Roof Area Calculator
Calculate true roof area and squares from your building footprint and pitch. Applies the pitch multiplier so shingle and underlayment estimates are accurate.
How this calculator works
The true surface area of a pitched roof is its flat footprint multiplied by a factor that depends on the slope. The calculator computes that pitch multiplier from your rise, applies it to the footprint, and converts the result into roofing squares.
Formula: Pitch multiplier = √(rise² + 12²) ÷ 12. Roof area = footprint area × multiplier. Squares = roof area ÷ 100.
Worked example
A 40 ft × 30 ft footprint with a 6/12 pitch:
- Footprint: 40 × 30 = 1,200 sq ft
- Pitch multiplier: √(6² + 12²) ÷ 12 ≈ 1.118
- Roof area: 1,200 × 1.118 ≈ 1,342 sq ft
- Squares: 1,342 ÷ 100 ≈ 13.4 squares
Multiplier reference
| Pitch | Multiplier | 1,200 sq ft footprint |
|---|---|---|
| Flat (0/12) | 1.000 | 1,200 sq ft |
| 4/12 | 1.054 | 1,265 sq ft |
| 6/12 | 1.118 | 1,342 sq ft |
| 12/12 | 1.414 | 1,697 sq ft |
How to measure
Measure the building’s length and width at the eave line, including overhangs, to get the footprint. Find your pitch with the Roof Pitch Calculator, then feed the roof area here into the Roofing Shingle Calculator to get bundle counts.
Frequently asked questions
Why is roof area larger than my house footprint?
A sloped roof covers more surface than the flat outline beneath it. The steeper the pitch, the bigger the gap: a 6/12 roof is about 12 percent larger than its footprint, while a 12/12 roof is over 40 percent larger. This calculator applies that pitch multiplier automatically.
What is a roofing square?
One square equals 100 square feet of roof surface. Shingles, underlayment, and labor are all quoted in squares, so the calculator converts your area into squares as well as square feet.
Do I measure the footprint at the ground or the eaves?
Measure the roof's horizontal footprint including any eave overhangs, since the roof surface extends over them. Measure at the eave line, not the wall line, for an accurate area.
How do I handle a complex roof with multiple sections?
Break the roof into simple rectangular sections, calculate each section's footprint and pitch separately, and add the areas. Hips, valleys, and dormers also add waste — plan 10 to 15 percent extra material for cut-up roofs.
Does this replace measuring each roof plane?
For a straightforward gable roof, footprint times pitch multiplier is accurate. For irregular roofs, measuring each plane directly is more precise, but this gives a fast, reliable starting estimate for ordering material.
Estimates only. Verify quantities with your supplier before purchasing.