Roofing calculator
Form 48 · Materials estimateFigures checked 2026-09-04
US customary
Measurements
What you need
Quantity · Amount- 01Footprint with overhang40 × 28 ft plus 12 in all round1,260 sq ft
- 02Roof area at this pitch× 1.118 pitch multiplier1,409 sq ft
- 03Squares to buyincluding 10 % waste; 100 sq ft per square15.5 squares
- 04Bundles of shinglesthree bundles per square at $3847 bundles$1,786
- 05Underlayment rollsabout 10 squares per synthetic roll2 rolls$120
- 06Ridge cap bundlesabout 20 linear ft per bundle2 bundles
Total, materials$1,906
- This is material only. Installed roofing is usually quoted per square, and tear-off, decking repairs, flashing and permits are extra.
- Steep roofs (above 8/12) and multiple valleys raise both the waste and the labour.
The material
Prices — put in what you were quoted
Where these figures come from
- Pitch multipliers are √(1 + (rise/12)²), the geometric ratio of slope to run.
- Three bundles per square and about 10 squares per synthetic underlayment roll: the common product figures.
- Prices are typical US retail figures at the date on this sheet.
Measure the building's footprint and pick the roof's pitch; the sheet applies the pitch multiplier to get the true roof area, converts it to squares (100 sq ft), and counts bundles at three per square, rolls of underlayment and the cost. Architectural shingles come three bundles to the square.
Worked examples
- 40 × 28 ft gable, 7/12 pitch
- 15.5 squares — squares to buy, about $1,906 in materials.
- Garage 24 × 24 ft, 4/12
- 7.67 squares — squares to buy, about $934 in materials.
How it is worked out
Footprint = (length + 2 × overhang) × (width + 2 × overhang). Roof area = footprint × the pitch multiplier. Squares = roof area × (1 + waste) ÷ 100. Bundles = squares × 3, rounded up. Underlayment rolls = squares ÷ 10.
The figures behind it
- Pitch multipliers are √(1 + (rise/12)²), the geometric ratio of slope to run.
- Three bundles per square and about 10 squares per synthetic underlayment roll: the common product figures.
- Prices are typical US retail figures at the date on this sheet.
Every figure here is an estimate. Materials vary with moisture, compaction and product; order a little extra and confirm quantities with your supplier.
Questions people ask
- How many bundles of shingles per square?
- Three for standard architectural or three-tab shingles. A square is 100 square feet of finished roof.
- How do I get the roof area from the footprint?
- Multiply the footprint by the pitch multiplier: 1.031 for 4/12, 1.083 for 6/12, 1.118 for 7/12, 1.202 for 9/12, 1.414 for 12/12. It is the ratio of the sloped length to the level run.
- How much does a roof cost?
- This sheet prices materials only. Installed asphalt roofing in the US typically runs several hundred dollars per square including tear-off, and varies widely by region and roof complexity; get quotes.
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