Roof pitch calculator

Rise over run, four ways: the rise-in-12 ratio, the angle in degrees, the slope percentage, and the pitch factor that turns a footprint into actual roof area. Imperial or metric.

01 How the math works

Pitch is rise over run — how far a roof climbs vertically for every unit it runs horizontally. Framers state it as rise in 12: a "6 in 12" roof rises 6 inches for every 12 inches of run. Four numbers describe that one slope, and they are not interchangeable.

The pitch ratio is the framing shorthand. The angle is the same slope in degrees. The slope percentage — rise divided by run, times 100 — is the grade a surveyor would quote. The pitch factor is the length of the sloped plane per unit of horizontal run, and it is the one number here that multiplies a footprint area into actual roof-plane area.

The two people mix up are the last two. The grade and the pitch factor are different numbers for the same roof: a 12/12 roof is a 100% grade but a 1.4142 pitch factor. Feed the grade into an area calculation and the shingle order comes out wrong by the difference. The site treats the pitch factor as the deliverable, which is a mild act of faith that anyone reads past the number they came for.

02 Worked example

Take a simple gable roof over a 1,200 ft² house footprint at a 6 in 12 pitch (111.5 m² at the same pitch). The pitch factor for 6/12 is 1.1180.

Roof-plane area is footprint × pitch factor: 1,200 ft² × 1.1180 = 1,341 ft² (124.6 m²), or 13.41 roofing squares. The same slope reads as 26.57° and a 50% grade. Had the shingles been ordered off the footprint instead — 1,200 ft², 12 squares — the order would have been 1.41 squares short before a single percent of waste was added.

03 When this calculator is wrong

The pitch factor is exact geometry, but the number it feeds — how much roofing to buy — is where the simplifications live.

• The factor gives plane area, not the amount to order. A hip roof over the same footprint at the same pitch has the same plane area as a gable, yet needs more shingles: the extra valleys and hip cuts push the waste factor from 10% (simple gable) to 15% (hip). The area is right; the order is area plus waste.
• One pitch assumes one plane slope. Roofs with a lower porch slope, a saltbox with two different pitches, or shed dormers need the factor computed per plane and the areas summed. A single factor for the whole roof under-counts the steeper section.
• Measured rise and run carry their own error. Reading rise off a rafter with a 2 ft level and a tape is good to roughly ±1/4 in on the rise. Over a 12 in run that is about a 1° swing — enough to move a 6/12 reading to a 6.25/12. Measure over the longest run you can reach and the error shrinks.
• Below 4/12, shingle rules change. This calculator reports the geometry for any slope, but asphalt shingles on a low slope call for different underlayment and lap details than a steep roof. Read the shingle spec, not just the pitch, before ordering for anything shallow.

04 What to do with the result

Take the pitch factor to the material math, not the angle. The pitch factor is the single most underestimated input in DIY roofing: roof area is always larger than the footprint by that factor — 1.0541 at 4/12, 1.1180 at 6/12, 1.2019 at 8/12, 1.3017 at 10/12 — and estimates that skip it and multiply the footprint by something close to 1.0 under-order by 10–30%.

So: multiply the footprint by the factor, divide by 100 ft² to get roofing squares, add the waste factor for the roof type (10% gable, 15% hip), and round up to whole bundles at 3 per square. The one exception is that none of this is a rule of thumb you can fudge — it is geometry. If the pitch factor isn't in the estimate, the estimate is wrong.

05 Common questions

What is a 6/12 roof pitch?

A roof that rises 6 inches for every 12 inches of horizontal run. In other terms that is a 26.57° angle, a 50% grade, and a pitch factor of 1.1180 — so the roof surface is 11.8% larger than the footprint it covers.

How do you calculate roof pitch?

Measure the rise over a level 12-inch run: hold a level out horizontally from the roof, mark 12 inches, and measure straight down to the roof surface. That vertical drop is the rise in 12. From any rise and run, the angle is atan(rise ÷ run) and the pitch factor is √(run² + rise²) ÷ run.

How do you convert roof pitch to degrees?

The angle equals atan(rise ÷ run). For the common pitches: 4/12 is 18.43°, 6/12 is 26.57°, 8/12 is 33.69°, and 12/12 is exactly 45°. The toggle above does this for any rise and run you enter.

What is the difference between roof pitch and slope?

Pitch is usually the rise-in-12 ratio; slope is often quoted as a percentage grade (rise ÷ run × 100) or in degrees. The number that matters for material is neither of those — it is the pitch factor, √(run² + rise²) ÷ run, which is the only one that multiplies a footprint into roof-plane area.

Does the pitch factor change between imperial and metric?

No. Pitch factor, angle, and grade are ratios — they depend only on rise divided by run, not on the unit. A roof measured as 6 in over 12 in and the same roof measured as 152.4 mm over 304.8 mm return the identical 1.1180 factor. The unit toggle only changes what you type in.