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Roofing Calculator
Use this free roofing calculator to estimate the materials for a new roof before you call a supplier. Enter the building footprint, the roof pitch, and the overhang, and it works out the roof area, the shingle bundles, the underlayment rolls, the drip edge length, and the estimated material cost.
One bundle covers about 33.3 sq ft (3 bundles per square). One underlayment roll covers 400 sq ft. Prices are editable — use your local supplier prices.
| Item | Amount |
|---|---|
| Roof area | 1,586 sq ft |
| Roofing squares (100 sq ft each) | 15.9 |
| Slope factor (6/12 pitch) | 1.118 |
| Shingle bundles (with waste) | 53 |
| Underlayment rolls (400 sq ft each) | 4 |
| Drip edge length | 160 ft |
| Estimated material cost | $2,845.00 |
How it works
A roof is bigger than the floor it covers, for two reasons. First, the roof hangs past the walls — the overhang that keeps rain off the siding. Second, the roof is sloped, and a sloped surface is always longer than the flat distance beneath it. This calculator accounts for both, so the area it gives is the real surface you have to shingle, not just the footprint of the house.
Start with the footprint: the length and width of the building as seen from above. Add twice the overhang to each side, because the roof extends past the wall on both ends. For a 40 by 30 foot building with a 1.5 foot overhang, the roof measures 43 feet long and 33 feet wide at the eaves. That adjusted rectangle is the base of the calculation.
Adjusted width = width + 2 × overhang
Slope factor = √(1 + (pitch ÷ 12)²)
Roof area = adjusted length × adjusted width × slope factor
Bundles = roof area × (1 + waste) ÷ 33.3, rounded up
Pitch is written as rise over run: a 6/12 pitch rises 6 inches for every 12 inches of horizontal run. The slope factor comes from that ratio. For a 6/12 pitch it is the square root of 1.25, about 1.118, so the roof surface is about 12 percent larger than the flat adjusted footprint. A low 3/12 pitch has a factor of about 1.031, while a steep 12/12 pitch has a factor of about 1.414 — the roof surface is more than 40 percent larger than the footprint.
Shingles are ordered by the bundle and the square. One roofing square is 100 square feet, and three bundles cover one square, so each bundle covers about 33.3 square feet. The calculator adds your waste allowance — 10 percent is standard for a simple gable roof, and 15 percent is safer for a roof with hips, valleys, dormers, or many cuts — then divides by 33.3 and rounds up, because a supplier will not sell part of a bundle.
Underlayment is the water-resistant layer under the shingles. One roll covers about 400 square feet, so the calculator divides the roof area by 400 and rounds up. Drip edge is the metal strip along the roof edges that guides water into the gutters; the estimate adds the two eave lengths to the sloped rake lengths around the perimeter. The cost line adds bundles, underlayment rolls, and drip edge at the prices you enter, and it covers materials only — labor, tear-off of the old roof, flashing, ridge caps, and deck repairs are separate and vary widely by location.
Worked example
You are re-roofing a house with a 40 by 30 foot footprint, a 6/12 pitch, and a 1.5 foot overhang. You choose architectural shingles at $45 per bundle, underlayment at $55 per roll, drip edge at $1.50 per foot, and a 10 percent waste allowance.
Step 1: Adjust for overhang — length 40 + 3 = 43 feet, width 30 + 3 = 33 feet.
Step 2: Apply the slope factor — √(1 + (6 ÷ 12)²) = √1.25 = 1.118, so roof area = 43 × 33 × 1.118 = 1,586 square feet, or 15.9 squares.
Step 3: Add waste and find bundles — 1,586 × 1.10 = 1,745 square feet to cover; 1,745 ÷ 33.3 = 52.4, so buy 53 bundles.
Step 4: Underlayment — 1,586 ÷ 400 = 3.97, so buy 4 rolls.
Step 5: Drip edge — 2 × 43 + 2 × 33 × 1.118 = 86 + 74 = 160 feet.
Step 6: Cost — 53 × $45 = $2,385 for shingles, plus 4 × $55 = $220 for underlayment, plus 160 × $1.50 = $240 for drip edge, for a total of $2,845.00.
The result is 1,586 sq ft of roof — 53 bundles, 15.9 squares, about $2,845.00 in materials. That is exactly what the calculator shows for these inputs.
FAQ
How many shingle bundles do I need for my roof?
A bundle of shingles covers about 33.3 square feet, so three bundles cover one roofing square of 100 square feet. Find your roof area, add about 10 percent for waste and cuts, then divide by 33.3 and round up. A 1,586 square foot roof with 10 percent waste needs about 53 bundles.
What is a roofing square?
A roofing square is 100 square feet of roof area. Roofers use it because it makes ordering simple: a 1,500 square foot roof is 15 squares. Shingles are sold by the bundle, with three bundles per square, and underlayment and labor are often priced per square too.
How does roof pitch change the roof area?
A steeper roof has more surface than a flat roof over the same footprint, because the sloped surface is longer than the flat distance it covers. A 6/12 pitch multiplies the area by about 1.12, an 8/12 pitch by about 1.20, and a 12/12 pitch by about 1.41. The calculator applies this slope factor automatically.
What is the difference between 3-tab and architectural shingles?
3-tab shingles are flat, single-layer shingles with a uniform look, and they usually cost less per bundle. Architectural shingles are thicker, layered shingles that look more dimensional and usually last longer, and they cost more per bundle. Both cover the same area per bundle, so the choice affects cost and appearance, not the bundle count.
How much does a new roof cost per square foot?
Materials alone often run $2 to $5 per square foot of roof, depending on the shingle type, while a full installed roof commonly runs $5 to $12 per square foot including labor, underlayment, and flashing. This calculator estimates materials only, so get a local quote for labor, tear-off, and any deck repairs.