How to calculate roof area
Start with the house footprint plus the overhang on every side. A 40 × 30 ft house with 12″ overhangs covers 42 × 32 = 1,344 sq ft from above. A sloped roof has more surface than its footprint, so multiply by the pitch factor for your roof's slope. At 6/12 that's 1.118, giving about 1,503 sq ft of roof. This works for any simple roof where every side has the same pitch, gable or hip.
| Pitch | Multiply by | Angle |
|---|---|---|
| 3/12 | 1.031 | 14° |
| 4/12 | 1.054 | 18° |
| 5/12 | 1.083 | 23° |
| 6/12 | 1.118 | 27° |
| 7/12 | 1.158 | 30° |
| 8/12 | 1.202 | 34° |
| 9/12 | 1.250 | 37° |
| 10/12 | 1.302 | 40° |
| 12/12 | 1.414 | 45° |
How do I find my roof pitch?
Pitch is how many inches the roof rises for every 12″ it runs horizontally. From the attic or at a gable end, hold a level horizontally against a rafter, mark 12″ along it, and measure straight up to the underside of the rafter. A 6″ rise means a 6/12 pitch. Most homes fall between 4/12 and 9/12.
What is a roofing square?
Roofers measure in squares: one square is 100 sq ft of roof. Most asphalt shingles, both 3-tab and architectural, come 3 bundles to a square. Some heavier designer shingles need 4 or 5, so check the wrapper, which lists the coverage.
How much extra for waste?
Plan on about 10% extra for a simple gable roof and 15% for a hip roof, which has more angled cuts along the hips. Roofs with dormers, valleys and multiple levels can need 15–20% or more. This calculator assumes a simple single-pitch roof, so measure each section separately for anything more complex.
What does a roof replacement cost?
For 2026, installed asphalt shingle roofs typically run about $425–825 per square for 3-tab and $450–1,250 per square for architectural shingles. The range is wide because labor rates, roof steepness, how many old layers come off, and decking repairs vary so much. A typical 2,000 sq ft home often lands somewhere between $9,000 and $24,000. Use these numbers to sanity-check quotes, not as one.
What this doesn't include
- Starter strip shingles along the eaves and ridge cap shingles along ridges and hips
- Ice and water shield, required at the eaves in cold climates
- Flashing for chimneys, walls and skylights; roof vents; nails
- Replacing damaged roof decking
Roofs below 4/12 need extra underlayment for shingles. Below 2/12, shingles aren't an option, and you'll need a low-slope roofing system.