How to use this calculator
- 1Measure the building's footprint including the eaves overhang, or take it from the plans.
- 2Enter the pitch in degrees or as rise per 12.
- 3Choose gable or hip; the extra fills in (hip roofs have more cutting).
- 4Choose your covering: for shingles keep 3 bundles per square unless the wrapper says otherwise; for tiles and slates check the number per m² against the maker's data for your pitch.
The formula
- 01Pitch factor = 1 ÷ cos(pitch angle) = √(1 + (rise ÷ run)²)
- 02Roof area = footprint × pitch factor
- 03Squares = roof area × (1 + extra %) ÷ 100 ft²
- 04Bundles = squares × bundles per square, rounded up
- 05Tiles = roof area × (1 + extra %) × tiles per m², rounded up
A sloping roof is bigger than its footprint. The pitch factor says how much bigger: a 6/12 roof (26.6°) is 1.118 times its footprint, a 45° roof 1.414 times.
For a simple gable or hip roof with one pitch everywhere, the footprint method is exact. Dormers, valleys and different pitches need each part measured separately.
Worked example
A 40 ft × 30 ft footprint at 6/12 has a pitch factor of 1.118, so the roof is 1,341.6 ft².
With 10% extra that's 14.76 squares — 45 bundles of shingles.
Tips before you buy
- Add starter strips, ridge caps, underlayment and flashing — they are separate items.
- Very low pitches may need a different roof covering; check the product's minimum pitch.
- Ask two roofers for quotes and compare their measured areas with this one.
Frequently asked questions
What is a roofing square?
100 square feet of roof. Shingles are priced and packed by the square, usually 3 bundles per square.
How do I convert pitch to degrees?
Angle = arctan(rise ÷ run). 4/12 ≈ 18.4°, 6/12 ≈ 26.6°, 8/12 ≈ 33.7°, 12/12 = 45°.
How much waste for shingles?
About 10 % for simple gable roofs and 15 % for hip roofs or roofs with valleys and dormers.
How many roof tiles per m²?
About 10–11 for large interlocking concrete tiles (Marley Modern: 9.9 at 75 mm headlap, 10.7 at 100 mm), 60 for plain tiles at 100 mm gauge, and 18.5–19.6 for 500 × 250 mm slates. The maker's figure for your pitch comes first.
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Sources
- Shingle packaging: most asphalt shingles are 3 bundles per 100 ft² square (manufacturers such as IKO)
- Marley – Modern concrete roof tile: 9.9 tiles/m² at 75 mm headlap, 10.7 at 100 mm
- Marley – Acme plain tile 265 × 165 mm: 60 tiles/m² at 100 mm gauge
- Welsh Slate – slate coverage: 1,000,000 ÷ ((length − headlap) ÷ 2 × (width + 5 mm))
Formulas last reviewed: September 30, 2026 ·How we check our formulas