Roofing
Roofing Calculator
Roofing squares and shingle bundles from your home footprint and pitch — measured from the ground.
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Adjust the inputs to see your result.
About this calculator
Roofing is priced by the square — 100 sq ft of roof surface — and shingles come 3 bundles to a square. Because the sloped surface is larger than the house footprint, this calculator applies the pitch multiplier for you, then adds waste for hips, valleys and starter courses.
What this number means
Roofing is bought in squares (100 sq ft of sloped surface) and bundles (a third of a square), but it is measured from the ground as a flat footprint. The bridge between those two facts is the pitch multiplier: at 6/12 pitch every square foot of house hides 1.118 sq ft of roof, and at 12/12 it hides 1.414. Getting the multiplier right is the whole estimate — everything after it is division.
The waste allowance is where roof shape enters. A simple gable roof wastes about 10% to starters, ridge cap and cuts; a hip roof, where every edge is a cut line, wastes 15% or more. Dormers and valleys push it higher still.
The formula
- Slope multiplier = √(1 + (rise ÷ 12)²) — 1.118 at 6/12, 1.054 at 4/12, 1.414 at 12/12
- Roof area = (Length + 2 × overhang) × (Width + 2 × overhang) × multiplier
- Squares = Roof area ÷ 100
- Bundles = Squares × (1 + waste %) × 3 bundles per square, rounded up
- Underlayment = Roof area ÷ 400 sq ft per roll (15 lb felt equivalent), rounded up
Worked example
A 30 × 40 ft house, 6/12 gable roof, no overhang counted.
- Footprint = 1,200 sq ft; multiplier at 6/12 = 1.118 → roof area = 1,342 sq ft
- Squares = 1,342 ÷ 100 = 13.4 squares
- With 10% gable waste: 14.8 squares → 45 bundles of shingles
- Underlayment: 1,342 ÷ 400 → 4 rolls
Add the typical 12 in overhang in the field above and watch the bundle count climb — the roof overhangs the walls on all four sides.
Assumptions and limitations
- A single rectangular roof plane pair is assumed. L-shaped plans, cross-gables and dormers each add surface and waste — measure each section and total the squares, adding 2–5% waste per major feature.
- Three bundles per square is the standard for architectural (laminate) shingles. Some heavyweight products run 4–5 bundles per square — check the wrapper and adjust by ordering squares, not bundles, with your supplier.
- Ridge cap is cut from the same shingles in many systems or bought as cap bundles; the waste allowance covers starter and cap cutting for standard systems.
- Underlayment rolls vary: synthetic underlayments cover ~1,000 sq ft per roll. The 400 sq ft figure is the conservative felt equivalent — divide accordingly if you buy synthetic.
Frequently asked questions
It depends on pitch, which is the point of the multiplier. At 4/12 the roof is 1,054 sq ft → about 35 bundles with gable waste. At 6/12 it is 1,118 sq ft → 37 bundles. At 12/12 it is 1,414 sq ft → 47 bundles. Same house, a third more roof.
One hundred square feet of finished roof surface. Shingles are priced and warrantied by the square and packaged in bundles — three bundles to a square for standard architectural shingles — so quotes in squares convert to bundles by multiplying by three.
The honest ways: measure in the attic (mark 12 in along a level from a rafter, then measure the vertical rise to the rafter), read it off the original plans, or use a pitch-finder app on a gable end. Our Roof Pitch Calculator converts between rise/run, degrees and percent once you have any one of them.
Cuts. A gable roof has two rake edges; a hip roof has four sloped edges plus valleys where planes meet, and every one of them consumes shingles as cut pieces and cap. The waste difference — roughly 15% versus 10% — is almost entirely cutting, not coverage.
Yes — the roof extends past the walls on every side, typically 12 in at eaves and often at gables too. On a 30 × 40 ft house, a 12 in overhang all round adds about 140 sq ft of footprint before pitch is even applied. Enter your overhang and the calculator extends both dimensions.