How many bundles of shingles are in a square?

Three, for both standard three-tab and most architectural shingles. Some heavyweight and designer shingles run four or five bundles per square — check the wrapper before ordering.

What is a roofing square?

One hundred square feet of finished roof surface. It is the unit roofers quote and suppliers price in, which is why every roofing estimate converts to squares before it converts to money.

Does the calculator include the overhang?

Only if you include it in the length and width you enter. Measure to the outside of the eaves and the outside of the rakes rather than to the wall line, or the result will be short by the overhang on all four sides.

Roofing Calculator

Convert a roof footprint and pitch into roofing squares, shingle bundles and underlayment rolls.

Draft — figures checked, awaiting an editorial read. Not indexed.

Roofing squares— squaresOne square = 100 ft² of roof surface.

Roof surface area: — ft²
Slope factorHow much longer the slope is than the footprint.: —
Shingle bundlesThree bundles per square.: — bundles
Underlayment rolls15 lb felt, 4 squares per roll.: — rolls
Ridge cap bundles: — bundles

Formula: Roof area = footprint × slope factor, where slope factor = √(rise² + 12²) ÷ 12. Squares = area ÷ 100; bundles = squares × 3.

What this calculates

This converts the footprint of a building and the pitch of its roof into the quantities a roofing supplier sells: squares, bundles and rolls. A "square" is the trade unit for 100 square feet of roof surface, and almost everything else is priced from it. The key step is the pitch adjustment. A roof always covers more surface than the ground beneath it, and the steeper it is the bigger that gap becomes — which is why measuring a building from the outside and ordering shingles for that area leaves you short on every roof that is not perfectly flat.

How it works

Pitch is expressed as inches of rise for every twelve inches of horizontal run. That rise and run form a right triangle whose hypotenuse is the actual slope, so the slope length is the square root of rise squared plus twelve squared, divided by twelve. This is the slope factor. Multiply the flat footprint by it and you have the true roof surface. Divide by one hundred for squares, multiply by three for bundles, since standard three-tab and architectural shingles both run three bundles to the square. Underlayment assumes 15 lb felt at four squares per roll.

Worked example

A 40 ft by 30 ft house with a 6/12 pitch. The footprint is 1,200 square feet. The slope factor is √(36 + 144) ÷ 12 = √180 ÷ 12 = 1.1180. So the roof surface is 1,200 × 1.1180 = 1,342 square feet. Add 10% for waste and you are at 1,476 square feet, which is 14.76 squares, or 45 bundles of shingles. Note how much the pitch matters: ordering for the 1,200 square foot footprint would have left you nearly two squares short.

Common mistakes

Ignoring the pitch entirely is the classic error and it always under-orders. The second is forgetting that this figure is the plan footprint, not the full roof — overhangs and eaves add area, so measure to the drip edge rather than to the wall. The third is using a flat 10% waste on a complex roof: a simple gable really does waste about 10%, but a roof full of hips, valleys and dormers can run 15–20% because every valley cut throws away part of a shingle.

Common questions

How many bundles of shingles are in a square?: Three, for both standard three-tab and most architectural shingles. Some heavyweight and designer shingles run four or five bundles per square — check the wrapper before ordering.
What is a roofing square?: One hundred square feet of finished roof surface. It is the unit roofers quote and suppliers price in, which is why every roofing estimate converts to squares before it converts to money.
Does the calculator include the overhang?: Only if you include it in the length and width you enter. Measure to the outside of the eaves and the outside of the rakes rather than to the wall line, or the result will be short by the overhang on all four sides.