How to calculate roofing
Shingles are estimated from the sloped roof area, not the ground footprint:
Roof area = footprint × pitch factor, then squares = area ÷ 100, and bundles = squares × 3
The pitch factor converts your flat footprint into the actual sloped surface. Add a waste allowance and round up — running short partway up a roof is not an option.
Pitch factors
| Roof pitch | Multiply footprint by |
|---|---|
| Flat to 3:12 | ~1.03 |
| 4:12 | ~1.06 |
| 6:12 | ~1.12 |
| 8:12 | ~1.20 |
| 12:12 | ~1.41 |
Roofing materials compared
Asphalt shingles cover the vast majority of homes, but they aren't the only option:
| Material | Relative cost | Lifespan |
|---|---|---|
| 3-tab asphalt | $ (lowest) | 15–20 yrs |
| Architectural asphalt | $$ | 25–30 yrs |
| Metal (standing seam) | $$$ | 40–70 yrs |
| Cedar shake | $$$ | 25–30 yrs |
| Clay / concrete tile | $$$$ | 50+ yrs |
| Slate | $$$$$ | 75–100+ yrs |
Don't forget the accessories
A roof is more than its shingles. A full order also needs underlayment (felt or synthetic) over the whole deck, ice-and-water shield at the eaves and valleys in cold climates, drip edge along every edge, a starter strip at the eaves, ridge cap shingles along the peaks and hips, flashing around chimneys and walls, and enough roofing nails (about 320 per square). Budget those separately from the field bundles.
How much does a new roof cost?
Shingle bundles run about $30–$60 each, but the installed cost is far more once you add tear-off of the old roof, underlayment, flashing, and labor. For a full breakdown by roof size and material, see our guide on how much a new roof costs.
Good to know
- A "square" = 100 sq ft. Roofers measure and quote in squares.
- 3 bundles per square is standard for architectural shingles — check the wrapper.
- Complex roofs waste more. Hips, valleys, and dormers mean more cuts — lean toward 15% waste.
- This is a material estimate. For steep or complex roofs, hiring a pro is strongly recommended.
Measuring a complex roof
The footprint-and-pitch method this calculator uses is ideal for a simple gable roof. For a roof with hips, valleys, dormers, or several sections, measure each plane separately and add them up. Rectangular planes are length × width; triangular hip sections are ½ × base × height. Sum every plane to get the true roof area, divide by 100 for squares, and add waste. If you can't get on the roof safely, a satellite-measurement report — or a roofer's own measurement — gives you the plane areas without climbing. Either way, lean toward 15% waste on a cut-up roof, since every hip and valley creates angled cuts and offcuts.