Roofing Calculator: Squares, Shingle Bundles and Cost

Type the house footprint, pick the pitch and roof shape, and get the roof area in square feet and squares, how many bundles of shingles to buy, the underlayment and trim that go with them, and what a replacement costs installed.

Measure the house at the outside of the walls (or read the footprint off the tax record), add the eave overhang, and pick the pitch — the calculator multiplies the footprint by the slope factor for the true roof surface, converts it to squares (100 sq ft each) and bundles (three per square), and adds the underlayment, drip edge, ridge cap and nails that go with them. The cost rows apply 2026 installed price ranges per square for the material you pick, plus tear-off per existing layer. Switch the roof shape to raise the waste allowance for hips, valleys and dormers.

How many square feet is a roof, and what is a roofing square?

A roof’s area is the house footprint times a pitch multiplier, so an 1,800 sq ft single-story house at a 6/12 pitch has about 2,012 sq ft of roof, or 20.1 squares, before overhangs. A roofing square is 100 square feet of roof surface; roofers price and order in squares, and three bundles of shingles cover one square. Add 10–20% waste for cuts and you need 67 bundles for that house.

The multiplier exists because a sloped surface is longer than the floor beneath it. At 6/12 (the roof rises 6 in for every 12 in of run) every 12 ft of horizontal span is 13.4 ft of sloped roof, so the factor is 1.118. A flat roof has a factor of 1.0 and a 12/12 roof (45°) is 1.414. Two-story houses use the footprint of the top floor, not the total living area, which is why a 3,000 sq ft two-story colonial often has a smaller roof than a 2,000 sq ft ranch.

Worked example: 40 × 45 ft house, 6/12, 18-inch overhangs

1. Footprint: 40 × 45 = 1,800 sq ft.
2. Add 1.5 ft of overhang on every side: (40 + 3) × (45 + 3) = 43 × 48 = 2,064 sq ft.
3. Multiply by the 6/12 factor: 2,064 × 1.118 = 2,308 sq ft of roof, which is 23.1 squares.
4. Add 10% waste for a simple gable: 25.4 squares → 25.4 × 3 = 77 bundles.
5. Underlayment: 25.4 ÷ 10 squares per roll = 3 rolls. Drip edge: the 192 ft of eave and rake ÷ 10 ft = 20 pieces. Ridge cap: 48 ft, 2 bundles.
6. Cost in architectural shingles at $450–$700 per square on the 23.1 real squares: $10,384–$16,153, plus $2,308–$3,461 to tear off and dump one layer (about 2.9 tons of old shingles).

Roof pitch multiplier table

Pitch is written as inches of rise per 12 in of run. Measure it with a level and a tape in the attic (hold a 12-in level horizontal against a rafter and measure down to the rafter from the far end) or from a ladder at the eave. Most houses built after 1970 sit between 4/12 and 8/12; Cape Cods, Tudors and A-frames run 10/12 and up. The multiplier is √(1 + (pitch ÷ 12)²).

If you only have the footprint from the county record, use the 6/12 row for a typical ranch or colonial and 8/12 for a house with a visibly steep roof — then confirm with a tape before you order. Steep roofs are also where overhangs matter most: an 18-inch eave adds about 15% to the area of a 40 × 45 ft house at any pitch, and 24-inch overhangs add 20%.

How many square feet does a bundle of shingles cover?

A bundle of standard asphalt shingles covers 33⅓ square feet, so three bundles make a square. That is true of 3-tab and most architectural (laminated) shingles; a few heavyweight “designer” lines come four bundles to the square, which the wrapper states. A bundle of 3-tab holds 26 or 27 shingles (36 × 12 in each, 5-in exposure); architectural bundles hold 20–22 larger pieces. A bundle weighs 60–80 lb, so a 24-square roof is close to two tons of shingles delivered to the roof, usually by the supplier’s boom truck.

Starter strip, ridge cap and valley material are separate line items. A box or bundle of starter runs about 100 linear feet of eave; hip-and-ridge cap bundles cover 20–33 linear feet depending on the product, or you can cut 3-tab shingles into caps at roughly 33 ft per bundle. Figure 2–2½ lb of 1¼-in roofing nails per square (four nails per shingle; six in high-wind zones, which is 3 lb).

Order whole bundles and keep one unopened bundle from the same lot in the garage for repairs — dye lots shift between production runs and a patch from a different lot shows. Suppliers take back unopened bundles, which is cheaper than a second delivery fee for a shortfall.

Roof replacement cost per square by material

Installed prices are quoted per square and include labor, standard underlayment, drip edge, ridge cap and ordinary flashing. In 2026 an architectural asphalt roof runs $450–$700 a square nationally, which puts the typical 20–25-square house at $9,000–$17,500 before tear-off. Metal roughly doubles that and lasts two to three times as long; tile and slate are premium products that also need rafters sized for their weight.

The cheapest material is rarely the cheapest roof per year of service. On the example house, architectural asphalt at a midpoint of $13,268 over 27 years is about $491 a year; standing-seam metal at $32,306 over 55 years is $587 a year — close, and metal usually earns an insurance discount in hail states. What tips the decision is how long you will own the house: the roof replacement ROI calculator estimates how much of the cost a sale recovers (usually 50–65% for asphalt), and the rebuild cost calculator shows what the roof represents in a full replacement-cost insurance figure.

What is in a roofing quote (and what is missing)

A complete quote lists the squares, the shingle brand and line, the underlayment (synthetic is standard now; felt is cheaper and worse), ice-and-water shield at eaves and valleys (required by code in snow climates for the first 24 in inside the warm wall), drip edge, starter, ridge cap, pipe boots, step and counter flashing at walls and chimneys, ridge or box vents, nails, tear-off, disposal and permit. Compare quotes on those line items, not the bottom line.

• Tear-off. $100–$150 per square per layer. Codes allow at most two layers of asphalt; if there are already two, tear-off is not optional. A one-layer tear-off on 23 squares produces about 3 tons of debris, one 10-yard dumpster.
• Decking. Quoted as an allowance, typically $70–$120 per 4 × 8 sheet of OSB or plywood replaced. Rotten decking is only found after tear-off, so ask for the per-sheet rate up front and expect 2–6 sheets on a 30-year-old roof.
• Flashing and penetrations. Reusing old step flashing is the most common shortcut; new flashing at every wall and chimney adds $300–$1,200 but is where leaks start.
• Ventilation. A ridge vent runs $8–$15 per linear foot installed; bath and kitchen fans should exit the roof, not the attic.
• Skylights. Budget $800–$2,500 each to replace while the roof is open; reflashing an old one is cheaper now than a leak later.
• Warranty. Manufacturer “lifetime” warranties cover material defects (prorated after 10 years); the labor warranty from the installer, 2–10 years, is the one that pays.

Metal roof cost calculator: what changes

Set the material to standing-seam metal and the installed range moves to $1,000–$1,800 per square; exposed-fastener panels (corrugated or ribbed, screwed through the face) run $500–$900. Metal is sold by the panel and the linear foot rather than the bundle, so the shingle-bundle row no longer applies, but squares, underlayment and drip edge still do. Two things change the estimate for metal:

• Waste is lower on a simple roof and higher on a complex one. Panels are cut to length at the factory for a gable, so waste can drop under 10%; hips and valleys waste whole panel ends and push it past 20%.
• Overlay is often allowed. Many jurisdictions permit metal over one layer of asphalt with a synthetic underlayment, which removes the tear-off line (set tear-off to none to see it). It also keeps the old shingles as a sound barrier.

Standing seam needs a high-temperature underlayment ($40–$60 per square more than standard synthetic) and snow guards in snow country ($300–$800). Exposed-fastener screws have neoprene washers that need checking at 15–20 years. Both metal types weigh a third of asphalt, which is why metal is the usual choice when the rafters are undersized for a second layer or for tile.

Repair or replace: how to decide

Replace when the roof is within five years of the end of the material’s typical life, when more than about a quarter of the shingles show curling, bald spots or granule loss, when there are already two layers, or when leaks come from more than one area. Repair when the roof is under 15 years old, the damage is local (a few wind-lifted tabs, one pipe boot, a chimney flashing) and the rest of the field looks uniform. A repair runs $300–$1,500; a full replacement on a typical house, $9,000–$20,000.

Two numbers help with the timing. First, a new roof on an older house is largely a sale-price condition: buyers and their inspectors price a 20-year-old asphalt roof as a near-term replacement, so the seller either does it or discounts for it. Second, homeowner policies in hail and wind states increasingly pay actual cash value on roofs over 15–20 years old rather than replacement cost, so an old roof that is “fine” may be insured for a third of what it costs to replace. If you are weighing solar, the roof comes first: panels add 20–30 years of load and a removal-and-reinstall bill of $2,000–$5,000 when the roof under them is replaced — size the array with the solar panel calculator after you know the roof’s remaining life.

For the rest of the house, the renovation cost tool and the home values map put the roof in context: on a $400,000 house, a $14,000 roof is 3.5% of value, roughly the premium a documented new roof earns in a sale.