How this calculator works
The calculator starts with the roof footprint. That is the flat length and width the roof covers, including overhangs if you know them.
It multiplies length by width to get flat footprint area. Then it adjusts that area for roof pitch. Home Depot explains pitch as vertical rise over horizontal run, often measured as inches of rise for every 12 inches of run.
The calculator uses that pitch to create a multiplier. A flat roof uses a 1.000 multiplier. A steeper roof uses a larger multiplier because the sloped surface is longer than the flat footprint.
The result shows roof area, order area with waste, and roofing squares.
Roof area formula
Start with the flat footprint:
Footprint area = roof length x roof widthThen calculate the pitch multiplier:
Pitch multiplier = sqrt(rise^2 + 12^2) / 12The roof area is:
Roof area = footprint area x pitch multiplierTo convert square feet to roofing squares:
Roofing squares = roof square feet / 100For a 40 ft by 30 ft roof footprint with a 6/12 pitch:
40 x 30 = 1,200 sq ft footprint
sqrt(6^2 + 12^2) / 12 = 1.118 pitch multiplier
1,200 x 1.118 = 1,342 sq ft roof areaIf you add 10% for overage and waste:
1,342 x 1.10 = 1,476 sq ft
1,476 / 100 = 14.76 roofing squaresCommon roof pitch multipliers
Use the pitch rise as the number before /12. For example, a 6/12 roof
rises 6 inches for every 12 inches of horizontal run.
| Roof pitch | Pitch multiplier | 1,000 sq ft footprint becomes |
|---|---|---|
| 0/12 | 1.000 | 1,000 sq ft |
| 3/12 | 1.031 | 1,031 sq ft |
| 4/12 | 1.054 | 1,054 sq ft |
| 6/12 | 1.118 | 1,118 sq ft |
| 8/12 | 1.202 | 1,202 sq ft |
| 10/12 | 1.302 | 1,302 sq ft |
| 12/12 | 1.414 | 1,414 sq ft |
These values are geometry-based estimates. They assume the same pitch across the roof area you entered.
What to measure before ordering
Home Depot says to measure the width and length of each roof section, multiply them together, add the section areas, then divide by 100 to get roofing squares.
GAF notes that one roofing square is 10 ft by 10 ft, or 100 sq ft. Roofing quotes, shingles, underlayment, and labor are often discussed in squares.
If you only know the footprint from the ground, use the footprint method in the calculator. If you can safely measure each roof plane, that direct plane method is usually better for complex roofs.
Home Depot suggests adding about 10% to 20% for overage and waste. Use the higher end for roofs with hips, valleys, dormers, or many cut edges.
You can also use the Concrete Block Calculator for wall material planning, the Drywall Calculator for interior panels, or the Fence Post Depth Calculator for outdoor posts.
Limitations
This calculator gives a planning estimate from a simple footprint and one pitch. It does not know the exact shape of your roof.
Dormers, valleys, hips, crickets, skylights, chimneys, rake edges, curved sections, multiple pitches, and missing overhang measurements can change the real roof area. For final ordering, use a professional roof measurement, manufacturer guidance, or contractor takeoff.
Roof work is hazardous. Do not climb onto a roof unless you have the right equipment, experience, and fall protection.