A roof pitch calculator converts between the ways a roof slope gets described: rise over run (like 6/12), degrees, and slope percentage. Contruva’s free roof pitch calculator does all three directions, and also gives you the pitch multiplier for turning a footprint into squares, plus the rafter length for any run and overhang. A 6/12 roof, for example, is 26.57°, a 50% slope, and has a multiplier of 1.118.
Open the free roof pitch calculator in your browser. No login, no email, nothing to pay.
Open the free toolIt works in inches and feet or millimetres and metres, and it shows a full chart from 1/12 to 18/12. No login. Below are the formulas behind every number, how to measure pitch on site, and worked examples you can check with a phone calculator.
How to use the roof pitch calculator
Enter the vertical rise over a level run you measured, in the same unit. 6 in over 12 in, 5 ft over 15 ft, or 250 mm over 1,000 mm all work.
Enter the roof angle in degrees from a digital level, an app or the plans, and get the pitch in x/12.
Enter a pitch like 7/12 and get the angle, slope percentage and multiplier.
In any mode, enter the horizontal run of the rafter and the overhang. The calculator gives the rafter line length and the rise at the ridge.
The result card always shows the pitch per 12, the angle, the slope percentage, the multiplier and the ratio (a 6/12 is 1 : 2). The chart underneath highlights the nearest whole pitch.
What is roof pitch?
Roof pitch is how many inches a roof rises for every 12 inches it runs horizontally. A 4/12 roof climbs 4 inches per foot; a 12/12 climbs a foot per foot, which is 45°. In North America roofers, shingle manufacturers and the building codes all state slope this way, which is why the calculator always reports x/12, even in metric.
Strictly, “pitch” once meant rise over the full span and “slope” meant rise over run, but on site and in most product literature the two words are used for the same x/12 number. Slope percentage is the same ratio times 100, so a 6/12 is a 50% slope. Engineers and metric drawings often use degrees or a ratio like 1 : 4 instead.
How to measure roof pitch
Hold a level horizontal with one end on the roof surface, mark 12 inches along the level, and measure straight down from that mark to the shingles. The measurement in inches is the pitch.
Same idea against the underside of a rafter. Safer in bad weather and not affected by bumpy shingles.
Measure the rise from the top plate to the ridge and half the building width, then enter both as rise and run.
Read the angle in degrees and use angle mode. Lay it on a straight edge, not on a single shingle tab.
Older houses sag and additions get framed at a different pitch. Check each roof plane you’re pricing; a porch or dormer at 3/12 needs a different system than a main roof at 8/12.
Roof pitch formulas
| To find | Formula | 6/12 example |
|---|---|---|
| Pitch (x/12) | Rise ÷ run × 12 | 6 ÷ 12 × 12 = 6/12 |
| Angle | arctan(rise ÷ run) | arctan(0.5) = 26.57° |
| Pitch from an angle | tan(angle) × 12 | tan(26.57°) × 12 = 6 |
| Slope % | Rise ÷ run × 100 | 50% |
| Pitch multiplier | √(1 + (rise ÷ run)²) | √1.25 = 1.118 |
| Rafter length | (Run + overhang) × multiplier | See below |
Worked examples
- Rise and run in feet: a gable end rises 5 ft over a 15 ft run. 5 ÷ 15 × 12 = 4/12, which is 18.43°, a 33.3% slope and a multiplier of 1.054.
- From an angle: plans say 30°. tan(30°) × 12 = 6.93/12, so just under 7/12, with a multiplier of 1.155.
- From a pitch: a 12/12 is exactly 45°, a 100% slope and a 1.414 multiplier.
- Metric: 250 mm of rise over 1,000 mm of run is a 25% slope, which is 3/12 or 14.04°.
Roof pitch chart: 1/12 to 18/12
The multiplier is what turns a flat footprint into real roof area: footprint × multiplier = sloped area. The same chart is in the calculator.
| Pitch | Angle | Multiplier | Pitch | Angle | Multiplier |
|---|---|---|---|---|---|
| 1/12 | 4.76° | 1.003 | 10/12 | 39.81° | 1.302 |
| 2/12 | 9.46° | 1.014 | 11/12 | 42.51° | 1.357 |
| 3/12 | 14.04° | 1.031 | 12/12 | 45.00° | 1.414 |
| 4/12 | 18.43° | 1.054 | 13/12 | 47.29° | 1.474 |
| 5/12 | 22.62° | 1.083 | 14/12 | 49.40° | 1.537 |
| 6/12 | 26.57° | 1.118 | 15/12 | 51.34° | 1.601 |
| 7/12 | 30.26° | 1.158 | 16/12 | 53.13° | 1.667 |
| 8/12 | 33.69° | 1.202 | 17/12 | 54.78° | 1.734 |
| 9/12 | 36.87° | 1.250 | 18/12 | 56.31° | 1.803 |
Read it like this: at 8/12, every 100 sq ft of footprint (overhangs included) carries 120.2 sq ft of roof. To go straight from footprint and pitch to squares, bundles, underlayment and ridge cap, use the roofing calculator, which applies the multiplier for you.
How to calculate rafter length
A common rafter’s line length is its horizontal run times the pitch multiplier. Add the overhang as a horizontal measurement and multiply that too, because the tail is on the same slope.
A 24 ft wide building has a 12 ft run to the centre of the ridge.
Measured level, out from the face of the wall.
For 6/12.
(12 + 1) × 1.118 = 14.53 ft, which is 14 ft 6 7/16 in.
The ridge sits 6 ft above the top plate at the centre line: 12 ft × 6/12.
In metric, a 3.6 m run with a 0.3 m overhang at 6/12 is (3.6 + 0.3) × 1.118 = 4.36 m.
That’s the line length to the centre of the ridge. Before you cut, take off half the thickness of the ridge board, measured level, and allow extra for the plumb cut and tail cut. Then round up to the next lumber length; a 14 ft 6 in rafter means buying 16-footers.
Low-slope, steep and minimum pitch for shingles
Pitch decides which roofing systems you can use, how the crew works, and what the job costs. A few thresholds worth knowing, checked November 2026:
| Rule | Source | Threshold |
|---|---|---|
| Minimum slope for asphalt shingles | IRC R905.2.2 (US) | 2/12; double underlayment from 2/12 up to 4/12 |
| Asphalt shingles, normal application | NBC Part 9 slope table (Canada) | 1 in 3 (4/12) minimum |
| Asphalt shingles, low-slope application | NBC Part 9 slope table (Canada) | 1 in 6 (2/12) minimum |
| Low-slope vs steep roof for fall protection | OSHA 29 CFR 1926.500 (US) | Low slope is 4/12 or less; steep is over 4/12 |
Below 2/12, shingles are out under both codes and you’re into membranes or metal systems rated for low slopes. Between 2/12 and 4/12, follow the manufacturer’s low-slope instructions on underlayment and exposure. Provinces adopt the National Building Code with their own amendments, and in Canada fall protection is set by provincial occupational health and safety rules, not OSHA, so check the regulations where the job is.
Pitch also drives labour. Most crews slow down past about 7/12 or 8/12 because they need roof jacks and harnesses on every move, and a steep roof often adds a steep charge per square. Track your own hours per square by pitch rather than guessing.
From pitch to a priced roof
Once you know the pitch, the rest of the roof follows: squares, bundles and accessories in the roofing calculator, then tear-off, disposal, labour and markup in how to write a roofing estimate. If you’re framing the roof as well, the same rise and run thinking carries over to stairs in the stair stringer calculator.
Contruva is built for trades that quote on site. Roofers use Contruva to build a quote from their price catalog, get it signed with a deposit, and see the job’s real cost when the receipts and hours come in. One plan, Contruva Pro, at $249.99 CAD a month. Try it free for 7 days, no credit card needed.