Roofing Calculator
You cannot measure a pitched roof from the ground, but you can calculate it: footprint area multiplied by a pitch factor gives the true roof surface. This calculator then converts area into shingle bundles and underlayment rolls.
Enter the house length and width (the footprint, including overhangs), choose the pitch, and add waste for hips, valleys and starter courses.
Enter valid values to see the results.
Formula
Pitch factors: low slope ×1.03, medium (4–6:12) ×1.12, steep (7–9:12) ×1.25, very steep (10–12:12) ×1.41. One roofing square = 100 ft² = 9.29 m² and takes 3 standard shingle bundles.
How to measure
- Measure the house footprint wall-to-wall and add overhangs (typically 30–60 cm / 1–2 ft on each side).
- Estimate pitch by measuring rafter rise over run in the attic, or use a pitch gauge app against the gable.
- For complex roofs (hips, dormers), calculate each plane separately and add them.
Worked example
Practical tips
- Order 10–15% extra for roofs with hips and valleys — every intersection doubles the cuts.
- Keep one bundle for future repairs; shingle lines are discontinued and colours fade.
- Check bundle coverage: most cover 3 m², but premium laminated shingles can cover only 2 m².
- Roofing over one existing layer is allowed in many regions, but two layers usually require a tear-off.
Waste allowance
Use 10% for simple gable roofs and 15% for hip roofs or roofs with valleys, dormers and skylights.
Pitch factors for common roof slopes
| Pitch | Angle | Factor |
|---|---|---|
| 3:12 | 14° | 1.03 |
| 4:12 | 18° | 1.05 |
| 6:12 | 27° | 1.12 |
| 9:12 | 37° | 1.25 |
| 12:12 | 45° | 1.41 |
Common mistakes
- 1 Using footprint area as roof area — a 6:12 pitch adds 12%, a 12:12 pitch adds 41%.
- 2 Forgetting overhangs, which on a 10 × 8 m house add 6–12 m² of roof.
- 3 Assuming all bundles cover 3 m² — check the label, premium shingles often cover 2 m².
- 4 Not ordering starter strips and ridge caps: they are separate products, about 10–15% of field shingle cost.
Frequently asked questions
How do I calculate roof area from the ground?
Measure the house footprint (length × width including overhangs) and multiply by the pitch factor: 1.05 for a 4:12 pitch, 1.12 for 6:12, 1.25 for 9:12, 1.41 for 12:12.
How many shingle bundles per square?
Three bundles per roofing square (100 ft² ≈ 9.3 m²) for standard shingles — one bundle covers about 3 m².
How many bundles for a 100 m² roof?
100 ÷ 3 = 34 bundles for field shingles, plus waste: order 37–39 for a gable roof, more for hips and valleys.
What is a roofing square?
A US unit equal to 100 ft² (9.29 m²) of roof surface. Contractors quote prices per square; one square takes 3 bundles of standard shingles.
Does this include underlayment and nails?
Underlayment rolls are included (75 m² or 400 ft² per roll). Nails: about 1 kg per 10 m² (320 nails per square) — buy 4 kg per 100 m².