Three bundles of a standard architectural shingle cover 98.4 square feet of roof, not 100. Owens Corning prints that figure for its Duration and Oakridge lines, and it means a 1,409 square foot roof needs 43 bundles before waste, or 48 once a 10% allowance is added. To get your own count, find the sloped roof area (plan area times the pitch multiplier), divide by the coverage per bundle printed on the wrapper, add waste for your roof shape, and round up. Starter strip and ridge cap are separate products, counted by the linear foot.
Every step of that can be checked with a tape measure and the product page. The tables below are built from the right-triangle geometry of a roof and from Owens Corning's own product data, checked October 30, 2026. Owens Corning is used because it publishes full coverage figures for each line on open product pages; it is cited as published data, not as a recommendation.
Sloped area: the number most estimates get wrong
A roof's plan area is what you would see from directly above: the length times the width of the roof outline, including the overhangs. The sloped area is the surface the shingles actually lie on, and it is always larger, because every rafter is the long side of a right triangle.
Pitch is written as rise over a 12 inch run. A 6/12 roof climbs 6 inches for every 12 inches it travels horizontally. The sloped length of that run is the hypotenuse: the square root of 12 squared plus 6 squared, or 13.416 inches. Divide by 12 and you have the pitch multiplier, 1.1180. In general:
The same idea gives the length of a hip or valley. A hip runs diagonally across the plan, so in plan it is longer than the common rafter run by the square root of 2, and it climbs the same height. Its sloped length, per foot of common-rafter run, is the square root of 2 plus (rise/12) squared.
Here are both factors for the pitches asphalt shingles are normally laid on, computed from those two formulas and rounded to four places.
| Pitch | Angle | Pitch multiplier | Area added over plan | Hip/valley factor (per ft of common run) | Sloped sq ft per 1,000 sq ft of plan |
|---|---|---|---|---|---|
| 3/12 | 14.0° | 1.0308 | 3.1% | 1.4361 | 1,031 |
| 4/12 | 18.4° | 1.0541 | 5.4% | 1.4530 | 1,054 |
| 5/12 | 22.6° | 1.0833 | 8.3% | 1.4743 | 1,083 |
| 6/12 | 26.6° | 1.1180 | 11.8% | 1.5000 | 1,118 |
| 7/12 | 30.3° | 1.1577 | 15.8% | 1.5298 | 1,158 |
| 8/12 | 33.7° | 1.2019 | 20.2% | 1.5635 | 1,202 |
| 9/12 | 36.9° | 1.2500 | 25.0% | 1.6008 | 1,250 |
| 10/12 | 39.8° | 1.3017 | 30.2% | 1.6415 | 1,302 |
| 11/12 | 42.5° | 1.3566 | 35.7% | 1.6853 | 1,357 |
| 12/12 | 45.0° | 1.4142 | 41.4% | 1.7321 | 1,414 |
Two things in that table catch people out. First, the penalty grows faster than the pitch. Going from 3/12 to 6/12 adds about 9 points of area; going from 9/12 to 12/12 adds more than 16. Second, at a 12/12 pitch the roof has 41% more surface than the floor plan suggests. Order shingles for a steep roof from the house's footprint and you will be short by roughly 4 bundles in every 10.
Measuring the plan area
You do not need to climb the roof to get the plan area. Measure the outside walls of the house at ground level, then add the overhang on each side. A 40 by 28 foot house with 12 inch overhangs all round has a roof outline of 42 by 30 feet, or 1,260 square feet of plan. Leaving out the overhangs on that house would drop 140 square feet of plan, about 156 square feet of 6/12 roof, or nearly 5 bundles.
For a house with an L shape, an attached garage or a porch roof at a different pitch, split the outline into rectangles, find the plan area of each, and multiply each one by its own pitch multiplier. The square footage calculator handles the rectangles; the pitch factors come from the table above.
Finding the pitch without a ladder
Pitch can be read from inside the attic or at a gable end. Hold a level horizontally against the underside of a rafter, mark 12 inches along the level from the point where it touches, and measure straight up from that mark to the rafter. That vertical distance in inches is the rise. A 6 inch reading is a 6/12 roof.
If you can photograph the gable end square on, the angle can also be measured from the photo and looked up in the angle column. And if you know any two sides of the triangle formed by a rafter, its run and its rise, the triangle calculator returns the angle and the third side.
Why roof shape does not change the area
A hip roof and a gable roof over the same rectangle at the same pitch have exactly the same sloped area. That surprises people, because a hip roof looks like more roof. It is not: every facet of a roof at a single pitch is tilted by the same angle, so every square foot of plan becomes the same number of square feet of slope, whatever the outline of the facet. The worked examples below show both roofs over a 42 by 30 foot plan coming out at 1,408.7 square feet.
What does change with shape is the waste, the starter, and the ridge and hip cap. A hip roof cuts shingles along four diagonal edges instead of two straight rakes, it has eaves on all four sides, and it has hips as well as a ridge to cap. Those are counted separately, below.
How many bundles make a square
A roofing square is 100 square feet of roof. Most shingles are sold so that three bundles make one square, but "three bundles per square" is a packaging convention, and the actual coverage is set by the size of the shingle and how much of it is left exposed to the weather.
Each shingle covers only its exposed strip: its length times its exposure. Everything above that strip is hidden under the next course. Owens Corning's product data for its Duration line lists a nominal size of 13 1/4 by 39 3/8 inches and an exposure of 5 5/8 inches. The exposed face of one shingle is 39.375 times 5.625, or 221.5 square inches, which is 1.538 square feet. The data lists 64 shingles per square, and 64 times 1.538 is 98.4 square feet. That is the coverage per square Owens Corning prints, and the arithmetic lands on it exactly.
The 3-tab Supreme shingle is a different size, so its square really is 100 square feet: 12 by 36 inches with a 5 inch exposure gives 1.25 square feet per shingle, and 80 shingles per square gives 100.
Here is what Owens Corning's US product pages list for five of its shingle lines.
| Owens Corning line (type) | Nominal size | Exposure | Shingles per square | Bundles per square | Coverage per square | Coverage per bundle |
|---|---|---|---|---|---|---|
| Supreme (3-tab) | 12 x 36 in | 5 in | 80 | 3 | 100 sq ft | 33.3 sq ft |
| Oakridge (architectural) | 13 1/4 x 39 3/8 in | 5 5/8 in | 64 | 3 | 98.4 sq ft | 32.8 sq ft |
| TruDefinition Duration (architectural) | 13 1/4 x 39 3/8 in | 5 5/8 in | 64 | 3 | 98.4 sq ft | 32.8 sq ft |
| TruDefinition Duration FLEX (specialty) | 13 1/4 x 39 3/8 in | 5 5/8 in | 64 | 3 | 98.4 sq ft | 32.8 sq ft |
| Duration Premium (architectural) | 13 1/4 x 39 3/8 in | 5 5/8 in | 64 | 4 | 98.4 sq ft | 24.6 sq ft |
The coverage per bundle column is the coverage per square divided by the bundles per square. It is the number to divide your roof area by.
The 98.4 versus 100 gap
On a small roof the difference between 98.4 and 100 square feet is a rounding error. On a large one it is not. A 3,000 square foot roof is 30 squares if a square is 100 square feet, which is 90 bundles. At 32.8 square feet per bundle the same roof needs 91.5 bundles, so 92. Estimating with 100 square feet per square, a convention many calculators use (ours included, more on that below), leaves you about 1.6% short before any waste is added.
Heavier lines packed four to a square
Duration Premium covers the same 98.4 square feet per square as standard Duration, with the same shingle size and the same 64 shingles, but Owens Corning packs it four bundles to the square. That makes each bundle cover 24.6 square feet. If you estimate a four-bundle product at three bundles per square, you will be short by a quarter of the field shingles, which on the 1,409 square foot roof below is about 14 bundles.
The packaging is not something you can infer from the product type: both lines in that pair are architectural shingles of the same size. The only safe source is the bundle wrapper or the product data for the exact line you are buying.
Reading the bundle wrapper
Shingle wrappers print coverage in one of a few ways: "3 bundles cover 98.4 sq ft", "33.3 sq ft per bundle", "bundles per square: 4", or a count of shingles per bundle with the exposure. Any of them can be turned into coverage per bundle:
- If the wrapper gives coverage for a set of bundles, divide that area by the number of bundles.
- If it gives shingles per bundle and the shingle size and exposure, multiply the shingle length by the exposure (in inches), divide by 144, and multiply by the shingles in the bundle.
Owens Corning's installation instructions for the Duration series also note that bundles should be laid flat on the roof and not bent over the ridge, which is worth knowing when you are deciding where the delivery truck should land them.
Waste: a stated allowance, not a measurement
Every roof loses some shingle to cutting. Pieces are trimmed at the rakes, along hips and valleys, around chimneys, vents and skylights, and at the top course under the ridge. Some offcuts can be reused across the roof; many cannot.
No shingle manufacturer page we cite publishes a waste percentage, and the right allowance depends on the roof and on the crew. The figures in this table are planning assumptions used by this page, not sourced values. Treat them as starting points and move up or down for your roof.
| Roof shape | Planning waste allowance (assumption) | Why it is higher or lower |
|---|---|---|
| Simple gable, two planes, few penetrations | 5% to 10% | Cuts happen mostly at two rakes, and the offset pattern reuses some cut pieces |
| Hip roof, four planes | 10% to 15% | Angled cuts on both sides of all four hips, instead of square cuts at two rakes |
| Cut-up roof: dormers, several valleys, multiple pitches | 15% to 20% | Every valley and dormer adds diagonal cuts and closed valleys add overlap |
That last row has a sourced reason behind it. Owens Corning's Duration instructions for a closed-cut valley say to lay the shingles on one side of the valley across its center line by at least 12 inches before cutting the other side back to a chalk line 2 inches from center. Roughly a foot of overlap along the full length of every valley is shingle that does no work on the field, and it has to come from the bundle count.
The offset pattern is also why a gable can run lean. In the Duration instructions each new course starts with a shorter first piece: 6 1/2 inches removed for the second course, 13 for the third, then 19 1/2, 26 and 32 1/2, before the pattern repeats. The pieces cut off at one rake are often long enough to start a course at the other.
What each extra 5% costs
Waste is multiplied onto the whole field count, so its effect is easy to see. For the 1,409 square foot roof in the examples below, at 32.8 square feet per bundle:
| Waste allowance | Bundles before rounding | Bundles to order |
|---|---|---|
| 0% | 42.95 | 43 |
| 5% | 45.10 | 46 |
| 10% | 47.24 | 48 |
| 15% | 49.39 | 50 |
| 20% | 51.54 | 52 |
Each 5 points of waste is a little over 2 bundles on a roof of this size. That is why it pays to choose the allowance from the roof's shape rather than applying 15% to everything.
Returns and leftovers
Unopened bundles are often returnable to the store that sold them, which makes rounding up cheap insurance, but return policies are the seller's, so check before you order. Keep two or three spare bundles from the same lot regardless. Shingle color varies between production runs, and a storm repair a few years later is easier with shingles from your own roof's lot.
Starter strip: count the eaves and, often, the rakes
The first course on the eaves needs a starter strip under it, with its sealant along the lower edge, so that the gaps between the first-course shingles are not open to the deck. Starter is sold by the linear foot of edge it covers, not by area.
Owens Corning's Starter Strip Plus product data lists about 105 linear feet per bundle (32 pieces, each 7 3/4 by 39 3/8 inches) and says it is for use with any shingle that has an exposure up to 6 inches. The Duration installation instructions call for an Owens Corning starter product along the eaves, overhanging the drip edge by 1/4 to 3/4 inch along both the rakes and the eaves, with 6 1/2 inches trimmed off the end of the starter course at the rake or hip. The Duration product page notes that its 160 mph wind rating applies when starter shingles are installed on both the eaves and the rakes.
So the starter count is:
Rake length is a sloped length. Multiply each rake's horizontal run by the pitch multiplier. A gable end with a 15 foot run on each side, at 6/12, has two rakes of 16.77 feet each.
A gable roof has eaves on two sides and four rakes. A full hip roof has eaves on all four sides and no rakes at all, so its starter is the whole perimeter.
Ridge and hip cap: counted by linear foot
Ridge and hip cap shingles fold over the peak and the hips and come in their own bundles or cartons. Count every ridge and every hip in feet, add them, and divide by the coverage on the carton.
Owens Corning's DuraRidge hip and ridge product data gives a nominal size of 12 by 10 5/8 inches, an 8 inch exposure, 30 pieces per carton and 20 linear feet per carton. The arithmetic checks: 30 pieces at 8 inches each is 240 inches, or 20 feet. Owens Corning lists DuraRidge with limited availability, and other cap products are packed differently, so take the linear feet from whichever carton you are buying.
Ridge length on a gable is simply the length of the roof. On a hip roof it is shorter: the length of the roof minus its width, when all four sides are at the same pitch. The hips are the long part. Each one is the common-rafter run times the hip factor from the pitch table.
Cutting cap from 3-tab shingles
Some 3-tab installations use field shingles cut into thirds as cap, instead of a dedicated cap product. The arithmetic for that depends on the exposure the shingle maker specifies for its cap pieces, which we have not sourced here. As an illustration only: if each 12 inch piece were laid with the same 5 inch exposure as the Supreme field, one 36 inch shingle would cap 15 inches of ridge, and an 80-shingle square would cap 100 feet. Check the maker's instructions for the actual cap exposure before relying on that figure. Architectural shingles are not cut for cap, which is one reason the dedicated products exist.
A gable roof, worked through
Take a 40 by 28 foot single-storey house with a simple gable roof running the long way, 12 inch overhangs all round and a 6/12 pitch. The shingle is a three-bundle architectural line at 32.8 square feet per bundle. Inputs and assumptions are stated for each step, and the arithmetic is ours.
- Plan area. The roof outline is 42 by 30 feet, so the plan area is 1,260 square feet.
- Sloped area. 1,260 times the 6/12 multiplier of 1.1180 is 1,408.7 square feet.
- Field bundles before waste. 1,408.7 divided by 32.8 is 42.95 bundles.
- Waste. A simple gable with a chimney and a couple of vents gets the 10% planning allowance from the waste table. 42.95 times 1.10 is 47.24, so 48 bundles.
- Starter. The eaves are the two 42 foot sides, or 84 feet. Each gable end has two rakes with a 15 foot run; at 6/12 each is 16.77 feet, so the four rakes total 67.1 feet. Eaves plus rakes is 151.1 feet. At 105 feet per bundle that is 1.44, so 2 bundles of starter.
- Ridge cap. One ridge, 42 feet long. At 20 feet per carton that is 2.1, so 3 cartons.
If the same roof were laid in a four-bundle line at 24.6 square feet per bundle, step 3 would give 57.26 bundles and step 4 would give 62.99, so 63. The starter and ridge counts would not change.
Where the roofing calculator differs
The roofing calculator on this site takes the sloped area and a waste percentage and assumes 3 bundles per 100 square feet. Enter 1,409 square feet and 10% and it returns 15.5 squares and 47 bundles, one fewer than the 48 worked above. The difference is the 98.4 square foot square. For a 100 square foot product such as a 3-tab, the calculator's figure is the right one; for a 98.4 square foot architectural line, add 1.6% to the area you enter, or use the per-bundle division above. For a four-bundle line, multiply its answer by 4/3.
A hip roof, worked through
Now put a full hip roof over the same 42 by 30 foot outline at the same 6/12 pitch, with the same shingle.
- Sloped area. Still 1,408.7 square feet. As a check, the two long facets are trapezoids with a 42 foot eave, a 12 foot ridge and a 15 foot plan depth, giving (42 + 12) / 2 times 15, or 405 square feet of plan each. The two end facets are triangles with a 30 foot base and a 15 foot depth, 225 square feet each. Together they make 1,260 square feet of plan, the same as before.
- Field bundles before waste. 42.95, unchanged.
- Waste. A hip roof gets the 15% planning allowance. 42.95 times 1.15 is 49.39, so 50 bundles, two more than the gable.
- Starter. The eaves run all the way round: 2 times (42 + 30), or 144 feet. There are no rakes. 144 divided by 105 is 1.37, so 2 bundles of starter.
- Ridge. With equal pitches all round, the ridge is 42 minus 30, or 12 feet.
- Hips. Each hip covers a common run of 15 feet. At the 6/12 hip factor of 1.5000, each hip is 22.5 feet, and the four hips total 90 feet.
- Cap. Ridge plus hips is 102 feet. At 20 feet per carton that is 5.1, so 6 cartons, double the gable's.
Side by side, the two roofs need the same shingle area but not the same order.
| Item | Gable, 42 x 30 ft, 6/12 | Hip, 42 x 30 ft, 6/12 |
|---|---|---|
| Sloped area | 1,408.7 sq ft | 1,408.7 sq ft |
| Field bundles before waste (32.8 sq ft each) | 42.95 | 42.95 |
| Waste allowance (planning assumption) | 10% | 15% |
| Field bundles to order | 48 | 50 |
| Starter edge (eaves + rakes) | 151.1 ft | 144 ft |
| Starter bundles (105 ft each) | 2 | 2 |
| Ridge and hip length | 42 ft | 102 ft |
| Cap cartons (20 ft each) | 3 | 6 |
The hip roof's extra cost is in the cap and the cutting, not the field. On a steeper hip the gap widens: at 12/12 each 15 foot hip would be 26 feet long instead of 22.5.
Roofs that do not fit the formula
Real roofs often break the tidy rectangle.
- Mixed pitches. A main roof at 8/12 with a porch at 3/12 is two calculations. Multiply each part's plan area by its own factor and add them.
- Dormers. A gable dormer adds its own two small planes and two valleys, and removes nothing from the main roof below unless the main roof was cut away under it. Measure the dormer's roof as its own gable and move the whole job to the cut-up row of the waste table.
- Low slopes. The Duration instructions treat 2:12 to less than 4:12 as low slope and call for two layers of underlayment there, against one layer at 4:12 and steeper. The shingle count is unchanged, but the underlayment roll count doubles on that part of the roof. The instructions give no low-slope method below 2:12, so a flatter section needs a different roofing product.
- Very steep sections. For slopes over 60 degrees, or 21 inches per foot, the same instructions require six fasteners and four spots of asphalt roof cement per shingle. At 64 shingles a square that is 384 nails per square, which matters when you buy coil nails.
- Tear-off versus overlay. This page counts shingles for a roof deck. Whether a second layer is allowed over existing shingles is a building code and permit question for your local building department, and it does not change the shingle count.
Building the order
Pull the numbers together in the same order the roof goes on:
- Plan area of each roof section, overhangs included.
- Pitch multiplier for each section, from the table; add up the sloped areas.
- Coverage per bundle for the exact product, from the wrapper or the manufacturer's data. Do not assume 3 bundles per 100 square feet.
- Field bundles: sloped area divided by coverage per bundle, times one plus your waste allowance, rounded up.
- Starter: eave length plus sloped rake length, divided by linear feet per starter bundle, rounded up.
- Cap: total ridge and hip length, divided by linear feet per cap carton, rounded up.
- Two or three spare field bundles from the same lot if your waste allowance did not already leave them.
Shingles are the largest line on the order, but not the only one. The underlayment, drip edge, flashing and vents are counted from the same measurements, and attic ventilation and insulation work is often done with a re-roof; the insulation calculator turns an R-value and attic area into batt or roll counts. If the re-roof is part of a wider exterior project, the guides to paint coverage by room and concrete bag counts use the same measure, convert and round-up approach. More home project tools are collected on the homeowners hub.
Where the figures come from
- Owens Corning, TruDefinition Duration Shingles product page (bundles per square, coverage per square, shingles per square, exposure, nominal size; 160 mph rating note on starter at eaves and rakes): https://www.owenscorning.com/en-us/roofing/shingles/trudefinition-duration (checked October 30, 2026)
- Owens Corning, Oakridge Shingles product page: https://www.owenscorning.com/en-us/roofing/shingles/oakridge (checked October 30, 2026)
- Owens Corning, Duration Premium Shingles product page: https://www.owenscorning.com/en-us/roofing/shingles/duration-premium (checked October 30, 2026)
- Owens Corning, TruDefinition Duration FLEX Shingles product page: https://www.owenscorning.com/en-us/roofing/shingles/trudefinition-duration-flex (checked October 30, 2026)
- Owens Corning, Supreme 3 Tab Shingles product page: https://www.owenscorning.com/en-us/roofing/shingles/supreme (checked October 30, 2026)
- Owens Corning, Duration Series installation instructions (web version: starter course, offset pattern, closed-cut valley, low-slope underlayment, steep-slope fastening): https://www.owenscorning.com/en-us/roofing/install-instructions/duration-series (checked October 30, 2026)
- Owens Corning, Starter Strip Plus product page (linear feet and pieces per bundle, exposure limit): https://www.owenscorning.com/en-us/roofing/components/products/starter-strip-plus (checked October 30, 2026)
- Owens Corning, DuraRidge Hip and Ridge Shingles product page (size, exposure, pieces and linear feet per carton): https://www.owenscorning.com/en-us/roofing/components/products/duraridge-hip-ridge (checked October 30, 2026)
- Pitch multipliers, hip factors, exposed-area checks, waste sensitivity and both worked examples are CalculatorVillage calculations from the formulas and inputs shown. The waste allowances are this page's planning assumptions, not sourced figures.