The job
Modelled, not recorded. Every figure here is the answer the Roofing Shingle Calculator gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
A roof is a bigger thing than the house it sits on, in two ways at once. It overhangs the walls on every side, and it rises on a slope, so the area that carries shingles is more than the rectangle of the walls. This job is a plain two-slope roof with no hips and no valleys, measured as a plan of 34 ft (10.36 m) by 26 ft (7.92 m), which is the walls plus the eave overhang, taken at the drip edge.
The pitch is 6/12, which the calculator lists as its common slope: the roof rises six units for every twelve it runs. The calculator turns that ratio into a slope factor, so the plan is multiplied by one number to give the area the shingles actually cover. A flat measurement from the ground is never the answer on a pitched roof, and that factor is what the calculator adds.
The allowance is left at the calculator's own figure. Shingles are cut at every eave, rake and ridge, and some are lost to a roof that is not perfectly square, so ordering the bare area leaves the last course short. What this calculator does not count is as important as what it does: the starter course and the ridge caps are sold separately and are not in the bundle figure.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Roof Footprint Length | 34 ft (10.36 m) | The plan to the drip edge, along the ridge: the house's wall length plus the overhang at each end, not the wall length alone. |
| Roof Footprint Width | 26 ft (7.92 m) | Across the ridge, from drip edge to drip edge over both slopes: the width of the house plus the overhang at each eave. |
| Roof Pitch | 6/12 — 26.6° (common) | Chosen from the calculator's own list; a roof at an in-between pitch is entered at the nearest rung, so measure the pitch before choosing. |
| Waste Factor (%) | 12 | The calculator's default, suitable for a simple two-slope roof with no valleys. |
The working, step by step
1. Roof footprint area
884 sq ft (82.13 m²)
Length times width: 884 sq ft of roof plan, the area the roof covers when seen from directly above. It is not the area to shingle. It is the number the slope factor multiplies, and it is only right if the two dimensions were taken to the drip edge.
2. Actual roof surface area
988.3 sq ft (91.82 m²)
The plan multiplied by the slope factor, which the calculator's second source gives as the square root of one plus the square of the rise over the run. At this pitch the roof surface is 988.3 sq ft, more than the plan because each slope is longer than the run it covers. A steeper roof grows the factor, which is why pitch is the input that moves the answer.
3. Bundles needed
34 bundles
The surface in squares, with the allowance on top, is the headline. Dividing it into bundles at the calculator's fixed three to the square and rounding up gives 34 bundles. The calculator's first source gives three bundles per square for the common shingles; a product that is packed differently needs this row worked again from the squares.
Estimated roofing shingle needed
11.07 squares
102.8 m²
The headline is in squares, the unit shingles are sold by, and the bundle row below it is what goes on the order: 11.07 squares and 34 bundles. The calculator rounds up only at the bundle, so the squares figure is the true requirement and the bundle count has a small surplus built in. The starter course, the ridge caps, the underlayment and the flashings are not in it; each has its own calculator.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Estimated roofing shingle needed | 11.07 squares (102.8 m²) | The roof's covering area in squares, with the calculator's allowance for cuts and waste. |
| Bundles needed | 34 bundles | Whole bundles, rounded up, at the calculator's three to the square. |
The allowance, and what it does here
A flat 12 per cent is added to the roof surface before it is divided into bundles. It turns more of the same field shingle and nothing else, so it is right for a plain roof where starters and caps are cut from field shingles and wrong for a roof where they are bought as strips. Because the calculator never learns how long the eaves, rakes or ridge are, the percentage cannot respond to how much cutting the roof needs; a roof with valleys and hips is charged the same unless the figure is raised, as the last section shows.
The common mistake
Measuring the house instead of the roof
The easy measurement is the walls, taken from the ground, and it is the wrong one: the roof extends past the walls at every eave and rake by the width of the overhang. The calculator multiplies exactly the rectangle it is given, so a plan taken at the wall faces leaves the overhangs out of the answer and the order comes up short by a row or more on every side. Take the length and width to the drip edge, or measure the roof from the ground by the method in the guide linked below.
Change the job, and see what moves
The same roof, steeper
Roof Pitch: 8/12 — 33.7° → 11.9 squares (110.5 m²)
The same plan at a steeper pitch, 8/12 — 33.7°, comes to 11.9 squares and 36 bundles. Nothing about the footprint changed: only the slope factor did. A pitch entered one rung too shallow therefore leaves the order short. The calculator lists only a few rungs, so a roof that falls between two of them is entered at the nearer rung, and the bundle count checked by hand against the true pitch.
The same roof, with a larger allowance
Waste Factor (%): 15 → 11.37 squares (105.6 m²)
Raise the allowance to 15 per cent for a roof with a valley or a hip, where more shingle goes into cuts, and the order becomes 11.37 squares and 35 bundles. The extra comes in whole bundles, so the larger allowance either changes the bundle count or does not, depending on where the area falls.
What to run next
- Roofing Underlayment Calculator
The felt or synthetic layer under the shingles is measured on the same roof surface and bought in rolls.
- Roof Ridge Cap Calculator
The ridge is finished with its own caps, which this calculator does not count.
- Ridge Vent Linear Footage Calculator
If the roof is vented at the ridge, the length of vent and the cap that covers it come from the ridge length.
Sources
The method and every constant behind these figures belong to the Roofing Shingle Calculator, version 1.0.1, last verified 2026-09-05. It rests on:
- 1 roofing square = 100 sq ft; standard 3-tab and architectural shingles use 3 bundles per square
- Roof surface area = footprint area x slope multiplier, where slope multiplier = sqrt(1 + (rise/12)²) for a given X/12 pitch
Frequently asked questions
- How many bundles of shingles does a roof 34 by 26 ft on plan need?
- At a 6/12 — 26.6° (common) pitch the roof surface is 988.3 sq ft. With the calculator's 12 per cent allowance that is 11.07 squares, which is 34 bundles at three bundles to the square.
- Why is the roof area larger than the plan?
- Because the slopes lean away from the plan. The calculator multiplies the plan of 884 sq ft by a slope factor taken from the pitch, which gives 988.3 sq ft of surface. A flatter roof gives a factor nearer one.
- Does the shingle count include the ridge caps and the starter strip?
- No. The count covers the field of the roof. Starter strips and ridge caps are sold by the length, are not in the bundle figure, and come from their own calculators.