Roof Pitch Calculator

How to calculate roof pitch

Choose the measurement you already know: rise per 12 inches, measured rise and horizontal run, or angle from horizontal. The calculator normalizes each method into an X:12 pitch, degree angle, percent slope, and pitch multiplier.

pitch rise per 12 = rise / run x 12; angle = arctan(rise / run); multiplier = √(1 + slope²)

Use a nonnegative pitch or rise, a positive measured run, and an angle from 0° up to but not including 90°. A flat 0:12 surface is valid geometry, not a recommendation for a roofing product. Horizontal rafter run must be positive; blank overhang means none. Leave optional plan area blank to calculate pitch and rafter length only. An entered plan area must be positive. Invalid or unsupported values show a correction message instead of being changed to zero or a different angle. The chart compares lengths in feet only; area and roofing squares stay in the table.

6:12 roof-pitch example

A 6:12 roof rises 6 inches for every 12 inches of horizontal run. Its slope is 50%, its angle is about 26.57°, and its pitch multiplier is about 1.118. A 12 ft horizontal rafter run with a 12 in horizontal overhang has an estimated sloped length of 14.53 ft.

Roof area and roofing squares

When a horizontal plan area is entered, the calculator multiplies it by the pitch factor to estimate sloped surface area. One roofing square equals 100 square feet. The estimate does not include waste, starter strips, ridge caps, valleys, hips, dormers, or multiple roof pitches.

Roof square footage with a known pitch

For roof planes with the same pitch, sloped area = horizontal roof projection x sqrt(1 + (rise / run)^2). A horizontal projection of 40 ft by 30 ft is 1,200 sq ft. At 6:12, rise / run = 0.5 and the multiplier is about 1.118034. The sloped area is 1,200 x 1.118034 = 1,341.64 sq ft, or about 13.4164 roofing squares before any material allowance. Do not double the full projected footprint again just because a gable has two sides.

Use the square footage calculator for horizontal plan area or the area calculator for individual projected shapes. A roof projection is not a home's total interior floor area: multiple stories, garages, porches and eaves can make them different. Obtain dimensions from suitable plans, reliable measurements or a qualified professional; do not climb onto a roof to use this example.

Include eaves in the projected area if they are part of the surface being estimated. The separate rafter-overhang field changes rafter length only; it does not enlarge the entered plan area automatically. For different pitches, calculate each roof section from its own horizontal projection and slope, then add the sloped areas. Overlapping projections, dormers and complex intersections require a proper takeoff rather than one whole-building multiplier.

A roofing square is an area unit, not a bundle count. GAF explains roofing squares and product-dependent shingle quantities. This tool does not specify bundles, cutting allowance, ridge/starter products, fasteners, underlayment, labor or cost. Check the selected product and professional takeoff before ordering.

Roof slope terminology

In common US field usage, X:12 describes inches of rise per 12 inches of horizontal run. The GAF steep-slope field guide explains rise, run, span, slope, and pitch conventions.

Frequently asked questions

Is run the same as span?

No. Run is horizontal distance from support to ridge. For a centered symmetrical gable, run is commonly half the full building span.

Does pitch determine the required rafter size?

No. Member size also depends on species, grade, spacing, loads, span, bearing, and code requirements. Use approved plans or a span resource such as the American Wood Council span calculator.