Roofing Calculator
The footprint is not the roof — pitch adds to it.
Work out Roofing. The footprint is not the roof — pitch adds to it. The exact conversions are exact; everything else is adjustable.
Ground area under the roof, not the roof surface.
10% for a simple gable; 15–20% for hips, valleys and dormers.
Three for most architectural shingles. Check the wrapper.
Waste depends far more on the roof's shape than its size. A simple gable wastes little; hips, valleys and dormers produce a cut at every intersection, and a complex roof can waste twenty percent or more.
Roofing squares
24.60
74 bundles · 2,460 sq ft of roof
A roofing square is exactly 100 square feet. The pitch multiplier is exact geometry — the square root of one plus the slope squared — and at 6/12 it adds 11.8% to the footprint. Ordering from the footprint instead is the commonest way a roofing order comes up short, and you find out on the roof.
How the Roofing Calculator works
The commonest roofing error is ordering from the building's footprint. A pitched roof covers more area than the ground beneath it, by exactly the square root of one plus the slope squared — at a 6/12 pitch that is 11.8% more, and at 12/12 it is 41.4%. Get that multiplier wrong and the order is short by an amount you discover on the roof.
Also known as: roof shingle calculator · roofing squares calculator · how many bundles of shingles · roof pitch multiplier
The calculation itself
The footprint is the ground area under the roof. The roof itself is larger, by exactly the square root of one plus the slope squared, where slope is rise over run.
That multiplier is pure geometry and therefore exact: it is the hypotenuse of a right triangle with legs 12 and the rise. Multiply the footprint by it, add waste, and divide by 100 — a roofing square is exactly 100 square feet.
In practice
A 2,000 square foot footprint at a 6/12 pitch: the multiplier is 1.1180, so the roof surface is 2,236 square feet before waste. With 10% it is 2,460 — 24.6 squares, or 74 bundles at three per square.
The pitch is doing 11.8% of that. At 9/12 the multiplier is 1.25, and at 12/12 — a 45-degree roof — it is 1.4142, adding 41.4%.
Order 20 squares for that roof because the footprint is 2,000 square feet and you are short by nearly a fifth. It is the commonest roofing error and you discover it on the roof.
Where the figure deceives
Waste depends on the roof's shape far more than its size. A simple gable wastes little. Hips, valleys and dormers produce a cut at every intersection, and each cut leaves an offcut that fits nowhere — a complex roof can waste twenty percent or more.
Bundles per square vary by product. Three is standard for architectural shingles, but heavier and specialty products differ, and the wrapper says which.
None of this includes underlayment, ridge cap, starter strip, flashing or fasteners, each of which is calculated differently and none of which is optional.
Acting on it
Measure pitch from inside the attic against a rafter — rise over a 12-inch level run — rather than from on the roof. Same answer, safer.
Count the planes before setting the waste allowance. Two planes is a gable; six is a job that wastes far more.
Order ridge cap and starter separately. They are not in the square count and running out of either stops the job.
Frequently asked questions
How do I calculate roofing squares?
Multiply the footprint by the pitch multiplier to get the actual sloped area, then divide by 100 — a roofing square is exactly 100 square feet. Add waste for cuts, hips and valleys.
What is the pitch multiplier?
The square root of one plus the slope squared, where slope is rise over run. A 6/12 pitch gives 1.118, a 9/12 gives 1.25, and a 12/12 gives 1.414. It is pure geometry, so it is exact.
How many bundles of shingles per square?
Three for most standard architectural shingles, but heavier or specialty products differ and some take four. Check the bundle before ordering — it is an input here for that reason.
How much waste should I allow?
Ten percent for a simple gable, more for a complex roof with hips, valleys and dormers where every cut leaves an offcut. A roof with many planes can waste twenty percent or more.
How do I find my roof pitch without climbing up?
Measure the rise over a 12-inch level run inside the attic against a rafter. That gives the same pitch as measuring on the roof surface, without being on the roof surface.
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Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/roofing-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Roofing Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.