Roofing Calculator

The formula

Coverage and yield assumptions

Standard three-tab and most architectural asphalt shingles come three bundles to a square; some heavy designer shingles need four or five. The Asphalt Roofing Manufacturers Association and NRCA describe underlayment, starter and ridge requirements that use separate products. The multiplier is exact for planar roofs of one pitch, whether gable or hip.

Pitch multipliers

Common multipliers: 3/12 = 1.031, 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414. Use the roof pitch calculator if you know rise and run but not the pitch. Low slopes below 2/12 need membrane roofing, not shingles; slopes 2/12 to 4/12 need double underlayment under most shingle instructions.

Accessories to add

Shingles are only part of the order. Add starter strip for eaves and rakes, ridge cap (roughly one bundle per 20–35 linear feet, depending on the product), drip edge for all edges, ice and water shield at eaves and valleys in cold climates, synthetic underlayment for the deck (about 10 squares per roll for many products), and roofing nails, typically four to six per shingle. Pipe boots, vents and flashing are counted by item.

Tear-off and decking

Removing old shingles costs extra and generates about 2–3 tons of debris on an average house. Many codes allow at most two layers of shingles, so a third layer needs a full tear-off. Rotten roof sheathing is replaced by the sheet (4 × 8 ft); contractors often price this per sheet in case it is needed.

Safety

• Use fall protection on any roof work; most falls are from ladders and edges.
• Do not work on wet, icy or very hot roofs.
• Check local permit rules; reroofing usually needs a permit.

Common mistakes to avoid

• Using house floor area instead of the roof footprint with overhangs.
• Forgetting starter strip, ridge cap and underlayment.
• Using too little waste on hip roofs with many valleys.
• Installing shingles below their minimum slope.
• Working on a roof without fall protection.

Waste factor

10% suits simple gable roofs. Use 15% for hip roofs, and 15–20% for roofs with many valleys, dormers or skylights, because each needs trimmed shingles.

Imperial and metric

Switch the toggle above the inputs to enter ft ↔ m, in ↔ cm. The page converts metric entries to imperial before calculating, using NIST factors (1 ft = 0.3048 m, 1 in = 2.54 cm), and shows key results in both systems.

Worked examples

Ranch house, 6/12 gable

Inputs: house length 40 ft; house width 30 ft; eave overhang 12 in; roof pitch (rise per 12 in) 6; waste factor 10%.

Result: You need 16.5 squares (50 bundles of shingles).

• Units to buy (rounded up): 50 bundles
• Footprint with overhang: 1,344 sq ft (124.9 m²)
• Pitch multiplier: 1.118
• Roof surface area: 1,503 sq ft (139.6 m²)

Two-story with steep hip

Inputs: house length 36 ft; house width 28 ft; eave overhang 18 in; roof pitch (rise per 12 in) 9; waste factor 15%; cost per unit $38.

Result: You need 17.4 squares (53 bundles of shingles).

• Units to buy (rounded up): 53 bundles
• Footprint with overhang: 1,209 sq ft (112.3 m²)
• Pitch multiplier: 1.25
• Roof surface area: 1,511 sq ft (140.4 m²)
• Estimated material cost: $2,014.00

Detached garage, 4/12

Inputs: house length 24 ft; house width 24 ft; eave overhang 12 in; roof pitch (rise per 12 in) 4; waste factor 10%.

Result: You need 7.8 squares (24 bundles of shingles).

• Units to buy (rounded up): 24 bundles
• Footprint with overhang: 676 sq ft (62.8 m²)
• Pitch multiplier: 1.054
• Roof surface area: 713 sq ft (66.2 m²)

Quick reference for common sizes

The table repeats the first worked example at smaller and larger sizes, keeping every other setting the same, so you can see how the roofing calculator result scales.

Limitations

The method assumes one pitch over the whole roof. For roofs with sections of different pitch, run each section separately. It does not include waste from the starter course or ridge.