Roofing Calculator

How much roofing material do I need?

Roofing is sold by the bundle or package and priced by the square, but a building is usually measured by its footprint, the flat area it covers on the ground. To get from one to the other you multiply the footprint’s length by its width, scale that up with a slope factor for your roof’s pitch to find the true sloped surface, add a waste allowance for cuts, then divide by what one bundle covers. The result is a whole bundle count for asphalt shingles, metal roofing, wood shakes, or tile.

Measure the footprint to the outer edge of the eaves and rakes, not to the walls, because the roofing runs out over the overhangs. For an L-shaped or multi-wing house, split the plan into rectangles, add them with the square footage calculator, and enter the total as length × 1. Sections with different pitches are best run separately and added together.

Formula & methodology

Slope factor = √(pitch² + 12²) ÷ 12, where pitch is the rise per 12 units of run. Roof area = footprint (length × width) × slope factor, plus a waste/cut allowance (default 10%, more for complex roofs with hips and valleys). Squares = that area ÷ 100 sq ft. In US mode you enter bundles per square, so each bundle covers 100 ÷ 3 = 33⅓ sq ft for a standard 3-bundle shingle. Bundles = area ÷ bundle coverage, rounded up, since the supplier won’t split a bundle for you.

The exact fraction matters. A rounded 33.3 sq ft per bundle turns one square into 3.003 bundles, which rounds up to 4. Working from bundles per square avoids that, so a 100 sq ft roof with no waste comes out at exactly 3 bundles.

Worked example: a 40 × 30 ft roof at 6/12

1. Footprint, measured to the drip edge: 40 ft × 30 ft = 1,200 sq ft.
2. Slope factor for a 6/12 pitch: √(6² + 12²) ÷ 12 = 1.118.
3. Sloped roof surface: 1,200 × 1.118 = 1,341.6 sq ft.
4. Add 10% waste: 1,341.6 × 1.10 = 1,475.8 sq ft, which is 14.76 squares.
5. Bundles at 3 per square: 1,475.8 ÷ 33⅓ = 44.27, so buy 45 bundles.

The same roof with a 4-bundle-per-square shingle needs 60 bundles, a third more bundles for the same area. These are the default values in the calculator above, so you can follow each step in “Show the math” and then swap in your own numbers.

Roof pitch multiplier table

Multiply the footprint by the factor for your pitch to get the sloped surface. The last column shows what a 1,000 sq ft footprint becomes, before waste.

Multiplier = √(rise² + 144) ÷ 12. Pitches below 4/12 usually need extra underlayment measures for asphalt shingles, and very low slopes call for a different roofing system; check the shingle manufacturer’s minimum slope.

Bundles vs squares

A square is a roofing unit of 100 sq ft of roof surface. Contractors quote and price jobs by the square, and shingle manufacturers state coverage per square. A bundle is the wrapped pack you actually carry up the ladder. Most standard asphalt shingles, both 3-tab and architectural, are packed 3 bundles to the square, but some heavier designer lines take 4 (GAF’s Woodland is one example), so the bundles-per-square field matters. The count is printed on the wrapper.

In metric mode the field becomes bundle coverage in m². One third of a square is about 3.1 m², which is the default, but metric-market products vary, so use the coverage printed on the pack. Results still show squares, because many suppliers outside the US quote that way too.

Roof pitch & metal roofing screws

Pitch is expressed as inches of rise per 12 inches of run — a 6/12 pitch rises 6 inches for every foot of horizontal run. The steeper the pitch, the more surface the roof has compared with its footprint: a 12/12 roof has about 41% more area than the ground it covers. You can find pitch from the attic by measuring the rise over a level 12-inch run along a rafter, or from the rake edge with a level and tape.

For metal roofing, select “Metal roofing” as the material to see an estimated screw count, based on a typical rate of about 120 screws per 100 sq ft at standard purlin spacing. Tighter purlin spacing in high-wind areas raises that number, so confirm it against your panel manufacturer’s fastening schedule. For metal panels, enter how many panels or packages cover one square in the bundles-per-square field.

Underlayment, ridge cap, starter strip & drip edge

The bundle count covers the field of the roof only. A complete shingle job needs a few more items, and most of them are measured differently:

• Underlayment goes over the whole deck and is sold in rolls. Divide your roof area (the squares figure above) by the coverage printed on the roll label, since roll sizes differ between felt and synthetic products. Ice-and-water barrier along eaves and valleys is sold separately.
• Ridge cap shingles cover the ridges and hips, so you need the total linear feet of every ridge and hip line. Each ridge-cap bundle lists how many linear feet it covers; divide and round up.
• Starter strip runs along the eaves, and often the rakes too, to seal the first course. Measure those edges in linear feet and check the per-bundle length on the product.
• Drip edge is metal flashing along the eaves and rakes, sold in fixed-length pieces. Total the perimeter edges and allow for overlap at the joints.

If you’re repainting fascia, soffits, or trim while the scaffolding is up, the paint calculator can size that job from the same measurements.

Roof replacement cost

An asphalt shingle roof replacement typically runs about $4–$8 per sq ft installed, or roughly $400–$800 per square (rough 2026 US ranges; prices vary by region, so get local quotes). Steep or complex roofs and tearing off old layers push it higher. For the 1,342 sq ft roof in the worked example, that suggests roughly $5,400–$10,700 installed.

The price field in the calculator only multiplies bundles by what you pay per bundle, so it gives a materials figure. Labor, underlayment, flashing, ridge cap, disposal, and permits are on top of that, which is why a contractor’s per-square quote is the better number for budgeting a full replacement.

Tips for roofing

• Increase waste to 15–20% for roofs with lots of hips, valleys, or dormers, since these add cuts and offcuts a simple rectangle doesn't need.
• Check your local building code for minimum pitch requirements and whether a permit or inspection is needed.
• Keep a bundle or two of the same shingle after the job for future repairs — matching an older color is hard once a line is discontinued.
• Get a professional inspection before a full re-roof — hidden deck damage or an extra layer to tear off can change material and labor needs significantly.
• Planning a gravel strip under the drip line to stop splash-back? The gravel calculator converts its length, width, and depth into tons.