Roofing Calculator — Squares and Bundles
Enter your footprint and roof pitch. The pitch multiplier is applied automatically — skipping it is the classic way to under-order shingles.
About the Roofing Calculator
A roof is always bigger than the house it sits on, and the steeper it is the bigger the gap. Measuring the building footprint and ordering to that number is the mistake that leaves a job short by a fifth. The pitch multiplier corrects for the slope, and this calculator applies it for you.
How to use it
- Enter the footprint — the area the roof covers when viewed from above.
- Enter the pitch as rise per 12 inches of run. A 6/12 roof rises 6 inches per foot.
- Leave waste at 10%, or use 15% for a roof with lots of hips and valleys.
- Order by bundles; three bundles make one square.
The formula
Pitch multiplier = √(rise² + 12²) ÷ 12
Roof area = footprint × multiplier
Squares = roof area × (1 + waste) ÷ 100
The multiplier is just Pythagoras on the roof triangle. A 6/12 pitch gives √(36+144)/12 = 1.118, so the real roof is 11.8% larger than the footprint. A 12/12 pitch is 41.4% larger.
Worked examples
| Footprint | Pitch | Multiplier | Roof area | Squares | Bundles |
|---|---|---|---|---|---|
| 1,600 sq ft | 4/12 | 1.054 | 1,687 | 18.55 | 56 |
| 1,600 sq ft | 6/12 | 1.118 | 1,789 | 19.68 | 60 |
| 1,600 sq ft | 9/12 | 1.250 | 2,000 | 22.00 | 66 |
| 1,600 sq ft | 12/12 | 1.414 | 2,263 | 24.89 | 75 |
Same house, four different roofs. The 12/12 needs 19 more bundles than the 4/12 — about a third more material for an identical footprint. This is why pitch is not an optional field.
Common mistakes
- Ordering to the footprint. The most expensive error in roofing. On a 9/12 roof that leaves you 25% short.
- Guessing the pitch. Measure it: hold a level out 12 inches from the roof surface and measure down to the roof. That drop is your rise.
- Forgetting starter and ridge cap. Starter strip along the eaves and cap shingles along the ridge are separate products with separate quantities.
- Using 10% waste on a complex roof. Every valley and hip generates cut waste. Fifteen percent is realistic once you have more than a simple gable.
Terms explained
- Square
- 100 square feet of roof. The standard unit — roofing is quoted in squares, not square feet.
- Pitch
- Vertical rise per 12 inches of horizontal run. Written as 6/12, 8/12 and so on.
- Bundle
- A package of shingles. Three bundles cover one square for standard architectural shingles.
- Underlayment
- The membrane between deck and shingles. Sold in rolls covering 2 to 4 squares each.
Common questions
- How many bundles of shingles do I need?
- Three bundles per square. A 20-square roof needs 60 bundles, plus starter strip and ridge cap.
- What is a roofing square?
- One hundred square feet of roof surface. All roofing quantities and quotes are based on it.
- How do I calculate roof pitch?
- Hold a level horizontally against the roof, measure 12 inches along it, then measure straight down to the roof surface. That drop in inches is the rise.
- Do I need to account for pitch?
- Yes, always. A 6/12 roof is 11.8% larger than its footprint, and a 12/12 is 41.4% larger. Ignoring it means ordering short.
- How much waste should I allow for roofing?
- Ten percent on a simple gable. Fifteen percent once there are hips, valleys or dormers generating cut waste.
- How many shingles come in a bundle?
- Around 26 for standard architectural shingles, though it varies by manufacturer. Coverage per bundle matters more than the count.
- Can I put new shingles over old?
- Many codes allow one extra layer, but it hides deck problems and shortens the new roof's life. Tearing off is nearly always the better job.