Roof Pitch Calculator

How to use this calculator

1. Pick what you know. Use Rise & run for a measurement you took yourself. Use Pitch for a number from plans or a building permit (for example 6 for 6:12). Use Angle for a reading from a digital level or angle finder.
2. Choose the roof type. On a gable roof, two slopes meet at a centered ridge, so each rafter covers half the span. On a shed roof, one slope covers the whole span.
3. Enter the building span and overhang if you want the rise and rafter length. The span runs from outside wall line to outside wall line, measured level and perpendicular to the ridge. The overhang is measured level, from the wall line out to the rafter tail.
4. Enter the ridge board thickness (gable roofs) to get the rafter length to the face of the ridge board. Enter 0 to skip it.
5. Say whether wind design is required where you build (IRC Figure R301.2.1.1). In those high-wind regions the underlayment rules change.

The 3D view draws the roof at your true pitch, span and overhang. Wall height and building length are only illustrative. The Section tab shows the rise-over-12 triangle, the angle, the rise and the rafter line.

How to measure roof pitch

From the attic or a rafter tail. Hold a level against the underside of a rafter and mark 12 inches along the level from where it touches. Measure straight up from that mark to the rafter. That distance is the pitch: 6 inches means 6:12. With a 24-inch level, measure over 24 inches and enter both numbers. The calculator scales them to 12.

From the roof surface. Lay the level on the shingles with one end touching, hold it level, and measure the gap from the 12-inch mark down to the roof. Shingle thickness hardly changes the result, because the roof surface runs parallel to the rafters.

With an angle finder. A digital level or angle-finder app on the rafter or roof surface reads the angle directly. Switch to Angle mode and enter it.

Only go on the roof if you can do it safely. See the safety section below.

The formulas

Pitch (x:12) = 12 × rise ÷ run

Angle = arctan(pitch ÷ 12)

Percent grade = pitch ÷ 12 × 100

Pitch multiplier = √(12² + pitch²) ÷ 12

Rafter line length = horizontal run × pitch multiplier

Hip or valley length = common run × √(2 × 12² + pitch²) ÷ 12 (both roof planes at the same pitch)

The pitch multiplier is the rafter’s sloped length for each foot of level run. It works for roof area too: a roof plane’s area equals its plan (footprint) area times the multiplier.

The IRC also measures rafter spans along the horizontal projection of the rafter (R802.4.1). That is the run used here.

Worked example: 24-foot gable at 6:12

You measure 6 inches of rise over a 12-inch level. The house is 24 feet wide, the eaves overhang 12 inches and the ridge board is 1 1/2 inches thick.

• Pitch: 12 × 6 ÷ 12 = 6:12
• Angle: arctan(6 ÷ 12) = 26.57°
• Percent grade: 6 ÷ 12 × 100 = 50%
• Pitch multiplier: √(144 + 36) ÷ 12 = 1.1180
• Run per rafter: 24 ÷ 2 = 12 ft, so the rise to the ridge line is 12 × 6 ÷ 12 = 6 ft
• Rafter line length: 12 × 1.1180 = 13.42 ft = 13 ft 5 in
• With the 12-inch overhang: 13 × 1.1180 = 14.53 ft = 14 ft 6 3/8 in
• Half the ridge board, 3/4 inch, is measured level, so it comes off the run before the multiplier: 0.75 × 1.1180 = 0.84 in shorter along the slope
• Rafter to the ridge-board face: (12 ft - 3/4 in) × 1.1180 = 13 ft 4 1/8 in, or 14 ft 5 5/8 in with the overhang

These are line lengths. If you subtract the 3/4 inch straight from the sloped length, the rafter ends up too long. The steeper the roof, the bigger the gap: at 12:12 the true deduction is 1.06 inches. The plumb cut and tail cut need extra length too, so buy boards longer than this. Rafter size, spacing and spans come from the IRC rafter tables or an engineer, not from this calculator.

Common roof pitches

What is the minimum slope for each roof covering?

Chapter 9 of the 2024 International Residential Code sets a minimum slope for each roof covering:

* Only where wind design is not required. Where IRC Figure R301.2.1.1 requires wind design, Table R905.1.1(2) calls for one of three wind-design underlayments at any slope (see the asphalt shingle question in the FAQ). That applies to asphalt shingles, tile, metal roof shingles, mineral-surfaced roll roofing, slate, wood shingles and shakes, metal panels and BIPV shingles.

A few details also change with the slope. Slate needs less headlap as the roof gets steeper (Table R905.6.6), and wood shingles have shorter maximum exposures between 3:12 and 4:12 (Table R905.7.6(1)). Where your area requires an ice barrier at the eaves, it must reach at least 24 inches inside the exterior wall line. On roofs of 8:12 or steeper, it must also reach at least 36 inches up the slope from the eave edge (R905.1.2). Rafter-framed roofs flatter than 3:12 need the ridge, hips and valleys designed as beams (R802.4.4).

These are minimums. The IRC also requires roof coverings to be installed per the manufacturer’s instructions (R905.1), and a product’s instructions may call for a steeper minimum slope than the code. States and cities adopt different code editions and amend them, so confirm with your building department.

Safety on the roof

Roof work is work at height. OSHA treats any roof steeper than 4:12 as a steep roof (29 CFR 1926.500(b)). On construction jobs, workers on a steep roof with edges 6 feet or more above a lower level need guardrails with toeboards, safety nets or a personal fall arrest system (1926.501(b)(11)). Low-slope roofs need fall protection at that height too (1926.501(b)(10)). These rules apply to employers, but the hazard is the same for a homeowner.

Measure pitch from the attic or from a ladder at the eave where you can. Keep off wet, icy or mossy roofs, and hire a roofer for work on steep or high roofs.

Last reviewed Oct 9, 2026. See how we calculate. Results are planning estimates - confirm with your supplier and local code.