Roofing Calculator
Roofing squares and shingle bundles from your building's footprint and roof pitch.
The roof
This works out roof area from the building footprint and the roof pitch, then converts it into squares and bundles of shingles.
Pitch is what separates roof area from floor area, and it is the step most estimates miss.
How roof area is calculated
Roof area is not the same as the building's footprint. A sloped roof covers more surface than the ground beneath it, and how much more depends entirely on the pitch.
Pitch is written as rise over run: a 6/12 pitch rises 6 inches for every 12 horizontal. The multiplier is √(1 + (rise/run)²), so a 6/12 roof is 1.118 times its footprint, or 11.8% larger. A steep 12/12 roof is 1.414 times — over 40% larger.
Roofing is sold in squares, where one square is 100 sq ft. A 2,000 sq ft footprint at 6/12 gives 2,236 sq ft of roof, which is 22.36 squares, and about 24.6 squares once you add 10% waste.
Using the 2,000 sq ft footprint directly would under-order by 236 sq ft, which is more than two squares.
What to enter
- Building footprint
- The ground area the roof covers, in square feet. Include overhangs, since the roof extends past the walls.
- Roof pitch
- Rise over run, such as 4/12 or 6/12. This sets the multiplier, so it is the input that matters most.
- Waste percentage
- 10% for a simple gable roof; 15% or more for hips, valleys, dormers and complex shapes.
- Number of layers
- Whether you are tearing off existing shingles. It affects disposal cost and sometimes the underlayment needed.
Pitch multipliers
- 4/12
- ×1.054. A shallow roof, walkable with care.
- 6/12
- ×1.118. Very common on residential builds. Around the limit for comfortable walking.
- 8/12
- ×1.202. Steep; usually needs roof jacks or staging.
- 12/12
- ×1.414. A 45-degree roof, over 40% larger than its footprint.
- 1 square
- 100 sq ft of roof. Shingles are usually three bundles per square.
What this assumes
The footprint must include roof overhangs, which extend past the exterior walls, typically by 12-24 inches.
Complex roofs with hips, valleys and dormers generate substantially more waste than a simple gable.
How to calculate your roof area and materials
Footprint times the pitch multiplier, divided by 100, plus waste.
- rise / run
- The pitch, so 6/12 means rise 6 and run 12
- √(1 + (6/12)²)
- √1.25 = 1.118, the 6/12 multiplier
Measure the footprint including overhangs. Roof overhangs typically extend 12-24 inches past the walls, and they are part of the roof.
Determine the pitch. Measure the rise over a 12-inch horizontal run in the attic or from a ladder, or use a pitch gauge.
Apply the multiplier. 2,000 sq ft × 1.118 = 2,236 sq ft of actual roof surface.
Convert to squares and add waste. 2,236 ÷ 100 = 22.36 squares, × 1.10 = 24.6, so order 25 squares.
See a worked example: why footprint is not roof area
- Footprint
- 2,000 sq ft including overhangs
- Pitch
- 6/12
Multiplier: √(1 + 0.5²) = √1.25 = 1.118.
Roof area: 2,000 × 1.118 = 2,236 sq ft, or 22.36 squares.
With 10% waste: 24.6 squares, so order 25.
At a steeper 12/12 pitch the same footprint would be 2,828 sq ft, nearly six squares more again.
2,236 sq ft, so 25 squares to order
Frequently asked questions
- What is a roofing square?
100 square feet of roof surface. It is the standard unit for pricing and ordering roofing materials.
Asphalt shingles usually come three bundles to a square, so a 25-square roof needs about 75 bundles.
- How do I find my roof pitch?
Measure the vertical rise over a 12-inch horizontal run. Hold a level out 12 inches from the roof surface and measure down to it; that measurement is the rise.
It is easiest and safest from inside the attic against a rafter. A pitch gauge or a phone app also works from a ladder without going onto the roof.
- Why is roof area bigger than the footprint?
Because a sloped surface is longer than its horizontal projection, which is the Pythagorean theorem applied to a roof.
The effect is substantial: 11.8% at 6/12 and 41.4% at 12/12. Ordering from the footprint alone under-orders by more than two squares on a typical house.
- How much waste should I allow?
10% for a simple gable roof with few penetrations. 15% or more for hip roofs, valleys, dormers and complicated shapes, which generate far more cuts.
Ridge caps and starter strips are usually separate products, so check whether your supplier includes them in the square count.
- Do I include overhangs?
Yes. The roof extends past the walls, commonly by 12-24 inches, and all of it needs covering.
Measuring the building's exterior walls rather than the roof's actual footprint is a common way to under-order, and on a large house the overhang alone can be a square or two.
- Can I roof over existing shingles?
Building codes commonly allow a maximum of two layers, and some jurisdictions require a full tear-off. Check locally before planning around it.
Layering saves disposal cost and hides any decking damage underneath. A tear-off lets you inspect and repair the deck, which is usually the better long-term choice.
Problems people actually run into
Ordering from the footprint
A 2,000 sq ft footprint is 2,236 sq ft of roof at 6/12, and 2,828 at 12/12. Ordering the footprint figure under-orders by two to eight squares.
Always apply the pitch multiplier. It is the entire difference between floor area and roof area.
Underestimating waste on a complex roof
A 10% allowance suits a plain gable. Hips, valleys and dormers all require cut shingles along every line, and the offcuts are largely unusable.
Count the valleys and hips before choosing an allowance. 15% is the sensible starting point once a roof has any of them.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026
