Roofing Calculator
Estimate roofing squares, bundles and material cost from the roof footprint and its pitch.
How to use this calculator
- 1Measure the area. For an irregular space, split it into rectangles, work each out, and add them.
- 2Set the unit you measured in — the calculator converts everything for you.
- 3Check the coverage figure against the bag or tin you are actually buying; it varies between brands.
- 4Leave the waste allowance on. Running short mid-job costs far more than one spare unit.
How the calculation works
Units = ⌈ (Area × (1 + waste)) / Coverage per unit ⌉- Area
- Surface area of the job
- waste
- Allowance for cuts, breakage and spillage
- ⌈ ⌉
- Round up — you cannot buy a fraction of a unit
Every dimension is converted to metres internally, so feet, yards and metres can be mixed with inches or centimetres without error.
A "roofing square" is 100 square feet (9.29 m²) — the trade unit for roof area.
Enter the actual sloped roof area, not the building footprint. A pitched roof is larger than the ground it covers: multiply the footprint by roughly 1.06 for a shallow 4/12 pitch, 1.12 for 6/12, and 1.20 for a steep 9/12.
Worked example
A 20 × 12 ft area
- 1.20 ft × 12 ft = 240 sq ft, which is 22.30 m².
- 2.Divided by the coverage per bundle, that gives the raw requirement.
- 3.A 15% waste allowance is added, then the figure is rounded up to whole bundles.
Result: See the result panel — it updates live as you change the numbers.
How roofing is measured and sold
Roofing has its own unit of measurement that has nothing to do with the size of the building underneath: a roofing square is 100 square feet (about 9.29 m²) of actual roof surface. Materials are bought and priced by the square, or by the bundle — a fraction of a square, most often a third, for standard asphalt shingles.
The number that matters is the sloped surface area of the roof, not the footprint of the building it sits on. Because a pitched roof is tilted, its surface area is always larger than the flat area it covers on the ground — the steeper the pitch, the bigger that gap becomes.
Common roofing materials
- Three-tab asphalt shingles — the most widely used residential roofing material in North America, valued for being inexpensive and straightforward to install.
- Architectural (dimensional) shingles — thicker and more textured than three-tab shingles, generally more durable and longer-lasting, but heavier and sometimes needing four bundles per square instead of three.
- Metal roofing — sold in panels or interlocking sheets rather than bundles, valued for longevity and performance in snow and fire-prone areas, at a higher up-front cost.
- Clay and concrete tile — very long-lasting and common in warmer climates, but heavy enough that the roof structure needs to be rated to carry the extra weight.
- Slate — among the longest-lasting roofing materials available, and priced accordingly — a specialist material for a specialist installer.
Why pitch changes the material estimate
The footprint of a house — what you’d measure by walking its perimeter — is not the number to plug into a roofing estimate, because the roof plane is tilted relative to the ground.
- Low slope (around 4/12) — a rise of 4 inches for every 12 inches of run — adds roughly 6% to the footprint area.
- Medium slope (around 6/12) — a common residential pitch — adds roughly 12% to the footprint area.
- Steep slope (around 9/12 or more) — adds roughly 20% or more, and starts to affect safe working conditions and installation cost on top of the material total.
Where estimates commonly go wrong
- Using the footprint instead of the sloped area — the single most common error, and one that quietly under-orders material on anything but a flat roof.
- Underestimating waste on a complex roof — a simple gable might only need about 10% extra, but hips, valleys, dormers and multiple roof planes push real-world waste up to 15–20% or more.
- Forgetting everything that isn’t shingles — underlayment, drip edge, ridge caps, flashing around chimneys and vents, and fasteners are all separate line items with their own quantities, not included in a bundle count.
What this assumes, and where it stops
Assumptions
- The area is flat and regular as described. Slopes, steps and obstructions change the quantity.
- Coverage figures are the manufacturer’s stated values, which assume good conditions and an even surface.
- The area entered is the true sloped surface area, not the flat footprint.
Limitations
- This estimates material only — not labour, tools, delivery, hire or disposal.
- Manufacturer coverage varies between brands and batches. Always check the packaging.
- Does not include underlay, drip edge, ridge caps, flashing, nails or disposal of the old roof.
- Roofing is dangerous work with real structural and weatherproofing consequences. This estimates material, not whether the job should be a DIY project.
Common questions
How many bundles are in a roofing square?
Three bundles of standard three-tab asphalt shingles cover one square (100 sq ft). Heavier architectural shingles sometimes need four, so check the wrapper.
Do I use the roof area or the floor area?
The roof area — the actual sloped surface, which is always larger than the footprint beneath it. Multiply the footprint by about 1.06 for a 4/12 pitch, 1.12 for 6/12, or 1.20 for 9/12 before entering it here.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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