CalculatorEnter your measurements below and it works the answer out.
Area divided by coverage, plus 10% waste, rounded up to whole squares. Enter what you measured and pick how it is sold.
The arithmetic. Work out the area, divide by what one square covers, add 10% waste, round up. Nothing more than that — the value of the page is the coverage figures and the waste allowance, both of which are stated here rather than hidden inside the tool.
Why the waste allowance is 10%. The area you can measure from the ground is the building footprint, not the roof. A pitched roof is longer up the slope, and the multiplier is pure geometry: the square root of 1 plus (rise ÷ 12) squared. A 6/12 roof is 1.118 times its footprint; a 12/12 roof is 1.414 times.
How it is sold. Shingles are sold by the bundle, three bundles to a square, and a square is 100 square feet. Ten per cent covers starter course, ridge caps and waste at hips and valleys — a simple gable roof needs less, a cut-up roof needs more.
Measure the openings honestly. Deduct large openings — a garage door, a picture window, a stairwell — and ignore small ones. Cutting a sheet or a board around a normal door wastes more material than the opening saves, which is why trades quote gross area and let the waste allowance absorb it.
Round up to how it is sold. A calculator that tells you 6.3 sheets is not finished. Everything below is rounded up to whole units, because that is what you can actually buy.
Divide your area in square feet by the coverage of one square, then add 10% and round up. At 100 sq ft each, a 200 sq ft job needs 3.
10% is what this calculator adds. The area you can measure from the ground is the building footprint, not the roof. A pitched roof is longer up the slope, and the multiplier is pure geometry: the square root of 1 plus (rise ÷ 12) squared. A 6/12 roof is 1.118 times its footprint; a 12/12 roof is 1.414 times.
Measure the footprint of the building, then multiply by the pitch factor for your roof. A 40 by 30 ft footprint is 1,200 sq ft; on a 6/12 pitch the roof surface is 1,342 sq ft, which is 13.4 squares before waste.
Pythagoras. A roof that rises R inches over 12 inches of run has a slope length of the square root of (12 squared plus R squared) for every 12 inches of run, so the factor is the square root of 1 plus (R ÷ 12) squared. A 6/12 roof gives 1.118. It is geometry, not a table anyone has to be trusted on.
Three, for standard three-tab and most architectural shingles. Heavier designer shingles can run four or five bundles to a square, so check the wrapper rather than assuming.