Roof Pitch Calculator: Rise, Run, Angle and Slope

How to use the roof pitch calculator

1. Choose input mode from the dropdown: rise per 12" run, angle in degrees, slope percentage, or pitch ratio.
2. Enter your known value in the field that appears. Use decimals for fractions (e.g., 6.5 for 6½:12).
3. Read all results instantly: pitch as X:12, angle in degrees, slope as a percentage, and the rafter length factor.
4. Use the rafter factor to find actual rafter length: multiply the horizontal run (in feet) by the factor.

How the calculations work

The roof pitch calculator uses the standard roof pitch definition: pitch is the rise in inches for every 12 inches of horizontal run. From this ratio the other values derive:

• Angle = arctan(rise ÷ 12) converted to degrees. For a 6:12 pitch: arctan(6/12) = 26.565°.
• Slope % = (rise ÷ 12) × 100. For 6:12: (6/12) × 100 = 50%.
• Rafter factor = √(1 + (rise/12)²). This is the hypotenuse of a right triangle with run = 1 and rise = rise/12. For 6:12: √(1 + 0.5²) = 1.1180.

When you enter angle or slope, the calculator reverses these formulas to find the rise, then computes the rest.

Common roof pitches reference

The table below shows standard pitches with their angle, slope and rafter factor. Values are rounded for readability.

Using the rafter factor

The rafter factor converts horizontal run to actual rafter length. Measure the horizontal distance from the outside wall to the ridge (the run), then multiply by the factor.

Example: A 6:12 roof with a 12-foot run. Rafter length = 12 ft × 1.1180 = 13.416 ft (13 ft 5 in). Add overhang and ridge board deductions as needed.

For hip and valley rafters, use the hip/valley factor = √(2 + (rise/12)²). This calculator gives the common rafter factor only.

Pitch categories and typical uses

• Flat to low slope (0:12 to 2:12): Commercial roofs, modern homes. Require membrane roofing (TPO, EPDM, built-up). Drainage is critical.
• Low slope (3:12 to 4:12): Ranch houses, sheds. Asphalt shingles need double underlayment below 4:12 per most codes.
• Conventional (5:12 to 8:12): Most residential roofs. Standard shingle installation, good drainage, walkable with caution.
• Steep (9:12 to 12:12): Victorian, Gothic, A-frames. Excellent shedding, more material, requires scaffolding or roof jacks.
• Very steep (>12:12): Specialty designs. Factor exceeds 1.414; rafters are much longer than run.

Measuring pitch on an existing roof

If you don't know the pitch, measure it safely from the attic or with a level:

1. In the attic, hold a 12-inch level horizontally against a rafter.
2. Measure vertically from the level's far end up to the rafter bottom. That measurement in inches is the rise.
3. Enter that rise in the roof pitch calculator (mode: Rise).

Alternatively, use a digital angle finder on the rafter or roof surface. Enter the angle in degrees (mode: Angle).

Never walk a roof steeper than 6:12 without fall protection. For steep roofs, measure from the gable end or use a smartphone app that reads the camera angle.

Common mistakes

• Confusing pitch with angle: A 12:12 pitch is 45°, not 12°. Always check which unit you have.
• Using slope % as pitch: 50% slope equals 6:12 pitch, not 50:12. Slope is rise/run; pitch is rise/12.
• Forgetting the rafter factor: Ordering lumber by horizontal run alone leaves you short. Multiply run by the factor.
• Ignoring code minimums: Many areas require 2:12 minimum for shingles, ¼:12 for membrane. Check local amendments.
• Measuring from the wrong plane: Measure rise vertically, run horizontally. Sloping the tape measure gives a larger number.

Worked examples