How it works
The calculator turns whatever you measured into one slope, the rise divided by the run, and writes it in the four ways roofers, plans and apps use.
Slope = Rise ÷ Run (Run = Span ÷ 2, or Run = √(Rafter² − Rise²), or Slope = tan Angle)
Pitch = 12 × Slope (inches of rise per 12 in of run)
Angle = arctan(Slope)
Percent grade = 100 × Slope
Slope factor = √(1 + Slope²) · Length along the slope = Run × Slope factor
Four ways to write the same slope
Pitch (x/12 or x:12) is the North American roofing convention: inches of rise per 12 in of run. Angle is what a phone level or angle finder shows and what most metric plans use. Percent grade is rise ÷ run × 100, common for drainage and low-slope roofs. The slope factor is not a slope at all but a multiplier: how much longer the roof surface is than the level distance under it.
| Pitch | Angle | Grade | Slope factor |
|---|---|---|---|
| 2:12 | 9.46° | 16.7 % | 1.014 |
| 4:12 | 18.43° | 33.3 % | 1.054 |
| 6:12 | 26.57° | 50 % | 1.118 |
| 8:12 | 33.69° | 66.7 % | 1.202 |
| 12:12 | 45° | 100 % | 1.414 |
All of these follow from the right triangle formed by rise, run and slope length, so they are exact for a flat roof plane.
Choosing a measuring method
Rise and run is the classic level-and-tape method on a rafter or the roof surface. Rise and span works from a gable end or from plans: the run is half the building width. Rise and rafter length uses the two lengths you may have from framing plans. The angle method is quickest with a phone or an angle finder laid on the roof – lay it on a straight board rather than on the shingles, which tilt.
Worked example in words
A rafter rises 6 in over 12 in of level run. The slope is 6 ÷ 12 = 0.5, so the pitch is 6:12, the angle is arctan 0.5 = 26.57° and the grade is 50 %. The slope factor is √(1 + 0.25) = 1.118, so the 12 in of run is 13.42 in along the roof.
What this page doesn’t decide
Whether a pitch suits a roofing material (most asphalt shingles have a minimum slope, for example) or a structure is set by the manufacturer, the building plans and local code. Use the roof area calculator to go on from the pitch to roof area and squares.
Worked example
Inputs
- Measure by
- Rise and run
- Rise
- 6 in
- Run
- 12 in
Result
- Pitch (rise per 12 of run, x:12)
- 6
- Angle (°)
- 26.57
- Percent grade (%)
- 50
- Slope factor
- 1.118
- Rise
- 0.5 ft (6 in)
- Run
- 1 ft (12 in)
- Length along the slope
- 1.12 ft (13.42 in)
Common mistakes
- Measuring the run along the roof. Run is the level distance; the distance along the roof is the slope length, which is always longer.
- Reading 6:12 as 6 degrees. A pitch is a ratio of rise to run – 6:12 is about 26.6°.
- Using the full span as the run. On a gable roof with equal sides the run is half the span.
- Confusing percent grade with degrees. A 100 % grade is 45°, not 90°.
- Measuring the rise on a sagging rafter or a fascia that isn't straight. Take the reading at a few places and use the typical value.
Assumptions and limits
- The roof plane is flat and has one slope. Curved, broken (gambrel, mansard) or uneven roofs need one measurement per plane.
- For rise and span the run is half the span, which assumes a symmetrical gable.
- Rise and run are measured to the same line (the top of the wall plate or the roof surface), so the ratio is not distorted by roof thickness.
- Lengths are shown only when you measured them; an angle alone gives the ratios but no lengths.
Frequently asked questions
What does a 6/12 roof pitch mean?
The roof rises 6 in for every 12 in of level run. That is a slope of 0.5, an angle of 26.57°, a grade of 50 % and a slope factor of 1.118 – IKO and Owens Corning both list 1.118 for a 6:12 roof in their estimating guides.
How do I convert roof pitch to degrees?
Divide the rise by the run and take the arctangent – angle = atan(rise ÷ run). 4:12 is 18.43°, 6:12 is 26.57°, 8:12 is 33.69°, 12:12 is 45°. To go back, pitch = 12 × tan(angle).
How do I measure roof pitch from the ground or the attic?
In the attic, hold a level against the underside of a rafter, mark 12 in along it and measure straight up to the rafter – that is the rise per 12 in. At a gable end you can measure the height from the wall plate to the ridge and the building width and use the rise-and-span mode.
What is the slope factor?
It is the length along the slope per unit of level run, √(1 + (rise ÷ run)²). Multiply a level measurement – a footprint area or a run – by it to get the true sloped size. The roof area calculator uses it to turn the footprint into roof area.
What is the difference between this and the roof area calculator?
This page finds the pitch from what you can measure. The roof area calculator takes a pitch you already know and turns the building footprint into sloped roof area and roofing squares.
Sources
- Estimating How Much Roofing Is Required — IKO. Actual area = horizontal area × area/rake factor; 6/12 → 1.118. The factor equals √(1 + (rise/run)²) (Pythagoras).
- DIY Roofing: How to Measure a Roof — Owens Corning. "Plane Area x Correction Factor = Roof Area"; slope correction factor 6:12 → 1.118.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 5, 2026