Shingle Calculator

How to use the Shingle Calculator

Enter the footprint under the roof and the pitch. The footprint is what you can measure from the ground; the sloped area is worked out from the pitch, because a 6/12 roof is 11.8% larger than the ground it covers.

The Trim tab handles everything bought by the linear foot, starter strip along the eaves, cap along the ridges and hips, drip edge round the whole perimeter. These cannot be derived from a footprint, so measure the edges.

If you already know the sloped roof area, use the roof pitch calculator to convert it and come back with squares.

Roof Cost prices shingles, underlayment and labour together, and lets you set bundles per square. Three is standard for architectural shingles, but heavier profiles run four or five, and getting that wrong skews the whole estimate.

Shingle formula

• roof area = footprint × √(1 + (pitch ÷ 12)²)
• squares = roof area ÷ 100
• bundles = squares × 3
• underlayment rolls = squares ÷ squares per roll
• nails = squares × 320

Three bundles to a square is standard for three-tab and ordinary architectural shingles, GAF Timberline HDZ and Royal Sovereign both ship at 33.33 sq ft a bundle, exactly three to the square. Heavy designer laminates are packed thinner: GAF Camelot II is 25 sq ft a bundle (four to the square), Grand Sequoia 20 sq ft (five), and CertainTeed Grand Manor is five. Set the bundles-per-square field to match your wrapper, leaving it at three on a designer shingle leaves the job a third to two thirds short.

Worked example

A 50 × 30 ft gable roof at 6/12, 10% waste:

• Footprint: 50 × 30 = 1,500 sq ft
• Multiplier at 6/12: √(1 + 0.5²) = 1.1180
• Roof area: 1,500 × 1.1180 = 1,677 sq ft
• Plus 10%: 1,845 sq ft = 18.45 squares → order 19 squares
• Bundles: 19 × 3 = 57 bundles
• Synthetic underlayment: 19 ÷ 10 = 2 rolls
• Nails: 19 × 320 = 6,080 → about 27 lb

Then the edges: 100 ft of eave needs 1 bundle of starter, 50 ft of ridge needs 3 bundles of cap, and 168 ft of eave plus rake needs 18 pieces of drip edge.

What the result means

Squares are ordered whole. Suppliers sell by the bundle, but they price and deliver by the square, and part-squares get rounded up. That is why the order figure here is a whole number and the exact one is shown beside it.

Waste is not padding. Every hip, valley and rake is a cut, and the offcut is rarely reusable. Ten percent covers a plain gable. A cut-up roof with dormers runs fifteen to twenty, and the estimate that fails is always the one that assumed a simple roof.

The field shingles are about two thirds of the delivery. Underlayment, starter, cap, drip edge, ice and water shield, vents and nails make up the rest. Pricing only the bundles is how a roof comes in over budget.

Common mistakes

• Using the footprint as the roof area. A 6/12 roof is 11.8% bigger than the ground under it; a 12/12 is 41% bigger.
• Using cap shingles as starter, or field shingles as cap. They are different products with different exposures, and a manufacturer will decline a warranty claim over it.
• Forgetting ice and water shield. Most codes require it at the eaves in cold climates, and it is not counted here, it is a separate roll and a separate line.
• Assuming three bundles per square. True for most, but heavy architectural and designer shingles run four or five, a 33 to 67 percent shortfall if you get it wrong. The field is on the calculator; read the wrapper and set it.
• Buying nails by the box without checking length. Nails must penetrate the deck by ¾ inch or through it. Over a re-roof that usually means longer nails.

Frequently asked questions