From footprint to roof area
A sloped roof has more surface than the floor under it, and the steeper the pitch, the bigger the gap. The calculator takes your home's length and width, adds the overhang on every side, and multiplies by a pitch factor to get the true roof area.
Roofers measure in squares. One square is 100 square feet of roof. Most architectural and three-tab shingles come three bundles to a square, so bundles are squares times three.
| Pitch | Multiply by |
|---|---|
| 4/12 | 1.054 |
| 6/12 | 1.118 |
| 8/12 | 1.202 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
Worked example: a 40 × 30 house with a 6/12 pitch
- With a 1 ft overhang: 42 × 32 = 1,344 sq ft
- × 1.118 for the pitch = 1,503 sq ft of roof
- That's 15 squares, or 16.5 with 10% waste
- Shingles: 50 bundles
How to find your roof pitch
From the attic, hold a level against the underside of a rafter. Mark 12 inches along the level, then measure straight up from that mark to the rafter. If it's 6 inches, you have a 6/12 pitch. A pitch app on your phone works from the ground too, or check the house plans if you have them.
Please don't walk a steep roof just to measure it. Anything above about 6/12 is hard to stand on without fall protection.
What this estimate leaves out
Dormers, valleys and multiple roof planes add area and waste that a single footprint can't capture, so pick the higher waste setting for those roofs. You'll also need starter strip, ridge cap shingles, drip edge and flashing, which are sold by the linear foot.
Frequently asked questions
What is a roofing square?
A square is 100 square feet of roof surface. A roof that measures 2,000 square feet is 20 squares.
How many bundles of shingles are in a square?
Three for most architectural and three-tab shingles. Some heavier designer shingles take four or five bundles, so check the wrapper.
Does the roofing calculator include the overhang?
Yes. Enter your eave overhang in inches and it's added to all four sides of the footprint before the pitch factor is applied.
How much waste should I add for roofing?
About 10 percent for a simple gable roof and 15 percent for a hip roof. Roofs with many valleys and dormers can need 20 percent.
Results are estimates for planning. Product coverage, site conditions and installation methods vary, so check manufacturer guidance before you buy.