Roof Pitch Calculator
Find your roof pitch from rise and run, or convert a known pitch, angle or percentage. Get the answer in X:12, degrees and percent, with a scale diagram and optional rafter measurements.
- Angle
- Slope
- Length / area multiplier
A 0:12 result describes a horizontal plane. It does not assess drainage or roofing suitability.
Estimated roof dimensions
Geometric lengths only. Ridge-board thickness, birdsmouth cuts, timber sizing and structural allowances are not included. Area excludes waste and overlaps.
Show the calculation
How to calculate roof pitch
- Measure a level horizontal run under the roof slope.
- Measure the vertical rise across that same horizontal distance.
- Enter both measurements and their units. A 12-inch level is convenient, but any positive run works.
- Select Calculate roof pitch to see the X:12 ratio, angle, percentage and multiplier.
For example, a roof that rises 6 inches over a horizontal distance of 12 inches has a 6:12 pitch, a 26.57° angle and a 50% slope. Six inches of rise over one foot of run gives exactly the same answer.
Roof pitch formulas
This tool uses “pitch” in the common X:12 sense: vertical rise per 12 units of horizontal run. Older references sometimes use pitch to mean rise divided by the full span; that is a different ratio.
Slope ratio = rise ÷ runPitch in 12 = (rise ÷ run) × 12Angle = atan(rise ÷ run) × 180 ÷ πSlope percentage = (rise ÷ run) × 100Rafter multiplier = √(1 + (rise ÷ run)²)
Measurements are converted to the same unit before calculation. Results are rounded for display; calculations use unrounded values.
Common roof pitches and angles
| Pitch | Angle | Slope |
|---|---|---|
| 1:12 | 4.76° | 8.33% |
| 2:12 | 9.46° | 16.67% |
| 3:12 | 14.04° | 25% |
| 4:12 | 18.43° | 33.33% |
| 5:12 | 22.62° | 41.67% |
| 6:12 | 26.57° | 50% |
| 7:12 | 30.26° | 58.33% |
| 8:12 | 33.69° | 66.67% |
| 9:12 | 36.87° | 75% |
| 10:12 | 39.81° | 83.33% |
| 12:12 | 45° | 100% |
| 18:12 | 56.31° | 150% |
| 24:12 | 63.43° | 200% |
Estimate common rafter length and roof area
Once the pitch is known, enable the dimension estimate and enter the full horizontal run from the wall to the ridge. On a symmetrical gable roof, run is half the horizontal building span. Do not enter the full span as the run.
For a 6:12 roof with a 10-foot run, the rise is 5 feet and the common rafter’s geometric length is 11.1803 feet before overhang. A 1-foot horizontal eave overhang increases the geometric sloping length to 12.2984 feet.
For area, also enter the roof length along the ridge. The calculator multiplies the sloping length by this roof length and by one or two equal roof planes. The two-plane option assumes a symmetrical gable roof with matching slopes and runs. It does not model hips, valleys, dormers or irregular roof sections. No waste factor, roofing overlap or trim allowance is added.
What this result can tell you
This is a geometry calculator. It does not determine a safe roof pitch, a minimum pitch for a particular covering, timber sizes, loads, code compliance or cut dimensions. Roofing suitability depends on the specific product instructions, assembly and applicable local requirements.
Rafter lengths exclude ridge-board thickness and birdsmouth adjustments. A multiplier applied to a horizontal footprint gives a sloped area only when that footprint belongs to the matching uniform roof plane. Do not use a diagram or a rounded result as a construction drawing.
Calculation reference
The rise/run, pitch conversion and common-rafter geometry follow the right-triangle methods illustrated in the Johnson Level CALC-1500 instruction manual, pages 16–17. This calculator is independently developed by SimCálculo.