Convert roof pitch between the x:12 notation carpenters use, degrees, and percent slope, then turn a building span into common rafter length, rise, and run — including overhang and a ridge-board deduction. A diagram draws the slope to scale, and the multiplier table covers 1:12 to 12:12 with hip and valley factors for quick layout.
Once you know the pitch, the roofing calculator turns roof area into shingle bundles and underlayment, and the gutter calculator sizes the drainage. For any other angle or percentage math, try the percentage calculator.
Frequently asked questions
How do I convert roof pitch to degrees?
Roof pitch is the rise in inches for every 12 inches of horizontal run. The angle is the arctangent of pitch ÷ 12. A 6:12 roof is atan(0.5) = 26.57°, a 4:12 roof is 18.43°, and a 12:12 roof is exactly 45°. Percent slope is simply pitch ÷ 12 × 100, so 6:12 is a 50% slope. Pick any input style — pitch, degrees, percent, or a measured rise and run — and the calculator fills in all the others. Results update as you type.
How do I calculate rafter length?
Take half the building span as the run (less half the ridge board's thickness), add any horizontal overhang, and multiply by the rafter multiplier for your pitch: √(1 + (pitch ÷ 12)²). For a 24-foot-wide building at 6:12 with a 2× ridge and a 1-foot overhang, the run is 11.94 ft plus 1 ft, times 1.118 — about 14' 5½". That is the line length along the top edge; allow extra for the plumb and tail cuts when buying stock. Rafter size (2×6, 2×8, and so on) comes from the span tables, not this calculation.
What is a rafter multiplier?
It is the length of rafter per foot of horizontal run for a given pitch — the same numbers stamped on the rafter table of a framing square. A 4:12 roof has a multiplier of 1.054, 6:12 is 1.118, 8:12 is 1.202, and 12:12 is 1.414 (the square root of 2). The same factor turns a roof's plan-view footprint into its actual surface area, which is why it is also called the roof area or slope factor. The table on this page lists 1:12 through 12:12. Hip and valley rafters use a larger factor because they run diagonally across the plan; that column is in the table too.
How do I measure the pitch of an existing roof?
Hold a level horizontally against the roof or a rafter, mark 12 inches along it, and measure straight up from that mark to the roof surface — that number is the pitch. From the attic, measure along the underside of a rafter the same way. If you can only measure the building, the rise from wall plate to ridge divided by half the span, times 12, gives the pitch. Enter the rise and run directly and the calculator does the division.
What pitch do asphalt shingles need?
The IRC (R905.2.2) allows asphalt shingles on slopes of 2:12 or steeper, but roofs between 2:12 and 4:12 need double underlayment (R905.2.7). Below 2:12, use a low-slope system such as a membrane or modified bitumen. Metal panels, tile, and slate each have their own minimum slopes from the manufacturer and code, so check before choosing a material for a shallow roof. Steeper roofs shed water faster but cost more to build and are harder to walk on, so most homes land between 4:12 and 9:12. Very steep roofs may also need extra fasteners per shingle; follow the manufacturer's high-slope instructions.