How the roofing calculator works
It takes the footprint of the roof, including overhangs, and multiplies by a pitch factor to account for the slope. Roofers price and order by the square, which is 100 square feet of roof.
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × √(1 + (pitch ÷ 12)²)
Squares = roof area ÷ 100 · Bundles = squares × 3
Pitch factors
| Pitch | Multiply footprint by |
|---|---|
| 3/12 | 1.031 |
| 4/12 | 1.054 |
| 6/12 | 1.118 |
| 8/12 | 1.202 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
Example: a 40 × 30 ft house with a 6/12 roof
With a 1 ft overhang the footprint is 42 × 32 = 1,344 square feet. Times the 6/12 factor of 1.118 that's 1,503 square feet of roof. Adding 10% gives 16.53 squares, which is 50 bundles of shingles and 2 rolls of synthetic underlayment. At $500 per square installed, expect around $8,265.
What this doesn't include
Ridge cap, starter strip, drip edge, flashing, tear-off and disposal are priced separately by most roofers. Use this figure to sanity-check quotes, not to replace them.
Frequently asked questions
How many bundles of shingles are in a square?
Three bundles cover one square (100 square feet) for most architectural and 3-tab asphalt shingles. Check the wrapper, as some heavy shingles need four.
How do I find my roof pitch?
Hold a level horizontally against the roof or a rafter, measure 12 inches along it, then measure straight up to the roof. That rise in inches is the pitch, for example 6/12.
How much does a new roof cost per square?
Installed asphalt shingle roofs commonly run several hundred dollars per square in the US, and metal or tile cost more. Enter a figure from local quotes for the best estimate.
Last reviewed: 2026-10-31