How many shingles do I need for my roof?

Roofing is sold in squares, and a square is 100 sq ft of finished roof. The trick is that roof area is always larger than the footprint, because a pitched roof is a slope: a 6/12 roof covers 1.118 times its plan area, and a steep 12/12 roof covers 1.414 times.

Building length (ft)

Building width (ft)

Roof pitch (rise per 12)

A 6/12 roof rises 6 in for every 12 in of run.

Overhang (each side) (in)

Waste allowance (%)

10% for a simple gable, 15% for hips and valleys.

Roofing squares

16.5squares

Bundles of shingles 50 bundlesRoof area 1,653 sq ftPitch factor 1.118 ×

Single-plane, uniform-pitch roofs only. Complex roofs should be measured by plane and summed, and a contractor's measured plan beats any footprint-based estimate.

This calculator starts from the building footprint, adds the overhang on all four sides, applies the pitch factor, and then adds a waste allowance. Ten percent is right for a simple gable where offcuts are reusable; fifteen is closer for a hip roof or anything with valleys, because every valley consumes shingles in cuts that cannot be used elsewhere.

Bundles are calculated at three per square, which is the standard packaging for three-tab and most architectural shingles — check the wrapper, because some heavier products are two or four bundles to the square. A square also needs roughly a roll of underlayment, 320 linear feet of starter strip and the same again of ridge cap, none of which is included here.

How the calculation works

Plan area = (length + 2 × overhang) × (width + 2 × overhang), with the overhang converted to feet. Pitch factor = √(144 + rise²) ÷ 12, where rise is inches per 12. Roof area = plan area × pitch factor × (1 + waste ÷ 100). Squares = area ÷ 100; bundles = squares × 3.

• The roof is a simple gable or hip over a rectangular footprint; dormers, porches and multiple roof planes must be measured separately and added.
• The pitch is uniform across the roof, and the overhang is the same on all four sides.
• Three bundles per square, which is standard for most asphalt shingles; confirm against the product data.
• Underlayment, starter strip, ridge cap, drip edge and fasteners are not included.

Sources: 1 square = 100 sq ft of roof area; 3 bundles per square for standard asphalt shingles; Pitch factor = √(144 + rise²) ÷ 12

Worked examples

Questions

How many bundles of shingles are in a square?

Three, for most three-tab and architectural shingles. Check the wrapper: some heavier products run two or four bundles to the square.

How do I calculate roof area from the footprint?

Multiply the plan area including overhangs by the pitch factor: √(144 + rise²) ÷ 12. A 6/12 pitch is 1.118, 4/12 is 1.054, 8/12 is 1.202, 12/12 is 1.414.

How much waste should I allow for shingles?

Ten percent for a simple gable, fifteen for hips, valleys and dormers. Valleys and rake edges consume shingles that cannot be reused.

Do I need to add starter strip and ridge cap?

Yes, and they are not counted here. Allow roughly one bundle of starter per 100 linear feet of eave, and one bundle of ridge cap per 35 linear feet of ridge.

Can I shingle over the old roof?

Sometimes, if there is only one layer and the deck is sound, but it shortens the life of the new roof and many codes limit it. It also means the deck cannot be inspected.

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Written and maintained by Sam Winters. Last updated 2026-11-04. Every figure is computed in your browser; nothing you enter is sent anywhere. Found an error? Tell me — accuracy reports get fixed first.