Squares and bundles
Roofing calculator
Measure each roof plane flat, the way it appears on a plan, then set the pitch. The calculator converts footprint to real surface area, then to squares and bundles.
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Pitch is the number people get wrong
A roof's footprint is not its area. A 40 by 20 foot plane at a 6-in-12 pitch covers 800 square feet on the ground and 894 square feet of actual roof — and if you order for the footprint you are nearly a square short on every plane.
The multiplier is the hypotenuse of the pitch triangle: for a rise of r inches over a 12-inch run, it is √(r² + 144) ÷ 12. That is pure geometry, so the figures in the table below are exact rather than approximate.
Slope factor by pitch
| Pitch | Multiplier | 1,000 sq ft of footprint becomes |
|---|---|---|
| 3 in 12 | ×1.031 | 1,031 sq ft |
| 4 in 12 | ×1.054 | 1,054 sq ft |
| 5 in 12 | ×1.083 | 1,083 sq ft |
| 6 in 12 | ×1.118 | 1,118 sq ft |
| 8 in 12 | ×1.202 | 1,202 sq ft |
| 10 in 12 | ×1.302 | 1,302 sq ft |
| 12 in 12 | ×1.414 | 1,414 sq ft |
How to measure without going up there
You can get every number you need from the ground and the attic. Pace or tape the building's footprint for the plane lengths. For the pitch, hold a level against a rafter in the attic, measure 12 inches along it, and measure straight down to the rafter — that drop is the rise.
Eave-to-ridge is the flat distance on the plan, not the distance up the slope. If you have measured up the slope by climbing it, divide by the multiplier instead of entering it directly, or the pitch gets counted twice.
Getting on a roof to measure is the single most dangerous part of a re-roof, and it is the part with the least to gain. Measure from the ground.
What the ticket does not cover
This is a materials count, not a roof design. How many layers are already up there decides whether you need a tear-off and a dumpster, and the answer is regulated — some jurisdictions allow a second layer, others ban it outright. Ice-and-water barrier at the eaves, ventilation, and flashing at walls and chimneys are all code questions with local answers.
Check with your building department before you order. A roof is also the one job on this site where the permit is usually not optional.
Common questions
What is a roofing square?
One hundred square feet of finished roof surface. Shingles are quoted, sold and estimated by the square, and coverage is printed on the wrapper in squares — which is why the ticket gives you both squares and bundles.
How many bundles of shingles are in a square?
Three for most shingles, four for heavy laminated ones. It is printed on the wrapper, and it is a field on this calculator rather than a fixed number, because it genuinely varies by product.
Do I use the footprint or the sloped area?
The sloped area, which is what this calculator works out for you. Enter the flat footprint of each plane and set the pitch, and the multiplier is applied. A 6-in-12 roof is 11.8 percent bigger than its footprint.
How much waste should I add for a roof?
Ten percent on a simple gable with two clean planes. Fifteen on anything with hips and valleys, because every valley means cutting shingles on an angle and throwing away the offcut. Twenty on a cut-up roof with dormers.
How do I measure roof pitch from inside?
Hold a level horizontally against a rafter, mark 12 inches along it, and measure vertically from that mark to the rafter. That measurement in inches is the rise, and the pitch is that rise in 12.
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