Roofers price jobs by the square, which is 100 sq ft of roof surface. You don’t need a ladder to get a good estimate. You need three things: your home’s footprint, the eave overhang, and the roof pitch.
1. Measure the footprint, then add the overhang
Measure the outside walls of the house (or use your property’s floor plan). Then add the overhang on each side. A typical eave overhangs 12 in, so a 40 × 30 ft house with 1 ft eaves has a roof “plan area” of 42 × 32 ft = 1,344 sq ft.
Include attached garages and covered porches that share the roof. Leave out separate structures; price them as separate jobs.
2. Find the pitch
Pitch is the rise in inches per 12 in of horizontal run. You can measure it from inside the attic with a level and a tape (mark 12 in along the level, then measure straight up to the underside of the rafter), or use a free smartphone inclinometer app on a gable end photo. Most US homes are between 4/12 and 9/12.
3. Multiply by the pitch factor
A sloped roof has more surface than its flat footprint. The multiplier is √(1 + (rise ÷ 12)²):
| Pitch | Pitch factor | 1,344 sq ft plan area becomes |
|---|---|---|
| 3/12 | 1.031 | 1,386 sq ft |
| 4/12 | 1.054 | 1,417 sq ft |
| 6/12 | 1.118 | 1,503 sq ft |
| 8/12 | 1.202 | 1,615 sq ft |
| 10/12 | 1.302 | 1,750 sq ft |
| 12/12 | 1.414 | 1,901 sq ft |
4. Add waste and convert to squares
Shingles are cut at hips, valleys, rakes and starter courses, so roofers add a waste allowance, commonly 10–15%. Simple gable roofs sit at the low end; hip roofs and roofs with dormers or many valleys go higher.
Example: 1,503 sq ft (6/12 pitch) × 1.10 = 1,653 sq ft = 16.5 squares. Asphalt shingles come 3 bundles per square, so that’s about 50 bundles.
Why your contractor’s number may differ
Contractors measure from aerial imagery or on the roof itself and count every hip, valley and ridge. Starter strip, ridge cap, drip edge, flashing and ice-and-water shield are priced separately from field shingles. If your estimate and a quote differ by more than about 10% in squares, ask how they measured.
Use the roof replacement cost calculator to turn squares into an installed cost range for asphalt, metal or tile.
Run the numbers for your project. The roof replacement estimator uses the method in this guide.
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