About this calculator
Free roofing calculator to estimate shingles, bundles, squares, underlayment, and total material costs. Includes waste factor and accessory materials.
How to use
Enter your values in the fields above and click Calculate to see your results. Click Clear to reset all fields.
What the roofing calculator does and when to use it
This calculator estimates the shingle bundles, roofing squares, underlayment rolls and shingle cost for a roof from a handful of inputs: the length and width of the building footprint, the pitch of the roof, a waste percentage and the price per bundle. It accounts for the fact that a sloped roof has more surface than the floor plan beneath it, and it builds in an allowance for cuts, hips, valleys and overlap.
Use it for budgeting before you request quotes, for checking a contractor's material list, or for planning a DIY re-roof of a simple gable or shed roof. It works best when the footprint is a rectangle and all planes share one pitch. Complex roofs with dormers, multiple pitches or large overhangs need each section measured separately, and the result is not a substitute for a supplier's takeoff or a professional roof inspection.
The formula and its variables
- Footprint = length × width, in square feet.
- Pitch (r) is the rise in inches per 12 inches of horizontal run, such as 6 for a 6/12 roof.
- Pitch multiplier = √(r² + 12²) / 12, which converts flat area to sloped surface area.
- Roof area = footprint × multiplier, and squares = roof area / 100, since one roofing square covers 100 square feet.
- Squares with waste = squares × (1 + waste / 100).
- Bundles = squares with waste × 3, rounded up, using 3 bundles per square for standard architectural shingles.
- Underlayment rolls = squares with waste / 4, rounded up, with a minimum of 1 roll.
- Cost = bundles × price per bundle.
Worked example
A house footprint is 40 ft by 30 ft with a 6/12 pitch, a 10 percent waste factor and shingles at $35 per bundle. The footprint is 1,200 square feet. The multiplier is √(36 + 144) / 12 = √180 / 12 = 13.4164 / 12 = 1.1180. Roof area is 1,200 × 1.1180 = 1,341.6 square feet, shown as 1342 sq ft or 13.42 squares. With 10 percent waste, squares become 13.416 × 1.10 = 14.76.
Bundles are 14.758 × 3 = 44.27, rounded up to 45. Underlayment is 14.758 / 4 = 3.69, rounded up to 4 rolls. The estimated shingle cost is 45 × $35 = $1,575.00. That is the headline figure shown by the calculator, along with the table of area, squares, bundles and rolls.
Common mistakes and how to interpret the result
- Measuring only the footprint. Roof eaves and rakes usually extend past the walls. If you have overhangs, add them to the length and width entered here.
- Entering the angle instead of the pitch. The field expects rise per 12 inches of run, such as 6, not a degree measurement like 26.6.
- Low waste on complex roofs. Simple gables can often use 10 percent, while roofs with many hips and valleys may need more. Adjust the waste field rather than accepting the default.
- Reading the cost as the whole job. The estimate covers shingles only. Underlayment, flashing, ridge caps, drip edge, nails, tear-off and labor are extra.
Related tools: Roof Shingles Calculator, Roof Pitch Calculator, Roof Underlayment Roll Planner and Gutter Calculator.