About this calculator
Roofing is measured in squares — 100 square feet of roof surface — and shingles are sold in bundles, usually three to a square. This calculator estimates roof area from something you can measure on the ground: the building’s footprint including the overhangs. It multiplies that by a pitch factor to account for the slope, then converts the result into squares, bundles and underlayment rolls.
The bundle count isn’t the whole order. Choose gable or hip and the calculator also measures the roof’s edges and lines: starter strip along the eaves (and rakes on a gable), hip and ridge cap along the ridge and hips, and drip edge in 10 ft pieces. The drawing shows the roof from above and from the end, to scale, with each of those runs, the ridge height and the slope length, and the cost comes from your local prices.
The pitch-factor method is exact for any roof whose planes all share the same pitch, whether it’s a gable, hip or a combination.
How to use it
- Choose the roof shape and measure the footprint of the roof: the building’s length along the ridge and its width, plus the overhangs on each side.
- Choose the roof pitch (rise in inches per 12 in of run). Use the roof pitch calculator if you don’t know it.
- Set the waste allowance: about 10% for simple gables, 15% for hip roofs and roofs with valleys and dormers.
- Pick bundles per square from the shingle packaging, and open the local prices to see the cost of the shingles and extras.
The formula
Pitch factor = √(1 + (pitch ÷ 12)²)
Roof area = footprint area × pitch factor
Squares = roof area × (1 + waste%) ÷ 100
Bundles = squares × bundles per square, rounded up
Gable: rakes = 4 × (width ÷ 2) × pitch factor, ridge = length
Hip: ridge = length − width, each hip = (width ÷ 2) × √(2 + (pitch ÷ 12)²)
Starter bundles = (eaves + rakes) ÷ 105 ft
Ridge cap bundles = (ridge + hips) ÷ 33 ft
Drip edge pieces = (eaves + rakes) ÷ 9 ft 10 in
A 6/12 roof has a pitch factor of 1.118, so every square foot of footprint holds 1.118 sq ft of roof. On a gable the eaves are the two long edges and the rakes are the four sloped edges of the gable ends. On a hip roof all four edges are eaves, and the four hips run from the corners up to the ends of a ridge that is shorter than the building by its width.
The coverage figures are Owens Corning’s: about 105 lineal feet per bundle of Starter Strip Plus, and 33 ft per bundle of ProEdge hip and ridge shingles. Other brands differ, so check the bundle label. Drip edge comes in 10 ft pieces, and the code laps each piece at least 2 in over the next. Below 4/12 the code asks for two layers of underlayment, so the roll count doubles.
Worked example
These are the calculator’s starting values and the results they produce:
| Roof shape | Gable (two slopes, ridge along the length) |
|---|---|
| Footprint length (incl. overhangs) | 40 ft |
| Footprint width (incl. overhangs) | 30 ft |
| Roof pitch | 6/12 |
| Extra for waste | 10 % |
| Bundles per square | 3 (most architectural & 3-tab) |
| Underlayment roll coverage | 1000 sq ft |
| Shingles, per bundle | $40.00 |
| Starter strip, per bundle | $55.00 |
| Hip and ridge cap, per bundle | $70.00 |
| Underlayment, per roll | $100.00 |
| Drip edge, per 10 ft piece | $9.00 |
Tips before you buy
- Roof work is dangerous. Use fall protection and consider hiring a licensed roofer for steep or high roofs.
- Check local code for how many shingle layers are allowed — many areas permit only one or two.
- Order a few extra bundles from the same lot; color can vary between production runs.
- Ice-and-water shield is typically required along eaves and in valleys in cold climates — measure those runs separately.
- Valleys, dormers, chimneys and skylights add flashing and cuts that this footprint method can’t see. Raise the waste and measure those parts on their own.
Frequently asked questions
How many bundles of shingles do I need for 1,000 square feet?
1,000 sq ft of roof is 10 squares. With 10% waste and 3 bundles per square, that’s 33 bundles.
What is a roofing square?
A roofing square is 100 square feet of roof surface. Shingles are usually packaged so that three bundles cover one square.
How do I calculate the area of my roof?
Multiply the building’s footprint (including overhangs) by the pitch factor. For a 40 × 30 ft footprint with a 6/12 pitch, that’s 1,200 × 1.118 = about 1,342 sq ft of roof.
How much starter strip, ridge cap and drip edge does a 40 × 30 ft gable roof need?
With a 6/12 pitch the roof has 80 ft of eaves and about 67 ft of rakes, so 2 bundles of starter strip at 105 ft per bundle and 15 pieces of 10 ft drip edge. The 40 ft ridge takes 2 bundles of ridge cap at 33 ft per bundle. With 45 bundles of shingles and 2 rolls of underlayment, that’s about $2,385 at typical prices.
How long are the hips on a hip roof?
Each hip runs from a corner to the ridge: half the width times √(2 + (pitch ÷ 12)²). On a 40 × 30 ft hip roof at 6/12 that’s 15 × 1.5 = 22.5 ft, so the four hips are 90 ft and the ridge is 10 ft: 100 ft of cap, 4 bundles at 33 ft each.
Sources
Areas and volumes are standard geometry. The published figures this calculator relies on come from these sources; other defaults (waste allowances, typical densities and prices) are common trade figures worth checking with your supplier.
- Owens Corning: DIY roofing guide: 3 bundles of shingles per roofing square for most shingles
- Owens Corning Starter Strip Plus starter shingles: about 105 lineal ft of eave and rake per bundle of starter strip
- Owens Corning ProEdge Hip & Ridge Shingles data sheet: 33 lineal ft per bundle of hip and ridge shingles
- International Residential Code 2021, Chapter 9 (R905, roof coverings): shingle slope limits, two layers of underlayment below 4/12, drip edge at eaves and rakes lapped 2 in, and the ice barrier
- OSHA 29 CFR 1926.501, fall protection: fall protection when working on a roof