Roof pitch is the steepness of a roof, written as rise over run: the inches a roof rises vertically for every 12 inches of horizontal distance. A 6/12 roof rises 6 inches per foot of run; a 12/12 roof is a 45-degree angle. This one number drives almost every roofing calculation, because the steeper the pitch, the more surface area a roof has over the same footprint — and roofing materials are sold by surface area.
The most common way to measure pitch is from inside the attic. Hold a 12-inch level horizontally against a rafter, then measure straight down from the 12-inch mark to the rafter's top edge. That vertical distance in inches is your pitch: 7 inches down means 7/12. No attic access? Measure from the ground with a long level and a tape, or check the building plans. The Roof Pitch Calculator converts your measurement between pitch (6/12), angle in degrees, and slope percentage instantly.
Here is why pitch matters for money. Roofing is quoted in squares — one square equals 100 square feet of roof surface — and the surface area of a pitched roof is always larger than the footprint of the house beneath it. The conversion uses the pitch multiplier: the square root of (1 + (pitch/12)²). For a 6/12 roof, the multiplier is about 1.118, meaning a 1,500-square-foot footprint becomes roughly 1,677 square feet of roof, or 16.8 squares. A 12/12 roof over the same footprint is 1.414× the area — nearly six extra squares of shingles.
Pitch also affects what you can install. Very low-slope roofs (below about 2/12) cannot use standard asphalt shingles at all — water does not shed fast enough, and manufacturers will not warranty it. Low slopes need membrane or rolled roofing instead. Steep roofs above 8/12 or 9/12 cost more to install because crews move slower, need safety equipment, and waste more material cutting around the geometry.
To estimate a full roof, work in this order. First, measure the building footprint (length × width of the roof's outline). Second, determine the pitch and apply the multiplier to get true roof area. Third, add a waste factor — 10% for simple gable roofs, 15% or more for hips, valleys, and dormers that force extra cutting. The Roofing Calculator does all three steps from footprint and pitch, and the Shingle Calculator converts the resulting squares into bundles (typically 3 bundles per square for standard shingles) with the waste factor stated.
One caution that matters more than the math: roofing is safety-sensitive and code-regulated work. Pitch affects ventilation requirements, underlayment choices, and in snowy climates, structural loading. Use these calculators to budget accurately and to check a contractor's quote — a quote in squares should roughly match your own calculation — but hire licensed, insured professionals for the installation, and never walk a steep or wet roof to measure it.