How to Measure a Roof in Squares: Pitch Factor Math
How to measure a roof in squares is straightforward with a pitch factor. One roofing square equals 100 square feet of roof surface. To estimate, measure the house footprint including overhangs, multiply by the pitch factor for your roof slope, and divide by 100. Then add about 10 percent waste for simple gable roofs and 15 percent or more for roofs with hips, valleys, and dormers. This method avoids climbing on the roof.
Asphalt tonnage calculator →Pitch factor table
| Pitch (rise per 12 in) | Pitch factor |
|---|---|
| 3/12 | 1.031 |
| 4/12 | 1.054 |
| 5/12 | 1.083 |
| 6/12 | 1.118 |
| 7/12 | 1.158 |
| 8/12 | 1.202 |
| 9/12 | 1.250 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
The factor converts flat footprint area into sloped roof area.
Step by step
- Measure the footprint of the roof from the ground, including overhangs. For a 40x30 house with 1-foot overhangs, use 42x32 = 1,344 square feet.
- Find the pitch, using a level and tape: measure rise over 12 inches of run on a rafter or the roof edge, or use a phone app.
- Multiply footprint by pitch factor: 1,344 x 1.118 (for 6/12) = 1,503 square feet.
- Divide by 100: about 15 squares.
- Add waste: 15 x 1.10 = 16.5, so order about 17 squares.
Complex roofs
Break the roof into sections with different pitches, such as main roof, porch, and dormers. Calculate each section separately and add them. Hips and valleys increase waste because of cutting.
Bundles per square
Most shingles come three bundles per square, so 17 squares is 51 bundles. Some heavy architectural lines use four. See how many bundles of shingles in a square.
Other materials
| Item | Measure |
|---|---|
| Underlayment | Squares, same as shingles |
| Ice and water shield | Eave length and valleys |
| Drip edge | Eave plus rake length |
| Starter strip | Eave length, plus rakes if used |
| Ridge cap | Ridge plus hip length |
See ridge cap shingles.
Low-slope sections
Shingles generally need a slope of at least 2:12. Flatter sections need other roofing. See minimum roof pitch for asphalt shingles.
Rainwater
Roof footprint area also drives runoff estimates for gutters and rain barrels. See rainfall roof runoff calculator.
Checking with satellite measurements
Many roofers now use aerial measurement reports, which calculate area, pitch, and edge lengths from imagery. You can compare your own estimate against a contractor’s report or quote. If the numbers differ by more than about 10 percent, ask how the contractor measured and whether they included porches, garages, or dormers. A rough estimate of your own helps you check bids and catch an order that is too small or padded.
Bottom line
Measure the footprint including overhangs, multiply by the pitch factor, divide by 100 for squares, and add 10 to 15 percent for waste. Most shingles come three bundles per square. Break complex roofs into sections for accuracy.
Questions people ask
What is a roofing square?
100 square feet of roof surface.
How do I find roof pitch?
Measure rise over 12 inches of run, or use a pitch app.
How much waste should I add for shingles?
About 10 percent for simple roofs and 15 percent or more for complex ones.
How many bundles are in a square?
Usually three.
Related guides
- How Many Bundles of Shingles Are in a Square?
- Minimum Roof Pitch for Asphalt Shingles
- Ridge Cap Shingles: Types, Coverage, Installation
- Rainfall Roof Runoff Calculator: Gallons From Your Roof
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