rise ÷ run × 12; slope factor = √(rise² + run²) ÷ run.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Angle = arctan(Rise ÷ Run)
Slope factor = √(Rise² + Run²) ÷ Run
Pitch (rise-in-12) = (Rise ÷ Run) × 12
Run the calculation to see each step with your own numbers.
A roof that rises 6 in over a 12 in run (a 6-in-12 pitch).
Pitch = (6 ÷ 12) × 12 = 6-in-12
Angle = arctan(0.5) = 26.6°
Slope factor = √(6² + 12²) ÷ 12 = √180 ÷ 12 = 1.1180
- Rise and run are measured in the same unit (the calculator normalises them, so mixed units also work).
- A planar roof section — the slope factor applies to simple gable and hip planes, not to curved or barrel roofs.
- What is the slope factor for?
- It is the number you multiply a roof's footprint (plan) area by to get the actual sloped surface area. A 6-in-12 roof has a slope factor of about 1.118, so its surface is ~11.8% larger than its footprint.
- My roof is measured as 'x/12'. What do I enter?
- Enter the rise as x and the run as 12 (any matching unit). A 9/12 roof: rise 9, run 12.
- How do I convert roof pitch to degrees?
- Take the arctangent of rise ÷ run. A 4/12 roof is about 18.4°, 6/12 is 26.6°, 8/12 is 33.7° and 12/12 is 45°.
- What slope factor does a 4/12 or 8/12 roof have?
- About 1.054 for 4/12 and 1.202 for 8/12, from √(rise² + 12²) ÷ 12. Multiply the roof's footprint area by the factor to get its sloped surface area.
- How do I measure rise and run on an existing roof?
- Hold a level horizontally against the roof or a rafter, mark 12 in along it from a fixed point, and measure straight up from that mark to the roof surface. That vertical distance is the rise per 12 in of run.
Trigonometry
Right-triangle identities: tan(θ) = opposite/adjacent; hypotenuse = √(a² + b²). Standard mathematics; no empirical constant.
About the Roof Pitch Calculator
The Roof Pitch Calculator converts a measured rise and run into the roof angle in degrees, the slope factor, and the standard rise-in-12 pitch notation. The slope factor it produces is the multiplier the Roof Area Calculator uses to turn a flat footprint into true sloped area, so the two are normally used together.
What you enter
- Rise — The vertical rise. For a standard pitch, this is the rise over a 12-unit run.
- Run — The horizontal run over which the rise is measured. Use the same unit as the rise.
What you get back
- Pitch (rise-in-12) — Read as “X-in-12” — e.g. 6.00 means a 6-in-12 roof.
- Roof angle — the exact result of the calculation.
- Slope factor (roof multiplier) — Multiply a roof's plan (footprint) area by this to get the sloped surface area.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Roof Area Calculator — Sloped surface area of a simple gable or hip roof from its footprint and pitch.
- Roof Shingle Calculator — Shingle bundles and roofing squares for a roof of a given surface area.
- Metal Roof Cost Calculator — Estimated cost of a metal roof from its area, panel type and installed rates.
- Rafter Length Calculator — Common rafter length from the building span and roof pitch.
Related articles
- Understanding Roof Pitch: 4/12, 6/12 and Other Ratios — Roof pitch notation like 6/12 tells you the rise over a 12-unit run — here's how to read it and what different ratios mean for a roof in practice.
- How Roof Pitch Affects Roofing Materials — Pitch doesn't just change how much material you need — it can determine which roofing products are rated to work at all on a given slope.
Part of the Roofing calculators.