Roof area & pitch calculator
Start with a horizontal plan or an already measured roof surface. Keep slope and ordering waste separate, and add roof planes when slopes differ.
The formula and a worked example
For each horizontal plane: surface area = plan area × √(1 + (rise ÷ 12)²). Add the plane surfaces, then divide square feet by 100 for roofing squares. An already measured surface gets no additional pitch factor.
Example: 1,800 sq ft of horizontal plan at 6:12 gives 2,012.46 sq ft of roof surface, or 20.12 squares. A reader-selected 10% waste scenario adds 201.25 sq ft, giving 2,213.71 sq ft (22.14 squares) before product-package rounding. Ten percent is an example, not an ordering recommendation.
For different pitches, calculate each horizontal projection separately. Do not multiply an entire two-slope roof by its steeper pitch or enter overlapping projected areas. A rectangular building calculation assumes the same overhang on all four sides; use plane mode for irregular footprints.
Inputs, precision, and limits
Geometry v1.0.0 accepts 1–20 planes, positive areas up to 1,000,000 in the selected area unit, rise from 0 to 24 per 12 of run, and a user-selected waste allowance from 0–100%. These are tool limits, not permitted roof-design limits. Full precision is retained until display, which uses two decimals; no package quantities are inferred.
Use plans or a professional measurement report. The tool is not a reason to climb onto a roof. It does not establish material suitability, structure, drainage, code compliance, or accessory quantities. Zero rise is supported for arithmetic only. Confirm actual slopes and product-specific details before ordering.
Source notes
GAF defines a roofing square as 100 square feet. NIST defines the international foot as exactly 0.3048 meter; this tool squares that conversion for area. Both sources checked September 8, 2026. Slope conversion follows right-triangle geometry.