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Roof Pitch Calculator

Convert roof pitch between the US "X in 12" format, degrees and percentage slope — and get the pitch multiplier and rafter length.

Formulas reviewed 13 November 2026 · See the formula

Roof Pitch Calculator

Pitch
Rafter
ft

Usually half the building width plus any overhang, measured horizontally.

Results

Pitch
6/12
Angle
26.57°
Slope (grade)
50%
Pitch multiplier (slope factor)Multiply plan area or horizontal run by this to get sloped area or rafter length.
1.118

Estimates only. Prices are your own inputs in USD; material properties vary by product and supplier.

About the roof pitch calculator

In the US and Canada roof pitch is usually written as rise over a run of 12 (for example 6/12), while the UK and Australia more often use degrees. Material instructions may use either, and minimum-pitch rules for shingles and tiles are stated in one or the other. This calculator converts between all three.

It also gives the pitch multiplier — the factor you multiply plan (horizontal) dimensions by to get lengths and areas along the slope.

How to use it

  1. Choose how you know the pitch: X/12, a measured rise and run, or an angle.
  2. For rise and run, measure a horizontal distance with a level and the vertical rise over that distance; any length works.
  3. Optionally enter the horizontal run of a rafter to get its sloped length.

Formula

Pitch (X/12)

X = 12 × rise ÷ run

Angle

θ = arctan(rise ÷ run)

Slope %

slope = rise ÷ run × 100

Multiplier

m = √(1 + (rise ÷ run)²)

Rafter length

rafter = horizontal run × m
  • Rafter length here is the line length along the slope; it does not include the ridge-board deduction, plumb cuts or tail details.

Worked example: A 6/12 roof with a 15 ft horizontal rafter run

  1. Angle = arctan(6 ÷ 12) = 26.57°
  2. Slope = 50 %
  3. Multiplier = √(1 + 0.25) = 1.1180
  4. Rafter = 15 × 1.1180 = 16.77 ft

Result: 6/12 = 26.57° = 50 % slope; rafter ≈ 16.8 ft.

Assumptions and limitations

  • Rise and run must be measured in the same unit.
  • Minimum pitch requirements for roofing materials come from the manufacturer and local code.

Frequently asked questions

What angle is a 4/12 roof?

About 18.43°. A 6/12 roof is 26.57°, an 8/12 roof is 33.69° and a 12/12 roof is 45°.

What is a pitch multiplier?

The ratio between a length along the roof slope and the same length measured horizontally. Multiply your footprint area by it to get roof area.

How do I measure roof pitch safely?

Measure at a gable end from a ladder, on a rafter inside the attic, or read it from plans. Avoid walking on the roof surface without proper fall protection.

Is pitch the same as slope?

The terms are often used interchangeably. Strictly, some trades use pitch for rise over total span; this calculator uses the common rise-over-run meaning (X/12).

Sources and further reading

Results are estimates for planning purposes. See our disclaimer for details.