Roof Area Calculator
Enter your building dimensions and roof pitch to instantly calculate total roof surface area in square feet, square meters, and roofing squares.
How to Use This Calculator
This calculator needs three inputs to compute your roof area:
- Building Length — The longest dimension of your house, measured at the eaves (not at the ridge). Measure the exterior wall-to-wall, including any overhangs you want to count.
- Building Width — The shorter dimension, measured perpendicular to the ridge. For a gable roof, this is the direction the roof slopes.
- Roof Pitch — Expressed as rise-over-run (e.g., 6/12 means 6 inches of rise for every 12 inches of horizontal run). If you don't know your pitch, use a level and tape measure: hold the level horizontally against a rafter and measure the rise over a 12-inch run.
Select your preferred unit system (imperial or metric), enter the values, and the calculator displays total roof area, number of roofing squares, and a recommended material order quantity including waste factor.
The Formula
Roof area is calculated by applying a pitch multiplier to the building's footprint area:
Pitch Multiplier = √(1 + (Rise/Run)²)
Roof Area = Footprint Area × Pitch Multiplier
Example: Rise/Run = 6/12 = 0.5
Multiplier = √(1 + 0.25) = √1.25 = 1.118
Roofing Squares = Roof Area ÷ 100
The pitch multiplier is the same as the slope factor used by contractors. It equals the length of the actual rafter for every 12 inches of horizontal run, divided by 12.
Example Calculation
Example: Ranch House — 40 x 28 ft with 6/12 pitch
You would order 15 squares of shingles for this roof — rounding up to the nearest half-square for safety.