Roofing Calculator
Enter the building footprint, overhangs included, and the roof pitch to get the true roof area, roofing squares and shingle bundles, with a waste allowance and optional ridge cap, starter strip and underlayment.
Quick answer: A 40 ft × 30 ft footprint at 6/12 pitch has a 1,341.64 ft² roof, or 13.42 squares. With 10% waste at 3 bundles per square, buy 45 bundles.
Roof area = footprint × pitch multiplier, where the multiplier is √(1 + (rise ÷ 12)²). One roofing square is 100 ft² of roof. Bundles = squares × (1 + waste) × bundles per square, rounded up.
Example: a 40 ft × 30 ft footprint at 6/12 pitch has a multiplier of 1.118, so the roof is 1,341.64 ft², or 13.42 squares. With 10% waste that is 14.76 squares, and at 3 bundles per square you buy 45 bundles.
Method
How this calculator works
The pitch math is exact trigonometry. Bundle and roll figures come from a manufacturer product guide, and the waste allowances are conventions you can change.
Roof area from pitch
A sloped roof is larger than the floor plan under it. Pitch is the rise in inches for every 12 inches of run, and the multiplier turns the flat footprint into the real surface. Use the footprint including the eaves and rakes, and add the planes of a compound roof together.
Multiplier = √(1 + (Pitch ÷ 12)²)Roof area = Footprint × MultiplierSquares and waste
Roofers price and order by the square: 100 ft² of roof. Waste covers cuts at edges, hips and valleys. The calculator starts at 10%, the usual figure for a simple gable. Hips, valleys and dormers call for 15% or more, and the slider runs from 0 to 40%.
Squares = Roof area ÷ 100Squares to order = Squares × (1 + Waste ÷ 100)Bundles per square
Most shingles ship 3 bundles per square, but not all. The Owens Corning product guide lists 3 for Supreme and TruDefinition Duration, 4 for Duration Premium and MAX, and 5 for Berkshire. The calculator starts at 3. Change it to the figure on your wrapper.
Bundles = round up (Squares to order × Bundles per square)Ridge, starter and underlayment
Optional extras. Starter strip is 100 lineal ft per bundle and a felt roll covers 400 ft² in the same guide; ridge cap is 20 to 33 lineal ft per bundle depending on the product. Ridge adds 5% for cuts. Underlayment uses your waste figure. Synthetic rolls cover more, so type your roll size.
Ridge = round up (Length × 1.05 ÷ Ft per bundle)Starter = round up (Eave and rake length ÷ Ft per bundle)Rolls = round up (Roof area × (1 + Waste ÷ 100) ÷ Roll coverage)Worked example: 40 ft × 30 ft footprint, 6/12 pitch, gable roof
Arithmetic checked10% waste, 3 bundles per square, 40 ft of ridge at 20 ft per bundle, 80 ft of eaves at 100 ft per starter bundle, 400 ft² felt rolls.
| Step | Math | Result |
|---|---|---|
| Footprint | 40 × 30 | 1,200 ft² |
| Pitch multiplier | √(1 + (6 ÷ 12)²) = √1.25 | 1.1180 |
| Roof surface area | 1,200 × 1.1180 | 1,341.64 ft² |
| Squares | 1,341.64 ÷ 100 | 13.416 squares |
| Add 10% waste | 13.416 × 1.10 | 14.758 squares |
| Shingle bundles | 14.758 × 3 = 44.27, round up | 45 bundles |
| Ridge cap | 40 × 1.05 ÷ 20 = 2.1, round up | 3 bundles |
| Starter strip | 80 ÷ 100 = 0.8, round up | 1 bundle |
| Underlayment (400 ft² rolls) | 1,341.64 × 1.10 ÷ 400 = 3.69, round up | 4 rolls |
| You would buy | 45 shingle bundles, 3 ridge, 1 starter, 4 underlayment rolls | |
Disclaimer
Results are planning estimates based on the dimensions and assumptions you enter. They are not engineering, structural or building-code advice. Product yields, densities and coverage vary by manufacturer, batch and site conditions. Check product labels, your supplier and your local building department before you order materials or start work. Read the full calculation disclaimer.
Accuracy
What affects your estimate
Roof geometry decides the area. Everything after that depends on the product and on how complicated the roof is.
Product coverage
The Owens Corning guide gives 98.4 ft² per square for Duration shingles and 100 ft² for Supreme 3-tab. The 1.6% difference is small, but a count near a whole number, such as 44.99, can tip over. The bundles per square on your wrapper wins.
Hips, valleys and dormers
Each hip and valley adds cutting. Raise the waste toward 15% or more on complex roofs. The calculator counts roof surface only, so add every plane to the footprint if the roof is compound.
Overhangs
Footprint includes the eave and rake overhangs. Measuring only the walls of the house leaves the roof short.
What is not counted
Ice-and-water barrier, drip edge, vents, flashing and nails are not in the totals. Eaves, valleys and ice-and-water barrier are not counted in the underlayment rolls, and the shingle maker’s instructions set the details.
Wind and fire ratings, decking, ventilation, ice-dam protection and minimum slope come from local code and the shingle maker. Roofs are a fall hazard, so measure from plans, the ground or photos when you can.
Common mistakes
Using the floor area as the roof area
A 6/12 roof is about 12% larger than its footprint, and a 12/12 roof about 41% larger. Always apply the pitch multiplier.
Assuming 3 bundles per square for every shingle
Some lines ship 4 or 5 bundles per square. Check the wrapper, or a line that ships 4 bundles per square leaves you a quarter short.
Forgetting ridge and starter
Shingle bundles do not include ridge cap or starter strip. Add the lengths of ridge and hips, and of eaves and rakes, in the options.
Questions people ask
How do you calculate your roof?
Multiply the footprint, overhangs included, by the pitch multiplier to get the roof area. Divide by 100 for squares, add waste, and multiply by bundles per square. If you do not know the pitch yet, see how to calculate roof pitch.
How many bundles of shingles are in a square?
Three for most shingles, four for some premium lines and five for some others. The wrapper says. The guide on shingles per bundle covers it in detail.
What is the multiplier for a 6/12 roof?
√(1 + 0.5²) = 1.118. For 4/12 it is 1.054, for 8/12 it is 1.202 and for 12/12 it is 1.414. The roof pitch calculator finds it from an angle or rise and run.
Is 10% waste enough?
It is the usual figure for a simple gable roof. Hips, valleys and dormers push it to 15% or more. It is a convention, not a manufacturer rule.
How big is a roofing square?
A square is 100 ft² of roof surface, so a 1,341.64 ft² roof is 13.42 squares.
Related calculators and guides
Sources
Where the numbers come from
- Owens Corning The Roofing Collection Product Guide (bundles per square, coverage, starter, underlayment), accessed October 16, 2026.