Free estimate — verify against local code before building
Roof Pitch Calculator
Turn rise over run or degrees into pitch, factor, rafter length, and ridge height.
What this calculator includes
Convert a measured rise over run — or an angle in degrees — into every common way of stating roof slope: X-in-12 pitch, degrees, slope percent, and the pitch factor used to turn plan dimensions into sloped lengths. Add the building span and overhang to estimate common rafter line length and ridge height above the plate, and see which walkability and underlayment band the slope falls in. The gable diagram redraws at the true angle as you type. Results are geometry only: verify rafter cuts, ridge deductions, and code requirements before building.
How to use this roof pitch calculator
- 01
Measure rise over run
Hold a level horizontally against the rafter, gable end, or roof surface, mark 12 inches out, and measure straight down to the slope. Enter the measured rise and run — uneven field measurements like 4.5 over 11.75 work; the calculator standardizes them to X-in-12.
- 02
Or enter the angle
Switch to angle mode when you have a degree reading from a digital level or plans. The calculator converts it back to rise per 12 inches of run.
- 03
Read pitch, angle, and factor
The results show the standardized X-in-12 pitch, angle in degrees, slope percent, and the pitch factor. Multiply any horizontal plan length by the pitch factor to get the length along the slope.
- 04
Estimate rafter and ridge
Enter the building span and overhang to estimate common rafter line length and ridge height above the plate. Deduct half the ridge thickness, lay out the seat and plumb cuts, and verify structure and code requirements before cutting.
Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- 2024 International Residential Code, Chapter 9 — Roof Assemblies (opens in a new tab)
Minimum slopes and underlayment requirements by roof covering (R905); locally adopted editions control.
- OSHA 29 CFR 1926.501 — Duty to have fall protection (opens in a new tab)
Fall protection is generally required in construction at 6 ft or more above a lower level, including steep- and low-slope roof work.
- National Roofing Contractors Association (opens in a new tab)
Industry classification of steep-slope versus low-slope roof systems and application guidance.
Frequently asked questions
What pitch is a 30-degree roof?
tan(30°) x 12 = 6.93, so a 30-degree roof is just under a 7-in-12 pitch. A 45-degree roof is exactly 12-in-12, and 26.57 degrees is 6-in-12.
What is a pitch factor and what is it for?
The pitch factor is sqrt(1 + (rise/run)^2) — the sloped length per foot of horizontal run. A 6-in-12 roof has a factor of 1.1180, so 100 sq ft of plan area is about 112 sq ft of roof surface, and a 12 ft horizontal run needs about 13.42 ft of rafter.
Is a 6/12 roof walkable?
A dry 6-in-12 roof is generally considered walkable with care, but footing depends on the covering, moisture, dust, and temperature. OSHA still requires fall protection for most work 6 ft or more above a lower level, and slopes above about 9-in-12 call for roof jacks or staging.
What is the minimum pitch for asphalt shingles?
Under IRC R905, asphalt shingles require at least a 2-in-12 slope, and from 2-in-12 up to 4-in-12 they need special underlayment (generally two layers or a self-adhering membrane). Below 2-in-12, use a low-slope system rated for the application.
How do I measure roof pitch from inside the attic?
Hold a level horizontally against the underside of a rafter, mark 12 inches from the contact point, and measure vertically from the mark up to the rafter. That rise over the 12-inch run is the pitch, with no need to get on the roof.