Roofing Calculator
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Result
Roofing squares
- Shingle bundles
- 45 bundles at 3 per square
- Roof surface area
- 1,342 sq ft
- Footprint area
- 1,200 sq ft
- Pitch multiplier
- ×1.1180 for 6/12
- Underlayment rolls
- 2 rolls (10 squares each)
- A 6/12 pitch makes the roof surface 12% larger than the footprint. Ordering against the footprint alone would leave you 142 sq ft short — the single most common error in roofing estimates.
Roof area by pitch
The same footprint at every common pitch. The multiplier is √(1 + (rise/12)²).
| Pitch | Multiplier | Roof area | Squares with waste |
|---|---|---|---|
| 3/12 | 1.0308 | 1,237 sq ft | 14 |
| 4/12 | 1.0541 | 1,265 sq ft | 14 |
| 5/12 | 1.0833 | 1,300 sq ft | 15 |
| 6/12 | 1.1180 | 1,342 sq ft | 15 |
| 7/12 | 1.1577 | 1,389 sq ft | 16 |
| 8/12 | 1.2019 | 1,442 sq ft | 16 |
| 9/12 | 1.2500 | 1,500 sq ft | 17 |
| 10/12 | 1.3017 | 1,562 sq ft | 18 |
| 12/12 | 1.4142 | 1,697 sq ft | 19 |
How do you calculate roofing squares?
To calculate roofing squares, multiply the building footprint by the pitch multiplier to get the roof surface area, then divide by 100. A 40 by 30 ft footprint at a 6/12 pitch is 1,342 square feet of roof, which is 13.4 squares before waste.
roof area = footprint × √(1 + (rise/12)²) squares = roof area ÷ 100- footprint
- Ground area the roof covers, including overhangs
- rise
- Inches of vertical rise per 12 inches of run
- 100
- Square feet in a roofing square
A roof's surface is always larger than the ground it covers, and the pitch multiplier is how much larger. It comes straight from Pythagoras: a run of 12 inches with a rise of 6 gives a rafter length of √(12² + 6²) = 13.42 inches, so the slope is 13.42/12 = 1.118 times the horizontal distance. That multiplier applies to the whole footprint. A 6/12 roof is 11.8% bigger than its footprint; a 12/12 roof is 41.4% bigger. Ordering against the footprint is the classic roofing error, and on a 1,200 square foot house at 6/12 it leaves you about one and a half squares short. Everything downstream is divisions: materials are sold by the square, which is 100 square feet of roof surface, and architectural shingles come three bundles to the square. Waste covers starter courses, hips and valleys, where every shingle is cut.
- Measure the building footprint including the eaves overhang.
- Determine the pitch as rise in inches per 12 of run.
- Multiply the footprint by √(1 + (rise ÷ 12)²).
- Add ten percent for starter courses, hips and valleys.
- Divide by 100 for squares, then multiply by three for bundles.
Worked example: A 40 × 30 ft footprint with a 6/12 gable roof, 10% waste
A 40 × 30 ft footprint with a 6/12 gable roof, 10% waste
The footprint is 40 × 30 = 1,200 square feet. A 6/12 pitch has a multiplier of √(1 + 0.25) = 1.1180, so the actual roof surface is 1,200 × 1.1180 = 1,342 square feet. That 142 square foot difference is what ordering from the footprint would miss. Adding the 10% waste allowance brings it to 1,476 square feet, which is 14.76 squares — so you buy 15. At three bundles per square that is 45 bundles of architectural shingles, plus 2 rolls of synthetic underlayment at 10 squares each. Changing nothing but the pitch to 12/12 would take the roof area to 1,697 square feet and the order to 19 squares: a 27% increase in material from the same house footprint, which is why pitch is the first thing a roofer asks about.
What each roofing input means
- Building length
- The footprint length in feet, measured on the ground including the eaves overhang — not the sloped roof surface.
- Building width
- The footprint width in feet, including the overhang.
- Roof pitch
- Rise in inches per 12 inches of horizontal run. Measure it with a level and a tape, or from inside the attic against a rafter.
- Roof type
- A hip roof has four sloping sides and more cut waste at the hips. A gable has two slopes and vertical end walls.
- Waste allowance
- Covers starter courses, hips, valleys and offcuts. Ten percent suits a simple gable; a complex roof with several valleys needs fifteen or more.
- Price per square
- Optional. Materials are priced per square — 100 square feet of roof surface.
What this roofing does not account for
- Measure the footprint including the eaves overhang, not the wall line. Overhangs of 12 to 24 inches on each side add meaningfully to the area.
- This assumes a simple rectangular footprint with a uniform pitch. Dormers, cross gables, turrets and varying pitches all need the roof split into sections and each calculated separately.
- Ridge cap, starter strip, drip edge, valley flashing, vents and nails are all separate purchases not counted here.
- Three bundles per square is right for architectural shingles. Some three-tab products and most heavyweight designer shingles run four or five bundles per square — check the wrapper.
- A tear-off adds disposal cost and may reveal deck damage. Local code also limits how many shingle layers can be stacked, commonly two.
- Above roughly a 6/12 pitch, a roof cannot be safely walked without staging and fall protection. That changes labour cost far more than materials.
Frequently asked questions
How many squares is a 2,000 square foot roof?
Twenty squares of surface area, since a square is 100 square feet. But a 2,000 square foot house does not have a 2,000 square foot roof — at a 6/12 pitch the roof surface is about 2,236 square feet, or 22.4 squares.
What is the pitch multiplier for a 6/12 roof?
1.1180, from √(1 + (6/12)²). So the roof surface is 11.8% larger than the footprint it covers. A 4/12 pitch is 1.054 and a 12/12 is 1.414.
How many bundles of shingles in a square?
Three, for standard architectural shingles. Some heavyweight designer and specialty products run four or five bundles per square, so check the wrapper rather than assuming three.
How do I measure my roof pitch?
Hold a level horizontally against the roof or a rafter, mark 12 inches along it, then measure straight down to the roof surface. That vertical distance in inches is the rise over a 12 inch run.
How much waste should I add for roofing?
Ten percent for a simple gable, fifteen or more for a hip roof or anything with several valleys and dormers. Waste covers starter courses and every cut shingle at a hip or valley.
Why is my roof bigger than my house footprint?
Because the roof is sloped. The surface follows the hypotenuse of a triangle whose base is the run and whose height is the rise, so it is always longer than the horizontal distance it spans.
Sources and review
Last reviewed .