Key takeaways
- Roof area is larger than your home's footprint because of pitch and overhangs.
- Footprint × slope multiplier gives the roof area for any roof with one pitch.
- One roofing square is 100 square feet.
Enter the outside length and width of your house, its pitch and the overhang. The calculator adds the overhang on all four sides, then applies the slope multiplier for your pitch.
How the calculator works
- Footprint: (length + 2 × overhang) × (width + 2 × overhang)
- Slope multiplier: √(1 + (pitch ÷ 12)²)
- Roof area: footprint × multiplier
- Squares: roof area ÷ 100
This works for gable and hip roofs alike, as long as every plane has the same pitch, because each one is stretched by the same amount.
Worked example
A 40 × 30 foot house with a 6/12 roof and 12-inch overhangs:
- Footprint: 42 × 32 = 1,344 square feet
- Multiplier for 6/12: 1.118
- Roof area: 1,344 × 1.118 = about 1,503 square feet
- Squares: about 15.0, or about 16.5 with a 10% waste allowance
Check it by measuring
For a more exact figure, measure each roof plane and add them up, as GAF's guide suggests. Include dormers and porches, and measure sections with a different pitch separately.
Footprints can miss things too: a garage, a change in roof height, or a wider overhang on the gable ends. If your result is far from a contractor's number, ask how they measured it.
How pitch changes the area
For the same 1,000 square feet of level footprint:
| Pitch | Roof area |
|---|---|
| 4/12 | 1,054 sq ft |
| 6/12 | 1,118 sq ft |
| 8/12 | 1,202 sq ft |
| 10/12 | 1,302 sq ft |
| 12/12 | 1,414 sq ft |
Next, turn your area into shingle bundles, or learn more about roofing squares.
Worked example with two pitches
Work out each section separately, then add them. Using footprints that already include overhangs:
| Section | Footprint | Pitch | Roof area |
|---|---|---|---|
| Main house | 1,200 sq ft | 6/12 | 1,342 sq ft |
| Porch | 240 sq ft | 3/12 | 247 sq ft |
| Total | 1,589 sq ft |
Why a contractor's number may differ from yours
- Waste is included. Ask whether the number is measured area or order quantity.
- They measured planes directly. That captures dormers and steps your footprint misses.
- Garages and porches. Check that you're both measuring the same roof.
- Overhangs. Gable-end overhangs can differ from the eaves.
A difference of a few percent is normal. A large one is worth asking about.
Frequently asked questions
Is my roof area the same as my home's square footage?
No. Pitch, eaves and other features usually make the roof larger than the footprint. A two-story home's living area also has little to do with its roof size, so measure the outside footprint instead.
Why is the waste allowance 10%?
It's a planning figure. GAF suggests adding 10–15% for trim and waste, and more complex roofs can need more. You can change the allowance in the shingle calculator.
Can I measure from satellite images?
They can give a useful starting footprint. Confirm the numbers before you order materials, because images may be out of date or distorted.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- GAF: How many bundles of shingles are in a square? (measuring roof planes, waste allowance)