Home Improvement
How To Calculate Shingles Needed
Shingle quantity starts from the roof area, not the floor area. Steeper roofs cover more surface, so the pitch multiplier has to be applied before anything is ordered.
Quick Answer
squares = roof length x roof width x pitch multiplier x (1 + waste) / 100
- squares
- Roofing area in units of 100 square feet
- bundles
- Three bundles make one square for standard shingles
- pitch multiplier
- The square root of one plus pitch over twelve squared
- waste
- An allowance for cuts, starter courses and ridge caps
Multiply the footprint by the pitch multiplier, add a waste allowance, then divide by 100 to get squares. A 40 by 30 foot roof at 6/12 pitch needs 14.76 squares, which is 45 bundles at three bundles per square.
What Is Shingles Needed?
Shingles are sold by the square, which is exactly 100 square feet of roof area. A typical bundle covers about 33.3 square feet, so three bundles make one square, and that is the number the supplier will quote.
The first step is the footprint. Measure the length and width of the house at the eaves and multiply. For a simple gable roof this is the plan area, and the pitch then stretches it into the real surface area.
Pitch is quoted as rise over run, so 6/12 means the roof rises six inches for every twelve inches it runs. The multiplier that converts footprint to surface is the square root of one plus the rise over twelve, squared.
For 6/12 pitch the multiplier is 1.118, so the real roof is about 12 percent larger than the floor plan. A 4/12 pitch gives 1.0541, and a steep 12/12 pitch gives 1.4142, which is nearly half again as much material.
Waste is not optional. Starter courses, hip and ridge caps, cuts around penetrations and the staggered pattern all consume shingles that never appear as field coverage. Ten percent is the usual allowance on a simple roof and fifteen percent on a complex one.
Complex roofs waste far more than simple ones. Valleys, dormers, skylights and multiple pitches all create offcuts, and a roof with several facets can easily need twenty percent more material than the area suggests.
Measure each facet separately for accuracy. Add the length and width of every plane, apply the pitch multiplier to each, and total the results. This is more work than multiplying the footprint but avoids large errors on irregular houses.
The ridge and hips need their own material. Ridge cap shingles usually come from the same bundles, and a long ridge consumes a noticeable number. Ordering by square footage only will leave you short at the peak.
Underlayment is a separate purchase. A roll of felt typically covers about 400 square feet, so divide the adjusted area by 400 and round up. Ice and water shield at the eaves and valleys is priced by the linear foot.
Nails are counted by the square. Roughly four nails per shingle at about 250 shingles per square gives about 1000 nails per square, and most suppliers sell them in one pound per square boxes.
Buy from the same batch where possible. Shingle dye lots vary slightly, and a roof assembled from two batches can show a visible colour difference even when both are technically the same product.
Return policies matter. Most suppliers accept unopened bundles, so ordering one extra square is safer than a second trip, but opened bundles usually cannot go back.
Formula
roof area = length x width x pitch multiplier
Formula page →The pitch multiplier converts the plan area into the sloped surface area.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| roof area | Surface area | sq ft | The sloped area to be covered, in square feet. |
| length, width | Footprint | feet | The plan dimensions of the roof, in feet. |
| r | Pitch rise | inches | Inches of rise per twelve inches of run. |
squares = area x (1 + waste) / 100, bundles = squares x 3
Formula page →Converts adjusted area into the units the supplier uses.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| squares | Squares | squares | Units of 100 square feet. |
| bundles | Bundles | bundles | Three bundles per square for standard shingles. |
| waste | Waste allowance | decimal | A decimal fraction, so 0.10 for ten percent. |
multiplier = sqrt(1 + (rise / 12)^2)
Formula page →The geometric factor that turns a flat measurement into a sloped one.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| multiplier | Pitch factor | unitless | 1.0541 at 4/12, 1.118 at 6/12, 1.4142 at 12/12. |
| rise | Pitch rise | inches | The first number in the pitch, such as the 6 in 6/12. |
How To Calculate Shingles Needed
- 1
Measure the footprint
A 40 by 30 foot roof has a footprint of 1200 square feet. For a gable roof this is simply the length times the width at the eaves.
- 2
Apply the pitch multiplier
At 6/12 pitch the multiplier is the square root of one plus 0.25, which is 1.118. The sloped area is 1200 times 1.118, which is 1341.64 square feet.
- 3
Add the waste allowance
Ten percent waste brings 1341.64 to 1475.8 square feet. Complex roofs with valleys and dormers should use fifteen percent instead.
- 4
Convert to squares and bundles
1475.8 divided by 100 is 14.76 squares, and at three bundles per square that is 44.27 bundles, so order 45 bundles. Underlayment at 400 square feet per roll needs four rolls.
Examples
Example 1: A 40 by 30 foot roof at 6/12 pitch with 10 percent waste
- Roof length
- 40
- Roof width
- 30
- Pitch rise
- 6
- Waste
- 10
| Step | Calculation | Result |
|---|---|---|
| Footprint | 40 x 30 | 1200 |
| Pitch multiplier | sqrt(1 + (6/12)^2) | 1.118 |
| Sloped area | 1200 x 1.118 | 1341.64 |
| Area with waste | 1341.64 x 1.10 | 1475.8 |
| Squares | 1475.8 / 100 | 14.76 |
| Bundles | ceil(14.76 x 3) | 45 |
Result: A 40 by 30 foot roof at 6/12 pitch with 10 percent waste needs 14.76 squares of shingles. At three bundles per square that is 45 bundles, and four rolls of underlayment cover the same area.
Example 2: A 24 by 20 foot roof at 4/12 pitch with 10 percent waste
- Roof length
- 24
- Roof width
- 20
- Pitch rise
- 4
- Waste
- 10
| Step | Calculation | Result |
|---|---|---|
| Footprint | 24 x 20 | 480 |
| Pitch multiplier | sqrt(1 + (4/12)^2) | 1.0541 |
| Sloped area | 480 x 1.0541 | 505.96 |
| Area with waste | 505.96 x 1.10 | 556.56 |
| Squares | 556.56 / 100 | 5.57 |
| Bundles | ceil(5.57 x 3) | 17 |
Result: A 24 by 20 foot roof at 4/12 pitch needs 5.57 squares with waste, which means 17 bundles. The shallower pitch adds only 5.41 percent to the footprint, compared with 11.8 percent at 6/12.
Calculator
Sloped roof area
1,341.64
- Squares needed
- 14.76
- Bundles needed
- 45
- Underlayment rolls
- 4
- Pitch multiplier
- 1.118
Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.
Prefer a full-width tool? Open the Shingles Needed calculator page.
Common Mistakes
Ordering shingles for the floor area
The sloped roof is always larger than the plan. At 6/12 pitch the difference is 11.8 percent, so ordering 12 squares for a 1200 square foot footprint leaves the job short.
Ignoring the waste allowance
Starter courses, ridge caps, hips and cuts all consume material that never shows as field coverage. Skipping waste is the most common cause of a second trip to the supplier.
Using the wrong pitch multiplier
Pitch is rise over run, so 6/12 is a rise of 6 in 12 of run. Reading it as 6 percent, or using the degree angle, gives a multiplier that is far too small.
Forgetting the ridge and hips
Ridge cap shingles come out of the same bundles and a long ridge uses several. A roof with hips and valleys needs more waste than a plain gable.
Assuming every bundle covers the same area
Standard three-tab shingles are about 33.3 square feet per bundle, but architectural and designer shingles vary. Check the label before converting bundles to squares.
Measuring only one facet of a complex roof
Dormers, porches and gable ends each add area. Add every plane separately, and expect a higher waste percentage on roofs with several intersections.
FAQ
How many bundles of shingles do I need per square?
Three bundles make one square of 100 square feet for standard three-tab shingles. Architectural shingles may differ, so check the coverage printed on the bundle.
How much waste should I add for shingles?
Ten percent suits a simple gable or hip roof. Add fifteen percent for valleys, dormers and skylights, and up to twenty percent for very complex roofs.
How do I convert roof pitch to an area multiplier?
Take the square root of one plus the rise over twelve squared. A 6/12 pitch gives 1.118, so multiply the footprint by 1.118 to get the sloped area.
How many squares of shingles for a 1000 square foot roof?
If 1000 square feet is the sloped area, that is 10 squares, or 30 bundles, before waste. With 10 percent waste it becomes 11 squares, so order 33 bundles.
How much underlayment do I need?
Divide the sloped area by the coverage of one roll, usually about 400 square feet, and round up. Eaves and valleys need separate ice and water shield.
Should I order extra shingles?
Yes, order the calculated amount plus one extra square. Unopened bundles can usually be returned, and repairs years later need a matching batch.
References
- [1]Wikipedia, Roof shingle — https://en.wikipedia.org/wiki/Roof_shingle
- [2]Wikipedia, Roof pitch — https://en.wikipedia.org/wiki/Roof_pitch
- [3]Wikipedia, Square (unit) — https://en.wikipedia.org/wiki/Square_(unit)