Roofing calculator
A roof is bigger
than its footprint.
A 6/12 pitch is about 12% larger than the ground it covers, and a 12/12 is 41% larger. Measure the footprint above, set the pitch, and the multiplier does the rest — in squares, which is how roofing is sold.
Not saving to this device — private browsing, or storage is full. Use Export takeoff in the footer to keep this.
Footprint, pitch, squares
Roofing is quoted in squares. One square is 100 ft² of roof surface — not of ground.
Measure the footprint
The building outline at ground level, which you can take from a tape on the ground or from a plan. You do not need to get on the roof to estimate it.
Pick the pitch
Rise in inches per 12 inches of horizontal run. A level and a tape in the attic gives it, or measure the rise over a 12-inch run against a rafter.
The multiplier
√(rise² + 144) ÷ 12. At 4/12 that is 1.054, at 6/12 it is 1.118, at 12/12 it is 1.414. Multiply footprint by it to get actual slope area.
Bundles
Three bundles to a square is the common convention for both three-tab and most architectural shingles, but heavier profiles run four. The wrapper tells you.
Waste is about shape, not size
Ten percent is the working minimum on a simple gable roof. Every valley, hip and dormer pushes it up, because each one means cutting shingles at an angle and throwing away the offcut. A complicated roof can justify 15% or more.
Starter course and ridge cap are separate purchases on top of the field shingles, and hip and ridge cap in particular is sold by the linear foot. Neither is included in a square count.
Underlayment, ice-and-water shield at the eaves, drip edge, flashing and vent boots are all separate line items, and on a re-roof the flashing is often the part that actually failed.
Estimating is not inspecting
This gives you a material quantity from geometry. It cannot tell you whether the decking underneath is sound, whether the existing layers can legally be overlaid, or whether the structure will take another layer of weight.
Most jurisdictions cap the number of shingle layers, commonly at two, and a tear-off changes both the cost and the disposal. Those are questions for a roofer and a permit office.
Roofing is also the highest-risk DIY job on this site by a wide margin. Estimate freely; get on the roof carefully, or not at all.
Common questions
How do I calculate roofing squares?
Measure the building footprint in square feet, multiply by the pitch multiplier to get actual slope area, add a waste factor, then divide by 100. One square is 100 square feet of roof surface.
What is the roof pitch multiplier?
The square root of the rise squared plus 144, divided by 12. For a 4/12 pitch that is 1.054, for 6/12 it is 1.118, and for 12/12 it is 1.414 — meaning a 12/12 roof has 41% more surface than the ground beneath it.
How many bundles of shingles are in a square?
Three bundles per square is the usual convention for three-tab and most architectural shingles. Heavier or premium profiles can run four bundles per square, so check the wrapper before ordering.
How much waste should I allow for a roof?
Ten percent is the working minimum on a simple gable. Valleys, hips and dormers each add cutting waste, so a complex roof can justify 15% or more. Starter course and ridge cap are separate purchases on top.
The rest of the toolbox
One measurement, then whichever material you are buying. Every tool reads the same ledger.