Roof Sqft Calculator
Measure your building footprint from the ground, enter the roof pitch, and get the actual sloped roof area plus the number of roofing squares to order.
To calculate roof square footage, multiply the building footprint by the pitch multiplier. A 40 ft × 30 ft footprint is 1,200 sq ft; on a 6/12 pitch the multiplier is 1.118, so the roof is 1,200 × 1.118 = 1,342 square feet, or 13.4 roofing squares.
One roofing square = 100 sq ft. Figures assume a simple gable roof; complex roofs with hips, valleys and dormers need more material.
Roof pitch and why it matters
A roof is always larger than the building underneath it, because it slopes. Pitch is written as rise over run — a 6/12 pitch climbs 6 inches for every 12 inches horizontally.
The multiplier is found with Pythagoras: √(rise² + 12²) ÷ 12. For a 6/12 pitch that is √(36 + 144) ÷ 12 = 13.416 ÷ 12 = 1.118.
So a steeper roof needs materially more shingles for the same house footprint — a 12/12 roof needs 41% more than a flat one.
Measuring without getting on the roof
You do not need to climb up. Measure the building's footprint at ground level, including the eaves overhang, then apply the pitch multiplier.
To find the pitch from inside the attic: hold a level horizontally against a rafter, mark 12 inches along it, then measure straight up from that mark to the rafter. That vertical distance in inches is the rise — if it is 6 inches, you have a 6/12 pitch.
Roofing squares and waste
Roofers order in squares, where one square covers 100 square feet. A 1,342 sq ft roof is 13.42 squares.
Always round up and add waste. Ten percent is standard for a simple gable roof; 15% or more is sensible once you have hips, valleys, dormers or skylights, because every one of those means cut shingles. Starter strips, ridge caps and valley flashing are ordered separately.
Roof pitch multipliers
Multiply your building footprint by this figure to get the actual roof area.
| Pitch | Angle | Multiplier | Roof area on a 1,500 ft² footprint |
|---|---|---|---|
| 3/12 | 14.0° | 1.031 | 1,547 sq ft |
| 4/12 | 18.4° | 1.054 | 1,581 sq ft |
| 5/12 | 22.6° | 1.083 | 1,625 sq ft |
| 6/12 | 26.6° | 1.118 | 1,677 sq ft |
| 7/12 | 30.3° | 1.158 | 1,737 sq ft |
| 8/12 | 33.7° | 1.202 | 1,803 sq ft |
| 10/12 | 39.8° | 1.302 | 1,953 sq ft |
| 12/12 | 45.0° | 1.414 | 2,121 sq ft |
Frequently asked questions
How do I calculate roof square footage?
Measure the building footprint including eaves, then multiply by the pitch multiplier for your roof. A 1,200 sq ft footprint on a 6/12 pitch gives 1,200 × 1.118 = 1,342 square feet of roof.
What is a roofing square?
A roofing square is 100 square feet of roof area. It is the standard unit for ordering shingles — a 2,000 square foot roof is 20 squares.
How do I find my roof pitch?
In the attic, hold a level against a rafter, measure 12 inches along it, then measure vertically from that point up to the rafter. That vertical measurement in inches is the rise, giving you a rise-over-12 pitch.
How much waste should I add for shingles?
Ten percent for a simple gable roof. Increase to 15% or more for roofs with hips, valleys, dormers or skylights, since each of those creates cut waste.
Does roof area include the overhang?
Yes. Measure the footprint out to the edge of the eaves rather than to the wall line, because the overhang is roofed too.