Roof Pitch Calculator

Roof pitch describes vertical rise relative to horizontal run. In North American construction it is commonly stated as inches of rise for every 12 inches of run, such as 4:12 or 6:12. This calculator accepts any rise and run measured in the same unit, divides rise by run, and scales the ratio to a 12-unit run. Using an actual measured run rather than assuming 12 also supports drawings, rafters, and existing roofs where the convenient reference length is unavailable. The same ratio can be expressed in two other useful forms. Multiplying by 100 gives slope percentage, a format often used for drainage and civil work. Taking the arctangent gives the roof angle in degrees from horizontal, which can help with saw setup, layout, and product limits. These formats describe identical geometry, but specifications may require one particular convention. Always retain enough precision during conversion before rounding the displayed result. Sloped surface area exceeds horizontal projected area. The calculator derives a slope factor by dividing rafter length by horizontal run, then multiplies the entered footprint area by that factor. For a simple gable roof, horizontal roof area is generally the building plan area covered by both slopes, including the horizontal projection of overhangs. Do not double the full footprint merely because there are two sides; each side covers one-half of the projected width. Complex roofs require separate treatment. Hips, valleys, dormers, intersecting gables, changes of pitch, and curved surfaces should be divided into suitable projected regions. Calculate each region using its own slope and add the sloped areas. Openings may be deducted for net coverage, although leaving small openings in the gross area can help provide a material allowance. Roofing products also require laps, side cuts, starter courses, ridge details, and waste that are not included in pure surface geometry. Measure safely from drawings, accessible framing, or approved instruments rather than climbing an unprotected roof. Existing structures may sag or vary across planes, so check several locations. Product suitability depends on pitch: shingles, tiles, metal panels, membranes, underlayments, and drainage details each have minimum-slope requirements. Structural rafter sizing depends on span, spacing, loads, species, grade, connections, and local code, not pitch alone. Use this calculator for dimensional planning and estimating, then apply overhangs, waste, bundle or panel coverage, and manufacturer instructions before purchasing or building.