Roofing Calculator
Estimate roofing squares and three-tab or architectural shingle bundles from the building footprint and roof pitch.
Estimates are for planning. Confirm quantities against your measured site and product packaging before ordering, and follow local building codes.
How to measure for a roof
You don’t need to crawl over every plane with a tape. Start with the footprint: measure the length and width of the building at ground level, then add the overhangs, since shingles cover the eaves and rakes, not just the walls. A 40×30 house with 1-foot overhangs all around has a 42×32 footprint, or 1,344 square feet.
Next, find the pitch. From the attic, set a level horizontally against the underside of a rafter, mark 12 inches out, and measure straight up from that mark to the rafter. If you measure 6 inches, you’ve got a 6/12 roof. No attic access? Hold a level and a speed square against the rake board on a gable end, or use a phone level app from a ladder at the eave.
For roofs with multiple sections, measure each footprint separately, note each section’s pitch, and add the results. Sketch the roof from above first. Five minutes with paper saves you from forgetting the porch roof.
The formula and what it assumes
The calculator converts your flat footprint into actual sloped roof area using a pitch multiplier: multiplier = √(1 + (rise/12)²). For a 6/12 roof that’s √(1 + 0.25) = 1.118, so 1,344 square feet of footprint becomes 1,503 square feet of roof surface. A steep 12/12 roof multiplies by 1.414, which is why steep roofs eat so much more material than they look like they should.
Roof area divided by 100 gives squares, the unit roofers price in. The calculator figures 3 bundles per square, the standard for three-tab and common architectural shingles, then adds 10% waste by default. Waste covers cut shingles at rakes, ridges, and edges, plus the starter course.
Run the example through: 1,503 square feet plus 10% is 1,653, which is 16.5 squares, times 3 is 49.6, so 50 bundles.
Two honest limitations. First, valleys and hips generate more cut waste than straight gables, so bump the waste setting to 15% on a cut-up roof. Second, this estimates shingles only. Underlayment, ice and water shield, drip edge, ridge caps, starter strips, nails, and flashing are all separate purchases, and on a full reroof they add up to a meaningful fraction of the shingle cost.
Buying tips
Shingles sell by the bundle at every US supplier, three bundles to the square for standard products. Bundles weigh roughly 60 to 80 pounds depending on the shingle, so 50 bundles is around 3,500 pounds. That matters twice: your truck can’t haul a whole roof in one trip, and your garage floor would rather you didn’t try to store it all.
For anything beyond a shed, get the order delivered. Suppliers and big-box stores both deliver, and roofing suppliers often offer rooftop delivery, where a boom truck places bundles along the ridge. Paying for rooftop delivery beats carrying 70-pound bundles up a ladder fifty times, and most pros consider it non-negotiable.
Check lot numbers when the order arrives. Shingle color varies slightly between production runs, and mixing lots on one roof plane can show as visible banding. Pros confirm all bundles share a lot number before the truck leaves.
Buy a bundle or two beyond the calculation and keep the extras. Future repairs from the same lot will match; shingles bought three years later won’t. Unopened bundles are usually returnable anyway, so overage costs you a receipt and a trip, not money.
Don’t forget the tear-off
If you’re stripping an old roof, plan disposal before you order. Asphalt shingles are heavy, around 200 to 350 pounds per square for a single layer, and most municipalities want them in a rented dumpster, not the weekly trash. Also check local code on layering: many jurisdictions allow a second shingle layer over the first but prohibit a third, and tearing off is almost always the better roof even where layering is legal.
Frequently asked questions
How many bundles of shingles do I need per square?
Three bundles cover one roofing square (100 square feet) for standard three-tab and most architectural shingles. Each bundle covers about 33 square feet. Some premium and specialty shingles pack four or more bundles per square, so check the wrapper before you multiply.
What does roof pitch mean and how do I find mine?
Pitch is rise over run: a 6/12 roof rises 6 inches for every 12 inches of horizontal distance. From inside the attic, hold a level horizontally against a rafter, measure 12 inches along the level, then measure straight up to the rafter. That vertical measurement is your rise. You can also eyeball it from a smartphone level app against a gable end.
Can I estimate my roof without climbing on it?
Yes. Measure the building's footprint from the ground, including eave and rake overhangs, then multiply by the pitch multiplier for your slope. A 6/12 roof multiplies the footprint by 1.118. It won't be surveyor-accurate on complex roofs, but it gets you within ordering range for a simple gable or hip.
Does this estimate include underlayment and flashing?
No. The calculator estimates shingles only. You'll still need underlayment for the full deck area, ice and water shield where your local code requires it (common along eaves in cold climates), drip edge, ridge caps, starter strips, and flashing at valleys, walls, and penetrations. Budget those as separate line items.