Roofing Calculator

How to calculate roofing squares and bundles

Roofing is ordered by the square — 100 sq ft of roof surface. Start from the outside footprint of the house (one floor, not the living area), add the overhang on every side, multiply length by width, and multiply by the pitch factor. That works for gable and hip roofs alike as long as every plane has the same pitch. Divide by 100 for squares, add waste, and divide by what one bundle covers.

Worked example — a 30 × 40 ft house, 6/12 pitch, 12-in overhangs: the roof covers 32 × 42 = 1,344 sq ft of plan. × 1.118 = 1,503 sq ft, or 15 squares. Add 5% waste and divide by 32.8 sq ft per bundle: 49 bundles. See it worked out for a 30×40 house at 6/12 or a 2,000 sq ft roof.

Pitch multipliers

Pitch is the inches of rise per 12 inches of run. The multiplier is √(1 + (rise/12)²); the hip/valley factor, √(2 + (rise/12)²), turns half the building width into the length of each hip. Don't know your pitch? The roof pitch calculator shows three ways to measure it without walking the roof.

Not every bundle covers 33⅓ sq ft

The old rule — three bundles make a 100 sq ft square — is only roughly true. GAF Timberline HDZ and Owens Corning Duration are "metric" shingles: 64 pieces of 39⅜ in at a 5⅝-in exposure cover 98.4 sq ft per three bundles, or 32.8 per bundle, even when the store label says 33.33. Ordering by the label leaves you about 1.6% short. Heavy designer lines pack less: CertainTeed NorthGate and Belmont are four bundles per square, Grand Manor and Carriage House five. Landmark PRO is three, not four.

How much waste to add

A flat 10% for gables and 15% for hips is what most people use, and CertainTeed's own applicator manual calls it too high for larger roofs. Its table scales with size and shape — and cut-up roofs with dormers and valleys need 3–5 points more:

The rest of the shopping list

• Starter strip along every eave — and along the rakes too if you want the enhanced wind warranties from GAF, Owens Corning or Atlas. Bundles cover 102–140 ft depending on the brand.
• Hip & ridge cap for the ridge plus every hip: 20–33 ft per bundle. CertainTeed Shadow Ridge comes in a 30-ft and a 45-ft pack under the same name, so read the wrapper.
• Underlayment: a "10-square" synthetic roll covers about 929 sq ft once the laps are taken out. Between 2/12 and 4/12 the IRC requires two layers, which roughly doubles the rolls; under 2/12 shingles aren't allowed at all.
• Ice & water shield must run from the eave to 24 in inside the exterior wall, measured level — not 24 in up the roof. With a 12-in overhang and 2×6 walls that is about 47 in up a 6/12 roof: two courses of 36-in membrane. Steeper than 8/12, it must also reach at least 36 in along the slope. Line valleys with a 36-in strip.
• Drip edge on eaves and rakes in 10-ft pieces lapped 2 in, nailed every 12 in. Underlayment goes over it at the eaves and under it at the rakes.
• Nails: the code minimum is four per shingle — 256 per square for GAF or OC — and six where the maker or high-wind rules require it. A 7,200-count coil box covers about 28 squares of field shingles.
• Ventilation: 1 sq ft of net free area per 150 sq ft of attic, half high and half low. At 18 sq in per foot, a 1,200 sq ft attic wants 32 ft of ridge vent and matching soffit intake.

What a new roof costs in 2026

Materials for architectural shingles run about $95–140 per square on their own. Installed, 2026 guides put architectural roofs at $400–$700 per square and 3-tab at $350–$550, including one layer of tear-off. A second old layer adds roughly $25–150 per square, 8/12 and steeper about $50–100 more, permits $150–500, and rotten decking $70–100 per 4×8 sheet — two to six sheets is normal on an older roof. Online figures of $2–4 per sq ft are years out of date.

Weight: delivery and tear-off

New shingles are heavy. CertainTeed lists Landmark at 217–229 lb per square, and IKO puts laminates under 230 lb and 3-tab around 190 lb — about 77 lb per architectural bundle. The 49 bundles in the example above weigh about 1.9 tons, so check your truck's payload before planning to haul them, and remember every bundle has to go up a ladder unless the supplier delivers to the roof.

One square of old shingles weighs about 200 lb for 3-tab, 240 lb for architectural and 300 lb for premium, and fills roughly half a cubic yard per layer. Dumpsters run out of weight allowance before they run out of space: a 20-yard bin usually includes about 2 tons, which is only around 16 squares of architectural tear-off. The calculator sizes the dumpster from both.

Mistakes that cost a second trip

• Entering living area instead of the footprint, or forgetting the overhangs (about 10% on a small house).
• Assuming every bundle covers 33⅓ sq ft, or that every line is three bundles per square.
• Ordering 20-in W-valley flashing for open valleys — the IRC calls for at least 24 in.
• Mixing roll sizes: GAF WeatherWatch is 150 sq ft per roll, StormGuard 200.
• Skipping the starter on the rakes — Owens Corning's 160 mph Duration rating and GAF's WindProven warranty both depend on it.

Putting on metal instead? Use the metal roofing calculator for panels, screws and trim.