How to use it
- Pick gable or hip. On a gable roof, the length is the side the ridge runs along.
- Enter the house length and width in feet, measured at the outside of the walls. Then set the overhang, the distance the roof sticks out past the walls. If your plans are in meters, switch Units to Metric first; the roof area and edges then show in m² and m.
- Tap a pitch or type your own. Decimals like 7.5 work.
- Set the waste factor and bundles per square, and pick your underlayment.
- To price the job, type the prices from your store or supplier in the cost section, plus labor if you're hiring it out.
Results update as you type. Copy result gives you a plain-text material list, and the page address saves your numbers so you can bookmark the job.
What a square is
Roofers buy and price shingles by the square, which is 100 square feet of roof. Shingles come in bundles, and most architectural shingles take 3 bundles to make a square. Some heavier designer shingles take 4 or more, so read the wrapper and change the bundles-per-square box to match.
Finding the pitch safely
Pitch is how many inches the roof rises for every 12 inches it runs level. A 6/12 roof climbs 6 inches per foot. You don't need to climb on the roof to find it.
- From the attic: hold a level against the bottom of a rafter, mark the level 12 inches from where it touches, and measure straight up from that mark to the rafter. That number is your rise.
- From the ground: stand back from a gable end and line up a pitch gauge, or a phone app that reads angles from a photo, with the slope of the roof edge. It's close enough to pick the right chip, and you can confirm it in the attic.
If your reading is in degrees, 18.4° is 4/12, 26.6° is 6/12, 33.7° is 8/12, 39.8° is 10/12 and 45° is 12/12.
Older plans and permit drawings often list the pitch too.
The formula
Pitch factor = √(1 + (Rise ÷ 12)²)
Roof area = (Length + 2 × Overhang) × (Width + 2 × Overhang) × Pitch factor
Squares = Roof area ÷ 100
Bundles = Squares × (1 + Waste % ÷ 100) × Bundles per square, rounded up
Hip length (each) = Half the short side × √(2 + (Rise ÷ 12)²)
The pitch factor turns the flat footprint into the sloped surface you actually shingle. It works for gable roofs and for hip roofs where every slope has the same pitch. On a gable roof the eaves run the length, the four rakes climb the gable ends and the ridge runs the full length. On a hip roof every edge is an eave, the ridge is the long side minus the short side, and four hips run from the corners up to the ridge. Drip edge covers the eaves and rakes, ridge cap covers the ridge and hips, and every count rounds up.
Worked example: a 40 ft × 30 ft house, 12 in overhang, 6/12 pitch
- Roof outline: 42 × 32 = 1,344 sq ft level. Pitch factor √1.25 = 1.118, so the roof is 1,502.6 sq ft, or 15.03 squares
- As a gable with 10% waste: 16.53 squares × 3 = 49.59, so 50 bundles
- Synthetic underlayment at 1,000 sq ft a roll: 2 rolls (or 4 rolls of #15 felt)
- Eaves 84 ft plus rakes 71.6 ft = 155.6 ft, so 16 pieces of 10 ft drip edge and 2 bundles of starter
- Ridge 42 ft, so 2 bundles of ridge cap at 25 ft each
Built as a hip roof with 15% waste, the area is the same, but that's 17.28 squares and 52 bundles. The ridge is 42 − 32 = 10 ft, and each hip is 16 × 1.5 = 24 ft, so ridge cap covers 106 ft: 5 bundles. All 148 ft of edge is eave, so that's 15 pieces of drip edge.
Common mistakes
- Leaving out the overhang. A foot of overhang on a 40 × 30 house adds 144 sq ft of level area before the pitch is even counted.
- Using too little waste on a cut-up roof. Every valley, hip and dormer means cut shingles you can't reuse. Raise the waste for those roofs.
- Forgetting the extra layers on a low slope. Below 4/12, manufacturers want more underlayment protection, which can double what you buy.
- Pricing only the shingles. Starter, ridge cap, drip edge, underlayment, nails, vents and flashing add up. Tear-off and a dumpster often cost more than people expect.
Questions people ask
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 that's 11 squares, or 33 bundles at 3 bundles per square. Make sure the 1,000 is the roof area, not the floor area of the house.
My house isn't a simple rectangle. What do I do?
Break it into rectangles, run each one, and add the results. Where two roofs meet, you'll have valleys, so use a higher waste factor. For an L-shaped or cut-up roof, this gets you a solid budget number, but measure the roof itself before you order.
Should I use synthetic underlayment or felt?
Both are common. Synthetic is lighter, covers more per roll and holds up better if the roof sits open for a few days. Felt costs less per roll. Many shingle warranties name approved underlayments, so check yours.
Do I need ice-and-water membrane?
In cold areas where ice dams form, building codes usually require an ice barrier along the eaves. It's also common in valleys and around chimneys. It isn't figured here, so ask your local building department what applies.
How do roofers charge for labor?
Most price by the square, often with a separate charge for tearing off old layers, plus disposal. Steep or tall roofs usually cost more per square. Type your quote in the labor boxes above to see the full job cost.