How to use this calculator
- Measure the footprint on the ground. Not the roof — the outline of the building underneath it, including the overhang at the eaves. Split an L-shaped house into rectangles and add a section for each.
- Pick the pitch. This is what converts a flat footprint into the actual sloped surface.
- Check the bundles per square. Three is standard, but heavier shingles take four or five, and that difference across a whole roof is enormous.
You should not need to get on the roof to use this. Working from the footprint and the pitch is how estimators produce a figure from a survey, and it is accurate for a simple gable.
The formula
slope factor = √(rise² + 12²) ÷ 12
roof area = footprint area × slope factor
with waste = roof area × (1 + waste %)
squares = area with waste ÷ 100
bundles = squares × bundles per square, rounded upWorked example — a 30 ft × 40 ft house with a 6:12 gable roof:
- Footprint: 30 × 40 = 1,200 sq ft
- Slope factor for 6:12: √(36 + 144) ÷ 12 = √180 ÷ 12 = 1.118
- Roof area: 1,200 × 1.118 = 1,342 sq ft
- With 10% waste: 1,476 sq ft
- Squares: 1,476 ÷ 100 = 14.76 squares
- Bundles at 3 per square: 45 bundles
Skip the slope factor and you would order 40 bundles for a roof that needs 45. That is the single most common way to get this wrong.
Slope factors
The factor is the hypotenuse of a right triangle with legs of 12 and the rise, divided by 12. It is derived rather than looked up here, because published contractor tables sometimes fold a waste allowance into the number and then disagree with each other.
| Pitch | Slope factor | Extra area over the footprint |
|---|---|---|
| 3:12 | 1.031 | 3.1% |
| 4:12 | 1.054 | 5.4% |
| 6:12 | 1.118 | 11.8% |
| 8:12 | 1.202 | 20.2% |
| 10:12 | 1.302 | 30.2% |
| 12:12 | 1.414 | 41.4% |
A 12:12 roof is 45 degrees, which is why its factor is exactly the square root of two.
If you do not know your pitch, you can measure it from inside the attic: hold a level horizontally against a rafter, mark 12 inches along it, and measure straight up from that mark to the underside of the rafter. That measurement is the rise.
What a “square” means
A roofing square is 100 square feet of roof surface. Shingles, underlayment and labour are all quoted by the square, which is why a roofing estimate talks about “a 15 square roof” rather than a square footage.
Most shingle lines are packaged three bundles to the square. Heavier architectural and designer shingles run four or five, and the wrapper states it. This is worth checking rather than assuming: on a 15 square roof, the difference between three and four bundles per square is 15 bundles.
How much extra to order
Ten percent for a straightforward gable roof — two planes, a ridge, no complications.
Fifteen percent or more once the roof has hips and valleys. Both are cut on the diagonal, and every course dies into them at an angle, so the offcuts are triangles that fit nowhere else. A complex roof with dormers, multiple valleys and a skylight or two can justify twenty.
The allowance also has to cover the starter course along the eaves and the cap along the ridge, unless you are buying purpose-made starter strip and hip-and-ridge shingles separately — which most people now do, because cutting field shingles into caps is slower and looks worse.
What this does not include
Starter strip and ridge cap are bought by the linear foot of eave and of ridge. They are not proportional to the roof area, so you have to measure those runs separately.
Underlayment is sold by the square, so the square count here works directly. Note that most codes require ice-and-water barrier along the eaves in cold climates, which is a different, more expensive product covering a specific width up the roof rather than the whole area.
Drip edge, flashing, vents and nails all scale with the perimeter and the number of penetrations, not with area.
Complex roofs. This calculation assumes a simple gable. Hips, valleys, dormers, turrets and roofs with multiple ridge heights all need to be broken into planes and measured individually — at that point, having a roofer measure it is worth more than any calculator.
A word about doing this yourself
Every other calculator on this site is about not wasting money. This one is different.
Roof work is one of the most dangerous jobs in residential construction. Falls from height are the leading cause of death in the industry, and steep pitches — anything from 8:12 up — need staging and fall protection, not a ladder and good intentions. Tearing off an old roof also has to be done in weather windows, because a roof that is open when it rains is a much bigger problem than an old roof that leaks.
Use this to understand a quote, to budget, or to plan a small outbuilding. For a house, the estimate is the cheap part.
This is an estimate for a simple gable roof. Confirm the bundles-per-square figure on the wrapper, measure starter and ridge separately, and have a contractor measure anything complicated.