A roof order usually goes wrong in the first two minutes, before any arithmetic happens. The building footprint gets measured, the answer gets read, and bundles are bought for a flat surface that does not exist anywhere on the job. The roof above a footprint is always longer than the ground it covers, and the pitch factor field is the only place that difference enters the math. A 10 by 6 meter footprint at a 6/12 pitch needs 24 bundles, while the same footprint treated as flat asks for 22. The five inputs below each have a physical source on the building, and one of them moves the order far more than the rest.
Footprint area and roof area are different numbers
The length and width fields want the plan dimensions of the roof, measured as though you were looking straight down at the building from above. They do not want the distance you would walk from eave to ridge. The pitch factor converts one into the other. A 6/12 roof multiplies plan area by 1.118, so a 60 square meter footprint becomes 67.08 square meters of shingle surface. A 12/12 roof multiplies by 1.414, which is the square root of two, and adds just over 41 percent to that same footprint. The expensive version of this mistake runs the other way. Climb the roof, measure the rafter line, enter that longer dimension, and then let the tool apply a pitch factor on top of it, and the slope correction lands twice. On a 6/12 roof that overstates the order by about 12 percent, which is three extra bundles on a small house. Measure on the ground or from a plan, and leave the slope correction to the pitch factor field.
Slope limits that decide whether shingles belong here
Before the bundle count matters, the slope has to permit asphalt shingles at all. The adopted code text checked for this page, Illinois Building Code 2021 Section 1507.2, allows asphalt shingles only on roof slopes of two units vertical in 12 units horizontal, a 17 percent slope, or greater. From that 2/12 floor up to four units vertical in 12 units horizontal, a 33 percent slope, the same section calls for a double underlayment application, which changes the underlayment order even though the bundle count is unaffected. The same section sets the fastening: roofing nails with a minimum 12 gage shank of 0.105 inch, or 2.67 mm, a head of at least 3/8 inch, or 9.5 mm, driven through the roofing and not less than 3/4 inch, or 19.1 mm, into the sheathing, with not less than four fasteners per strip shingle or two per individual shingle. Code editions differ by jurisdiction, so confirm the edition adopted where the work happens. A porch or addition that measures below 2/12 needs a different roofing system and a roofer, not a larger bundle count.
| Roof pitch (rise in 12) | Roof angle | Pitch factor |
|---|---|---|
| 2/12 | 9.46 degrees | 1.014 |
| 3/12 | 14.04 degrees | 1.031 |
| 4/12 | 18.43 degrees | 1.054 |
| 5/12 | 22.62 degrees | 1.083 |
| 6/12 | 26.57 degrees | 1.118 |
| 7/12 | 30.26 degrees | 1.158 |
| 8/12 | 33.69 degrees | 1.202 |
| 9/12 | 36.87 degrees | 1.250 |
| 10/12 | 39.81 degrees | 1.302 |
| 12/12 | 45 degrees | 1.414 |
Source: Pennsylvania Department of Education, Carpentry Math T-Chart: Compute Roof Slope.
Multiply the footprint area by the factor for the plane you are covering. Roofs with two different slopes need two calculations.
Getting the footprint and the pitch off the building
- Take plan dimensions, including the eaves. Measure the roof as it appears from above, from drip edge to drip edge, not from wall to wall. A 0.4 meter overhang on all four sides of a 10 by 6 meter house turns a 60 square meter reading into 10.8 by 6.8 meters, or 73.44 square meters, which is about 22 percent more roof and six more bundles. Tool: Long tape or a dimensioned plan.
- Read the pitch against a rafter. Hold a spirit level horizontal with one end touching the underside of a rafter or the rake board, mark 300 mm along the level, and measure straight down from that mark to the rafter. The drop over a 300 mm run is the rise per 12 units, so a 150 mm drop is a 6/12 pitch. Tool: Spirit level and tape.
- Convert the pitch into a factor. Look the pitch up in the table on this page and copy the factor into the pitch factor field. A 6/12 pitch is 1.118 and sits at 26.57 degrees, a 3/12 pitch is 1.031, and a 10/12 pitch is 1.302. Enter the factor, not the angle and not the fraction. Tool: Pitch factor table.
- Copy the coverage from the wrapper. Read the coverage per bundle printed on the shingle wrapper or the product data sheet and enter it in the same unit system as the length and width fields. Mixing a square foot coverage with metric dimensions produces a result that looks plausible and is wrong by roughly ten times. Tool: Product wrapper or data sheet.
- Set the waste percent last. Count the hips, valleys, dormers, and penetrations before touching this field. Leave 10 percent for a plain gable, and raise it for cut up shapes so the allowance stays visible in the inputs instead of being hidden in a manual adjustment to the bundle count. Tool: Roof plan or a sketch.
Sixty square meters of footprint at a 6/12 pitch
| Field | Value |
|---|---|
| length | 10 |
| width | 6 |
| pitchFactor | 1.118 |
| coverage | 3.1 |
| waste | 10 |
The footprint is measured from drip edge to drip edge as 10 by 6 meters, giving 60 square meters of plan area for the whole roof, both planes of the gable included. The pitch is read as 6/12 against a rafter, so the factor is 1.118 and the shingle surface is 67.08 square meters, or about 722 square feet. The 10 percent waste allowance lifts that to 73.79 square meters, and dividing by the 3.1 square meter coverage printed on the wrapper gives 23.8 bundles, which rounds up to 24. Two of the five inputs dominate this answer. Moving the pitch from 6/12 to 8/12 changes the factor to 1.202 and pushes the order to 26 bundles, while raising waste from 10 to 15 percent adds one bundle. A coverage entered as 31 instead of 3.1 collapses the answer to 3 bundles, which is the fastest sanity check available on this page.
Result: 24 bundles.
Bundles are the purchase unit, not square meters
The coverage field is the one number that does not come off the building. It comes off the wrapper, and it is printed there in whatever unit the manufacturer chose. The default of 3.1 square meters per bundle converts to 33.4 square feet using the exact NIST factor of 9.290304E-02 square meters per square foot, so a metric coverage figure and an imperial one can look wildly different while describing identical product. Copy the printed figure rather than assuming it. The tool then rounds up, because a partial bundle is not a purchasable item: a raw result of 23.8 becomes 24 bundles and leaves about 0.61 square meters of shingle in hand. That leftover is a feature, not a rounding error, since the first damaged bundle on site comes out of it. What the bundle count does not include is everything that is not field shingle. Starter strip, ridge cap, hip cap, underlayment, drip edge, valley metal, and nails are separate lines, and ridge and hip cap is normally sold by length rather than by area.
Common mistakes
- Measuring the walls instead of the roof and losing the eave overhang.
- The roof extends past the wall line at every eave and rake, and the shingles have to cover that projection. A 0.4 meter overhang on four sides of a 10 by 6 meter house adds about 22 percent to the plan area. Measure from drip edge to drip edge, or take the wall dimensions and add twice the overhang to each direction, which turns 10 by 6 into 10.8 by 6.8 and the order from 24 bundles into 30.
- Measuring up the slope and then also entering a pitch factor.
- The eave to ridge distance already contains the slope correction, so applying a factor to it counts the same geometry twice and inflates the order by the factor itself, about 12 percent at 6/12. Keep the two apart: length and width are plan dimensions taken from the ground or a drawing, and the pitch factor field is the only place the slope is allowed to enter the calculation.
- Mixing a square foot coverage figure with metric footprint dimensions.
- Coverage is printed in whichever unit the manufacturer chose, and 3.1 square meters and 33.4 square feet describe the same bundle, so an unnoticed swap changes the answer by roughly a factor of ten. Convert first with the exact NIST factor of 9.290304E-02 square meters per square foot, then enter length, width and coverage in one unit system and check that the printed answer is plausible.
- Expecting starter strip and ridge cap to be inside the bundle count.
- The formula covers field shingle only. Starter course, ridge and hip cap, underlayment, drip edge, valley metal and fasteners are separate items, and cap is usually sold by length rather than by area. Take the bundle number to the supplier as one line, then measure ridge and hip length, eave and rake length, and the full sloped area separately for cap, drip edge and underlayment.
- Applying one pitch factor to a roof that has more than one slope.
- A main roof at 6/12 with a porch at 3/12 has factors of 1.118 and 1.031, and a single average understates one plane and overstates the other while hiding a slope that may fall under the 2/12 limit. Split the roof into planes of equal slope, run the calculation once per plane with its own footprint and factor, and add the rounded bundle counts only at the end.
Waste on a cut up roof is not the default ten percent
The 10 percent default suits a plain gable with two rectangular planes, straight eaves, and one ridge. Every departure from that shape raises the off-cut rate. On a hip roof each course dies into a diagonal, so a piece is cut and part of it is discarded on every run, and the loss grows with the number of hips. Valleys do the same thing on both planes that meet there. Dormers, chimneys, skylights, and pipe penetrations each add cut waste out of proportion to the area they occupy, because the cuts are fussy rather than large. Raising the waste field is better practice than buying extra bundles afterward: at 15 percent the 60 square meter example moves from 24 to 25 bundles, and the reason is recorded in the input rather than lost in a note. Steep roofs add a second cost that this field does not carry at all, which is staging and the slower installation rate that comes with working above a 6/12 pitch.
Frequently asked questions
How do I find the pitch factor without getting on the roof?
Work from inside or from the gable end. Hold a level horizontal against the underside of a rafter in the attic, mark 300 mm along it, and measure the vertical drop from that mark to the rafter. A 150 mm drop is a 6/12 pitch, which the table on this page converts to a factor of 1.118. From the ground you can do the same against the rake board at the gable end with a short level and a tape.
Does the footprint cover both planes of a gable roof?
Yes. The plan area of the building already contains the ground beneath both planes, so multiplying it by the pitch factor returns the total sloped surface of the whole gable rather than one side. For a 10 by 6 meter footprint at 6/12 that is 67.08 square meters across both planes. Split the calculation only when the planes have different slopes, or when part of the roof is a separate structure such as a lower porch.
Can asphalt shingles go on a roof that measures 1.5 in 12?
No. The code text checked for this page allows asphalt shingles only on slopes of two units vertical in 12 units horizontal, a 17 percent slope, or greater, so a 1.5 in 12 plane is below the limit and needs a different roofing system. Between 2/12 and 4/12 shingles are permitted but a double underlayment application is required. A slope near either threshold is worth measuring twice and worth a roofer looking at before material is ordered.
Why does the answer jump a whole bundle after a small change?
The formula rounds up at the last step, because a supplier sells whole bundles. The example lands on 23.8 and is reported as 24, so it sits only 0.2 of a bundle below the next boundary. Adding five points of waste, or a slightly steeper pitch factor, is enough to cross it. When a result sits close to a boundary, note the unrounded figure as well so a later change in coverage or waste can be re-checked without redoing the measurement.
Should coverage be entered in square feet or square meters?
Either works, as long as length, width and coverage all use the same system. A bundle printed as 33.4 square feet and one printed as 3.1 square meters are the same product, since the exact conversion factor is 9.290304E-02 square meters per square foot. If the footprint was measured in meters, convert the wrapper figure before entering it. A result that is out by roughly a factor of ten is almost always a mixed unit rather than a bad measurement.
Related calculators
Sources
- Carpentry Math T-Chart: Compute Roof Slope Pennsylvania Department of Education. Provides the official rise-run roof-slope geometry. The pitch factors in this page's table are calculated from that geometry as sqrt(1 + (rise/12)^2), including 1.118 at 6/12 and 1.414 at 12/12. Accessed 2026-09-07.
- 2021 International Building Code, Chapter 15 Roof Assemblies and Rooftop Structures International Code Council. Source of the 2/12 minimum slope and 17 percent figure, the 4/12 and 33 percent double underlayment threshold, the 12 gage 0.105 inch shank, the 3/8 inch head, the 3/4 inch sheathing penetration and the four fasteners per strip shingle minimum. Accessed 2026-09-07.
- NIST Guide to the SI, Appendix B.9 Factors for Units Listed by Kind of Quantity National Institute of Standards and Technology. Provides the exact square foot to square meter factor of 9.290304E-02 used to show that a 3.1 square meter bundle and a 33.4 square foot bundle describe the same coverage, and the exact foot to meter factor of 3.048E-01. Accessed 2026-09-07.
This is a planning result, not a supplier quote; sellers use different pack, bag, board, roll, and bundle conventions, so confirm the exact product label and selling unit before you order.