How the roofing calculator works
Plans and tape measures give the flat footprint of a roof, but shingles cover the sloped surface, which is always larger. The calculator multiplies the footprint, measured to the outside of the eaves and gable overhangs, by a pitch factor, adds a cutting allowance, then converts the result to roofing squares.
Roof area = footprint × √(1 + (rise ÷ 12)²) · Squares = roof area × (1 + waste) ÷ 100 ft²A square is 100 ft² (9.29 m²) of roof, and standard asphalt shingles come three bundles to the square. Because every plane of a regular hip roof has the same pitch, the same factor works for hip and gable roofs; the difference lies in the waste, since hips and valleys need many more angled cuts. Underlayment is counted on the real area plus 10% for side and end laps.
Worked example
A house measuring 10 m × 8 m including overhangs has an 80 m² footprint. At 6/12 (26.6°) the pitch factor is 1.118, so the real roof is 89.4 m², or 962.8 ft². A simple gable roof adds 10%: 1,059 ft², which is 10.59 squares and 31.8 bundles – buy 32. Underlayment with laps covers 98.4 m², so two 75 m² rolls. In feet, a 32 × 26 ft footprint is 832 ft²; at the same pitch it becomes 930 ft² of roof, 1,023 ft² with waste, 10.23 squares and 31 bundles.
Pitch factors
| Pitch | Angle | Multiply footprint by |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Asphalt shingles are normally installed from 4/12 upward; between 2/12 and 4/12 manufacturers and building codes require extra underlayment, and flatter roofs need membrane roofing instead.
What else to order
- Starter strip along every eave and rake, measured in linear feet or metres.
- Ridge and hip caps, also measured by length; one bundle typically covers somewhere between 20 and 35 linear ft depending on the product, so read the wrapper.
- Drip edge, sold in 10 ft (about 3 m) lengths, for eaves and rakes.
- Ice and water membrane at the eaves in cold climates and down every valley.
- Nails: four per shingle in normal conditions, six in high-wind areas or on steep slopes – roughly 250–330 nails per square.
Some heavy architectural shingles come four or five bundles to the square; check the coverage printed on the bundle before converting squares.
Common mistakes
- Measuring the walls, not the roof edge. 50 cm overhangs on a 10 × 8 m house take the footprint from 80 m² to 99 m².
- Forgetting porches and dormers. Add each separate roof plane, with its own pitch, to the total.
- Roofing over old shingles. Local codes often limit a roof to two layers, and a tear-off shows rotten decking that would otherwise be hidden.
- Underestimating the danger. Roof work causes serious falls. Use proper fall protection or hire a licensed roofer, especially above 6/12.
Frequently asked questions
How many bundles of shingles do I need for a 1,000 sq ft roof?
1,000 ft² is 10 squares, or 30 bundles of three-bundle shingles. Adding 10–15% waste brings it to 33–35 bundles.
How do I calculate roof area from the house footprint?
Multiply the footprint including overhangs by the pitch factor. At 6/12 the factor is 1.118, so a 1,500 ft² footprint has about 1,677 ft² of roof.
What is a square in roofing?
A roofing square is 100 ft², or 9.29 m², of roof surface. Standard asphalt shingles need three bundles per square.
How much waste should I add for shingles?
About 10% for a simple gable roof, 15% for hip roofs or roofs with valleys, and up to 20% for cut-up roofs with dormers.
How many rolls of underlayment do I need?
Synthetic underlayment rolls usually cover 10 squares (1,000 ft²), #15 felt about 4 squares and #30 felt about 2. In Europe breathable membranes often come in 1.5 × 50 m rolls (75 m²). Add 10% for laps.
Last reviewed: 2026-09-27 · How we calculate