Roofing Calculator
Convert your house footprint and roof pitch into roof area in squares and the exact number of shingle bundles.
Roofing Calculator — input your values
What does this construction calculator do?
The roofing calculator estimates the sloped surface area of a gable roof from the house footprint and the roof pitch, in square feet and roofing squares.
It then converts the area into the number of 3-bundle shingle squares you need to order, with a waste allowance for ridge caps, cuts and starter courses.
Formula and variables
Hip roofs, gambrels and complex valleys don't fit a simple footprint formula — measure each plane separately.
| Symbol | Meaning |
|---|---|
| 1 square | 100 sq ft of roof |
| 1 bundle | 1/3 square (about 33 sq ft) |
| 4/12 pitch | factor ≈ 1.054 |
| 6/12 pitch | factor ≈ 1.118 |
| 12/12 pitch | factor ≈ 1.414 |
Worked example
A 30 ft × 20 ft house with a 6/12 pitch, ordering shingles with 10% waste.
- Footprint = 30 × 20 = 600 sq ft.
- Pitch factor for 6/12 = 1 ÷ cos(atan(0.5)) = 1.118.
- Roof area = 600 × 1.118 = 670.8 sq ft.
- Squares = 670.8 ÷ 100 = 6.71 squares.
- Bundles = 6.71 × 3 = 20.1, plus 10% waste = 22.1 → 23 bundles.
Answer: Order ≈ 23 bundles of shingles (7.7 squares).
Tips and common mistakes
Tips
- Add 15% waste on complex roofs with valleys, hips and multiple planes instead of the standard 10%.
- Shingles are sold in bundles of 1/3 square — three bundles cover one square, so always buy in multiples of three.
- Buy all bundles from the same production run so color variations are hidden.
- A 4/12 pitch is about the minimum for standard asphalt shingles; below that, use a specialty low-slope product.
Common mistakes to avoid
- Ordering based on footprint area alone — a steep roof has far more surface than the house plan suggests.
- Ignoring overhangs and eaves, which can add 5–10% to the real surface area.
- Forgetting the underlayment: the felt or synthetic membrane must be ordered at the same coverage as the shingles.
Roofing Calculator — frequently asked questions
How many bundles of shingles are in a square?
Three bundles of standard three-tab or architectural shingles cover one roofing square (100 sq ft), so a 2,000 sq ft roof needs about 60 bundles plus waste.
How do I calculate roof area from the footprint?
Multiply the footprint length by width, then multiply by the pitch factor: 1 ÷ cos(atan(pitch/12)). For a 6/12 roof, multiply the footprint area by about 1.12.
What is a roofing square?
A square is a unit used in roofing that equals 100 sq ft of surface, regardless of the roof shape or pitch.
What pitch do I need for asphalt shingles?
Asphalt shingles need at least a 2/12 pitch with special low-slope installation, and 4/12 is the common minimum for standard application.
How much waste should a roofer add?
Most roofers add 10–15% for starter strips, ridge caps, cuts at hips and valleys, and mistakes — more on complex roofs.
Does the roof area calculation include overhangs?
No — the footprint formula covers the wall footprint only. Add the eaves overhang width on each side when measuring for a real roof.