Roofing calculator
Roof area, squares and bundles from the footprint and pitch
Type the length and width of the roof seen from above, including the overhang, and its pitch. The example is 40 x 30 ft at a 6 in 12 pitch.
–
- Roofing squares (100 sq ft)
- –
- Bundles or tiles
- –
- Area seen from above
- –
- Slope factor
- –
An estimate of quantity from the sizes you typed, not an order list or a price. Working on a roof is dangerous. Check structural work, and anything covered by a building code, with your local code or a qualified person.
How the roof area is worked out
A sloping roof has more surface than the ground it covers. The slope factor is the length along the slope divided by the level distance under it: √(rise² + run²) / run. For a 6 in 12 pitch that is √(36 + 144) / 12 = 1.118. Roof area = area seen from above x slope factor. This holds for gable and hip roofs alike, as long as every face has the same pitch.
Squares and bundles
In North America roofing is sold by the square: 100 sq ft of roof. Most asphalt shingles come three bundles to the square, but heavier ones may need four or five, so check the wrapper and change the box. The waste allowance covers cutting at hips, valleys, rakes and starter rows.
Worked example
40 x 30 ft from above is 1,200 sq ft. At 6 in 12: 1,200 x 1.118 = 1,341.6 sq ft of roof, or 13.42 squares. Add 10%: 14.76 squares. At 3 bundles per square, 44.3, so 45 bundles.
Metric and tiles
In metric the pitch is usually in degrees. Roof tiles are counted per square metre, a figure the tile maker publishes for each tile and batten gauge. Type it in.
What it leaves out
Ridge caps, starter strips, underlay, flashing and fixings are not counted. Different pitches on different faces need each face worked out separately. It says nothing about whether a roof can carry a covering, or which pitch a product allows; check the maker's guide and the local code.
Defaults and where they come from
Every default is only a starting point. Change any box to match what you are buying or building.
| Default | Value | Source |
|---|---|---|
| Bundles per square | 3 | Most standard asphalt shingles (3-tab and architectural) are packed three bundles to a 100 sq ft square, the convention the Asphalt Roofing Manufacturers Association describes. Heavier products differ: check the wrapper. |
| Tiles per m² | 10 (example) | Varies by tile and batten gauge: use the tile maker's figure. |
| Waste allowance | 10% (usual 10 to 15%, more for complex roofs) | Common practice in roofing estimates. No standard sets a figure. |