Roofing calculator
Work out your roof's surface area from its footprint and pitch, then how many squares and bundles of shingles to buy.
- Roof surface area
- Pitch factor
How to use it
- Measure the house's footprint from the outside edge of the overhangs, the way the roof would look from directly above.
- Choose the roof's pitch: how many inches it rises for every 12 inches across.
- Pick a waste allowance: 10% for a simple gable roof, 15% for roofs with hips, valleys or dormers.
- Check the bundles per square on your shingle wrapper. Most asphalt shingles are 3.
You don't need to climb on the roof: the footprint and pitch are enough for any roof where all the slopes share the same pitch.
How it's calculated
- Pitch factor = √(1 + (rise ÷ 12)²).
- Roof area = footprint area × pitch factor.
- Squares = roof area × (1 + waste) ÷ 100. A roofing square is 100 sq ft.
- Bundles = squares × bundles per square, rounded up.
| Pitch | Pitch factor | A 1,200 sq ft footprint becomes |
|---|---|---|
| 4/12 | 1.054 | 1,265 sq ft |
| 6/12 | 1.118 | 1,342 sq ft |
| 8/12 | 1.202 | 1,442 sq ft |
| 10/12 | 1.302 | 1,562 sq ft |
| 12/12 | 1.414 | 1,697 sq ft |
Is your roof steep enough for shingles?
Asphalt shingles need a pitch of at least 2/12. Between 2/12 and 4/12, building codes and shingle makers require extra underlayment to stop water backing up under the shingles. Flatter roofs need a different roofing system, such as membrane or rolled roofing, so the calculator starts at 2/12.
Finding your roof pitch
From the attic or a ladder at the gable end, hold a level horizontally against the underside of the roof, measure 12 inches along the level, and measure straight up from that point to the roof. That height in inches is the rise: 6 inches means a 6/12 pitch.
What else to buy
This calculator covers shingles only. A full re-roof also needs underlayment, starter strips, ridge cap shingles, drip edge, flashing and nails. Ridge caps and starter strips are usually sold separately and measured in linear feet of ridge and eaves.
Safety
Roofing is dangerous work. Measure from the ground or the attic where you can, and use proper fall protection on the roof. Many areas also require a permit for a re-roof.