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Roof Pitch Calculator: Pitch, Angle and Rafter Length

Enter the rise and the run of the roof, the eave overhang and the footprint of the roof. The calculator converts the slope into every common notation and gives the rafter length and the true sloped area.

m
m
On a symmetrical gable roof the run is half the building span.
m
m²
The area seen from above, measured to the edge of the eaves.

Includes the waste margin explained below. Check your product label for coverage and sizes.

How the roof pitch calculator works

Pitch describes how steeply a roof climbs: the vertical rise for a given horizontal run. North American builders write it as inches of rise per 12 inches of run (a 6/12 roof rises 6 in every foot), much of Europe uses the angle in degrees, and drainage and flat-roof products often use a percentage. All three come from the same ratio, so the calculator divides the rise by the run once and converts.

Pitch = 12 × rise ÷ run · Angle = arctan(rise ÷ run) · Multiplier = √(1 + (rise ÷ run)²) · Rafter = √(rise² + run²) + overhang × multiplier

The rafter is the hypotenuse of a right triangle whose sides are the rise and the run, extended by the eave overhang measured horizontally. The same multiplier turns a flat plan area into the real sloped area that roofing, underlayment and insulation have to cover.

Worked example

A gable roof over an 8 m wide building rises 1.5 m from the wall plate to the ridge, so each side has a run of 4 m. The ratio is 1.5 ÷ 4 = 0.375: a pitch of 4.5 in 12, an angle of 20.56° and a slope of 37.5%. The rafter from plate to ridge line is √(1.5² + 4²) = 4.27 m, and a 0.5 m overhang adds 0.5 × 1.068 = 0.53 m, for 4.81 m in total. If the roof covers 80 m² seen from above, the sloped surface is 80 × 1.068 = 85.4 m². In imperial terms, a 5 ft rise over a 12 ft run is exactly 5/12 (22.62°), with a 13 ft rafter plus 1.63 ft for an 18 in overhang.

Pitch conversion table

PitchAngleSlopeArea multiplier
2/129.46°16.7%1.014
3/1214.04°25%1.031
4/1218.43°33.3%1.054
5/1222.62°41.7%1.083
6/1226.57°50%1.118
8/1233.69°66.7%1.202
9/1236.87°75%1.250
10/1239.81°83.3%1.302
12/1245°100%1.414

What the pitch decides

  • Roof covering. Each material has a minimum pitch. Under the US International Residential Code, asphalt shingles need at least 2/12 with double underlayment below 4/12, and standing-seam metal can go much lower. Clay and concrete tiles in Europe often need 17.5° to 30° or more depending on the profile, so always check the manufacturer's minimum for your exposure.
  • Material quantities. Multiply the plan area by the multiplier before ordering sheathing, shingles or membrane, then add the usual waste for hips and valleys.
  • Rafter cut length. The figure is the line length to the ridge centreline. Deduct half the ridge board thickness (measured along the slope) before cutting, and buy the next stock length up.
  • Safety. Roofs steeper than roughly 6/12 are hard to stand on and anything above about 8/12 needs roof brackets or scaffolding. Working at height is dangerous; hire a professional if in doubt.

Measuring an existing roof

From inside the loft, hold a level horizontally against the underside of a rafter, mark 12 in (or 30 cm) along it and measure straight up to the rafter: that height is the rise per 12 in, or per 30 cm. A phone inclinometer on a rafter gives the angle directly. Rafter sizes and spans are structural and must follow local building codes.

Frequently asked questions

What is a 4/12 roof pitch in degrees?

A 4/12 pitch is 18.43°, a slope of 33.3%. It is the lowest pitch at which asphalt shingles are usually installed with a single layer of underlayment under US codes.

How do I calculate roof pitch from rise and run?

Divide the rise by the run and multiply by 12. A 6 ft rise over a 15 ft run is 6 ÷ 15 × 12 = 4.8, so the pitch is about 4.8/12, or 21.8°.

What is the most common roof pitch?

In North American houses, pitches between 4/12 and 9/12 are most common, with 6/12 (26.57°) a frequent choice. Many European tiled roofs are built between 30° and 45°.

How long is a rafter for a 24 ft wide building with a 6/12 pitch?

The run is 12 ft and the rise 6 ft, so the rafter is √(144 + 36) = 13.42 ft to the ridge line; with a 1 ft overhang it is about 14.54 ft, cut from 16 ft stock.

What is a roof pitch multiplier?

It is the factor that converts flat plan area into sloped roof area: 1.054 at 4/12, 1.118 at 6/12 and 1.414 at 12/12. A 1,500 ft² footprint at 6/12 has about 1,677 ft² of roof.

Last reviewed: 2026-09-27 · How we calculate