Roof Pitch Calculator: Pitch, Angle, Slope and Rafter Length

How to use the roof pitch calculator

1. Pick what you know. You might have a rise and run you measured, a pitch from the plans (“6/12”), an angle from a phone level or angle finder, or a slope in percent.
2. Enter the number. The calculator converts it to the other three forms right away. It also shows the nearest carpenter’s fraction, such as 6 3/8 in 12.
3. Add the span for rafters (optional). For a symmetrical gable, the run of each rafter is half the building span, measured wall to wall. Add the horizontal overhang to get the length of the rafter tail too.
4. Read the roofing note. Under the results you’ll see where the pitch falls against the shingle slope rules in the residential code and the OSHA low-slope definition.

Going on to order shingles? The roofing calculator uses the same pitch to turn your house footprint into squares, bundles, ridge cap and underlayment.

How to measure roof pitch

Pitch is the rise in inches for every 12 inches of horizontal run. You don’t have to measure a full 12 inches, because the calculator scales any rise and run you give it. These methods are listed from safest to least safe:

• From the plans or permit drawings. Look for a small triangle symbol on the elevations with 12 on the flat side and the rise on the upright side.
• In the attic. Hold a level horizontally against the underside of a rafter, with one end touching it. Mark 12 in along the level and measure straight up from that mark to the rafter. That measurement is the rise.
• At a gable end or rake board. From a ladder, hold the level against the rake board and measure the same way. You can also measure the vertical drop over any level distance and enter both numbers as rise and run.
• With an angle reading. Many phone level apps and digital angle finders read degrees. Set one on a rafter or the rake and choose “Angle in degrees.”

Please don’t walk a roof just to measure it. At 8/12 and steeper, footing is poor even in dry weather.

Roof pitch formulas

The pitch multiplier (also called the slope factor) is the length of sloped roof per foot of level run. It does two jobs. Multiply a common rafter’s run by it to get the rafter length, and multiply the roof footprint by it to get the true roof area. The hip and valley factor works the same way for a hip or valley rafter on a regular hip roof. Because that rafter crosses the plan on a 45° diagonal, its plan length is the common run × √2.

The rafter length here runs along the top edge, from the outside of the wall to the center of the ridge. Framers shorten it by half the ridge board’s thickness, measured level, before cutting. The bird’s-mouth cut and the plumb cuts change the board you buy but not this line length. To pick lumber, round the rafter-with-overhang length up to the next even length.

Worked examples

You measured 7 in of rise over 16 in of run

r = 7 ÷ 16 = 0.4375, so the pitch is 12 × 0.4375 = 5.25 in 12 (5 1/4). That is 23.6° or a 43.8% slope, with a pitch multiplier of 1.092.

A phone app reads 30°

tan 30° = 0.577, so the pitch is 6.93 in 12, which is nearly 7/12. Shingles overlap, so a reading taken on the shingle surface can be slightly off. A reading on a rafter or the rake board is more reliable.

Rafters for a 24 ft wide gable at 6/12 with a 1 ft overhang

The run is 24 ÷ 2 = 12 ft. The ridge sits 12 × 0.5 = 6 ft above the wall plates. Each rafter is 12 × 1.118 = 13.42 ft from wall to ridge, or (12 + 1) × 1.118 = 14.53 ft with the overhang, so you would buy 16 ft stock.

A 28 ft wide garage at 8/12 with 16 in overhangs

Run 14 ft, multiplier 1.202. Rafter to the ridge is 16.83 ft, and 18.43 ft including the 1.33 ft overhang, so you need 20 ft boards. The ridge is 9.33 ft above the plates.

Roof pitch to degrees chart

What your roof pitch means for roofing

• Under 2/12: residential code (IRC section R905.2.2) allows asphalt shingles only on slopes of 2/12 or more. Flatter roofs need a low-slope system such as a membrane or roll roofing.
• 2/12 to under 4/12: shingles are allowed, but the code requires a double layer of underlayment. Plan for twice the underlayment when you order.
• 4/12 and up: standard shingle installation with one layer of underlayment. OSHA’s construction rules call a roof of 4/12 or less “low-slope” and anything steeper a “steep roof,” and the fall protection rules differ between the two.

Pitch also changes how much material you buy. A 12/12 roof has 41% more surface than its footprint, while a 4/12 roof has only about 5% more. Local codes and the shingle maker’s instructions control the final details, including any minimum slope that is stricter than the code.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Pitch = 12 × rise ÷ run; angle = arctan(rise ÷ run); percent slope = 100 × rise ÷ run. These are exact geometric conversions.
• Pitch multiplier = √(1 + (rise ÷ run)²); hip/valley factor = √(2 + (rise ÷ run)²) for a regular hip roof with equal pitches (45° in plan).
• Rafter length is the line length from the outside of the wall to the ridge centerline for a symmetrical gable (run = half the span). It does not deduct half the ridge board or add plumb and bird's-mouth cuts.
• The shingle and safety notes summarize the code and OSHA thresholds below; local codes and the shingle maker's instructions govern.

Sources

1. City of Seattle, Department of Construction and Inspections, 2018 Seattle Residential Code, Chapter 9 Roof Assemblies (adopts IRC Section R905.2.2). R905.2.2: asphalt shingles only on slopes of 2:12 or greater; double underlayment required from 2:12 up to 4:12.
2. U.S. Occupational Safety and Health Administration, 29 CFR 1926.500, Fall Protection: Scope, application, and definitions. Low-slope roof: slope less than or equal to 4 in 12; steep roof: slope greater than 4 in 12.

Last reviewed October 25, 2026. Product figures vary by brand; the label on what you buy wins.