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Roof Pitch Calculator

This roof pitch calculator takes a rise and run, an angle or an X:12 pitch and gives every other form — plus the rafter length from the run or span, with a labeled diagram.

Your roof

Vertical height gained

Horizontal distance, same unit

Rafter length (optional)
ft

Leave blank to measure to the wall only.

Roof pitch

6 in 12, 26.57 degrees

6:12

Angle
26.565°
Slope
50%
Rise : run
1 : 2
26.57°Run 12Rise 6Slope
  1. angle = atan(rise ÷ run) = atan(6 ÷ 12)
  2. slope % = rise ÷ run × 100 · X:12 = rise ÷ run × 12

Geometry only. Rafter length here is the measured slope line — it doesn't size timber, allow for the ridge board or birdsmouth cuts, or check building-code requirements.

The formulas

  • Angleangle = atan(rise ÷ run)
  • Pitch and slopeX:12 = rise ÷ run × 12 · slope % = rise ÷ run × 100
  • Rafter length√(rise² + run²)

Worked example

A roof rises 6 for every 12 across: 6 ÷ 12 = 0.5, so it's a 6:12 pitch, 50% slope and atan(0.5) = 26.565°. On a 24 ft span the run is 12 ft, the rise 6 ft, and the rafter √(6² + 12²) = 13.42 ft to the wall line, before any overhang.

Frequently asked questions

How do I calculate roof pitch?+

Measure the rise (vertical height) over a horizontal run, then divide rise by run. Multiply by 12 for the X:12 pitch, by 100 for percent slope, and take the arctangent for the angle. A 6-inch rise over 12 inches of run is 6:12, a 50% slope and 26.57°.

What's the difference between run and span?+

The span is the full width of the building, wall to wall. On a symmetrical gable roof each side covers half of it, so the run — from the outside of the wall to the center of the ridge — is half the span.

Is the rafter length the length of timber to buy?+

No. It's the geometric slope length from the wall line to the ridge center (plus any overhang you enter). Cutting length also depends on the ridge board, birdsmouth cut and the timber's depth, and structural sizing is a job for building rules or an engineer.

How do I measure pitch from inside the loft?+

Hold a level horizontally against the underside of a rafter, mark 12 inches (or 300 mm) along it, and measure straight up from that mark to the rafter. That vertical distance over 12 is the pitch.

What pitch is a “low slope” roof?+

Roughly anything below 3:12 (about 14°) is usually called low slope, and many roofing materials have minimum pitches — check the manufacturer's instructions for the covering you plan to use.

How do I convert roof pitch to degrees?+

Take the arctangent of the pitch divided by 12. Common pitches: 3:12 is 14.04°, 4:12 is 18.43°, 6:12 is 26.57°, 8:12 is 33.69° and 12:12 is 45°. Choose “X in 12” in the calculator to convert any pitch.

How do I calculate rafter length?+

Use Pythagoras on the run and the rise. A 24 ft span gives a 12 ft run on each side; at 6:12 the rise is 6 ft, so the rafter is √(12² + 6²) = 13.42 ft. A 1 ft horizontal overhang adds 1.12 ft along the slope, for 14.53 ft in total.

Are roof pitch and roof slope the same thing?+

They describe the same steepness in different ways. Pitch is usually written as rise per 12 of run (6:12), slope as a percentage (50%) and the angle in degrees (26.57°). The calculator shows all three at once.

What is a pitch multiplier?+

It's the length along the slope for every 1 unit of horizontal run: √(1 + (pitch ÷ 12)²). It is 1.054 for 4:12, 1.118 for 6:12 and 1.414 for 12:12. Multiplying a roof's flat plan area by it gives a rough sloped area for a simple roof.

Can I use metric measurements?+

Yes. Rise and run can be in any unit as long as both use the same one, and the rafter length works in feet, inches, meters, centimeters or millimeters. Outside North America, roof pitch is usually given in degrees, so the Angle option may be the quickest.

Is anything I enter saved or sent?+

No. Everything is calculated in your browser and nothing is uploaded or stored.

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