Roofing Calculator

How to use the roofing calculator

1. Keep Imperial selected (feet and inches). Switch to Metric if your plans are in meters.
2. Enter the length and width of the building footprint, measured along the outside walls. A tape measure on the ground is enough.
3. Enter the roof pitch as inches of rise per 12 inches of horizontal run. A 6/12 roof rises 6 inches for every foot. If you don't know it, hold a level and a tape against a rafter in the attic, or use a pitch gauge app.
4. Enter the overhang: how far the eaves and rakes stick out past the walls, out to the drip edge.
5. Set the waste allowance. The default is 10%; raise it for hips, valleys and dormers.
6. Under Advanced options, change bundles per square if your shingle wrapper says something other than 3.

You get the roof area with and without waste, the number of squares and bundles, the slope factor and the pitch in degrees.

How roof area is calculated

The calculator adds the overhang to all four sides of the footprint, then multiplies by a slope factor that turns flat plan area into sloped surface area:

• Plan area = (length + 2 × overhang) × (width + 2 × overhang)
• Slope factor = √(1 + (pitch ÷ 12)²)
• Roof area = plan area × slope factor
• Squares = roof area × (1 + waste %) ÷ 100
• Bundles = squares × bundles per square, rounded up to a whole bundle

Every plane with the same pitch is tilted by the same amount, so this works for gable and hip roofs alike, as long as all planes share one pitch. If your roof mixes pitches or has big dormers, run each section separately and add them up.

A roofing square is 100 sq ft of roof surface. According to GAF, most standard asphalt shingles, both 3-tab and architectural, come three bundles to the square, while some specialty lines need four or five. The coverage printed on the wrapper is what counts.

Roofing estimate examples

40 × 30 ft ranch, 6/12 pitch

With a 1 ft overhang the plan area is 42 × 32 ft. At a slope factor of 1.118, the roof area is 1,502.6 sq ft. Adding 10% waste gives 1,652.9 sq ft, or 16.53 squares, which rounds up to 50 bundles at three per square.

52 × 28 ft two-story, 8/12 hip roof

With 1.5 ft overhangs and 15% waste for the hips and valleys, the roof area is 2,049.2 sq ft, or 2,356.5 sq ft with waste. That is 23.57 squares and 71 bundles. The pitch works out to 33.7°.

24 × 20 ft detached garage, 4/12

With a 1 ft overhang and 10% waste, the roof area is 602.9 sq ft, or 663.2 sq ft with waste. That's 6.63 squares and 20 bundles. At 4/12 you are at the edge of the low-slope range, where the residential code requires double underlayment.

What the estimate leaves out

The bundle count covers the field shingles. Most jobs also need:

• starter strip along the eaves and rakes, and ridge cap shingles for ridges and hips, sold by the linear foot;
• underlayment, ice and water barrier where your code calls for it, drip edge and flashing;
• roofing nails and vents.

Check the result against a real measurement before you order, and keep the receipt so unopened bundles can go back. If you are tearing off several old layers, switching to a heavier material, or see sagging or rotten decking, have a licensed roofer or a structural engineer look at the framing first. Roof work is dangerous; use fall protection or hire a professional.