Key takeaways

• Order by true roof area, not footprint. A 40 ft by 30 ft house has a 1,200 sq ft footprint, but at a 6/12 pitch the roof surface is 1,341.64 sq ft.
• Roofing squares simplify ordering. That roof is 13.42 squares before waste and 14.76 squares with the default 10% waste allowance.
• Buy 45 bundles for the worked example. At the standard 3 bundles per square, 14.76 squares rounds up to 45 bundles, plus 4 underlayment rolls and 2 ridge cap bundles.
• Waste is already in the result. The default 10% waste factor covers cuts, valleys, hips, and starter rows. Change it under Advanced options for simple or complex roofs.
• Always round up. Partial bundles and rolls cannot be bought, so every material count here rounds up to the next whole unit.

Contents

• How it works
• The formula
• Roof pitch slope factor table
• Worked examples
• Buying tips: bundles, underlayment, and ridge cap
• Common mistakes
• Frequently asked questions

Related tools

The formula

Roofing material math has three steps. First, find the flat footprint. Second, correct it for the slope of the roof. Third, convert to roofing squares and add waste.

Footprint area = roof length x roof width.

Slope factor = square root of (1 + (rise / run) squared). For a 6/12 pitch, rise / run is 6 / 12 = 0.5, so the slope factor is the square root of 1.25 = 1.1180.

True roof area = footprint area x slope factor.

Roofing squares = true roof area / 100. One roofing square always equals 100 square feet.

Area with waste = true roof area x (1 + waste factor). With the default 10% waste, multiply by 1.10.

Bundles = squares with waste x bundles per square, rounded up. Standard 3-tab and architectural shingles are commonly sold at 3 bundles per square, so one bundle covers about 33.3 square feet.

Underlayment rolls = area with waste / coverage per roll, rounded up. The default roll covers 400 square feet.

Ridge cap bundles = ridge length / 33, rounded up. A typical ridge cap bundle covers about 33 linear feet of ridge.

These are planning estimates for a simple gable-style rectangle. Dormers, valleys, hips, skylights, and multiple roof planes add cutting waste and should push the waste factor toward 15%.

Roof pitch slope factor table

Pitch is written as rise over a 12 inch run. A higher pitch means a steeper roof and a larger true surface for the same footprint.

Pitch	Slope factor	Extra area vs flat
0 / 12 (flat)	1.0000	0%
2 / 12	1.0138	1.4%
3 / 12	1.0308	3.1%
4 / 12	1.0541	5.4%
5 / 12	1.0833	8.3%
6 / 12	1.1180	11.8%
7 / 12	1.1580	15.8%
8 / 12	1.2019	20.2%
9 / 12	1.2500	25.0%
10 / 12	1.3020	30.2%
12 / 12	1.4142	41.4%

Ignoring pitch is the most expensive roofing mistake: a 12/12 roof has 41.4% more surface than its footprint, so ordering by footprint alone would leave you more than 40% short.

Worked examples

Example 1: 40 ft x 30 ft, 6/12 pitch, 10% waste

Footprint = 40 x 30 = 1,200 sq ft. Slope factor = square root of (1 + 0.5 squared) = 1.1180. True roof area = 1,200 x 1.1180 = 1,341.64 sq ft = 13.42 squares. With 10% waste: 1,475.80 sq ft = 14.76 squares. Bundles = 14.76 x 3 = 44.27, rounded up to 45 bundles. Underlayment = 1,475.80 / 400 = 3.69, rounded up to 4 rolls. Ridge cap for a 40 ft ridge = 40 / 33 = 1.21, rounded up to 2 bundles. At $35 per bundle, shingles cost about $1,575.

Example 2: 50 ft x 28 ft, 8/12 pitch, 10% waste

Footprint = 1,400 sq ft. Slope factor for 8/12 = 1.2019. True roof area = 1,682.59 sq ft = 16.83 squares. With waste: 1,850.85 sq ft = 18.51 squares. Bundles = 18.51 x 3 = 55.53, rounded up to 56 bundles.

Example 3: Flat roof, 30 ft x 20 ft, 10% waste

Footprint = 600 sq ft and the slope factor is 1.0000, so the true area is also 600 sq ft = 6.00 squares. With waste: 660 sq ft = 6.60 squares. Bundles = 6.60 x 3 = 19.80, rounded up to 20 bundles.

Example 4: Steep roof, 40 ft x 30 ft, 12/12 pitch, 15% waste

Footprint = 1,200 sq ft. Slope factor for 12/12 = 1.4142. True roof area = 1,697.06 sq ft = 16.97 squares. With 15% waste: 1,951.61 sq ft = 19.52 squares. Bundles = 19.52 x 3 = 58.56, rounded up to 59 bundles. Steep roofs also deserve the higher waste factor because cuts and staging are harder.

Buying tips: bundles, underlayment, and ridge cap

• Check bundles per square on the product you buy. Most architectural shingles are 3 bundles per square, but some heavier or designer shingles are 4 or 5 bundles per square. Change this under Advanced options before you trust the bundle count.
• Pick the waste factor by roof complexity. Use 5% only for a simple single rectangle with no valleys. Use 10% for a typical home. Use 15% for hips, valleys, dormers, or skylights.
• Do not forget starter shingles and flashing. This calculator covers field shingles, underlayment, and ridge cap. Starter strips, drip edge, valley flashing, and vents are separate line items on a real quote.
• Buy one extra bundle if the count is exact. A damaged bundle or a late design change is cheaper to cover from stock than from a second delivery.
• Confirm underlayment roll size. Rolls vary by brand. If your roll covers 500 square feet instead of 400, update the coverage under Advanced options and the roll count updates.

Common mistakes

• Ordering by footprint area. This under-orders by the slope factor, from 11.8% short at 6/12 to 41.4% short at 12/12.
• Forgetting waste. Cutting around valleys, hips, and edges consumes real material. A zero-waste order always runs short.
• Mixing squares and bundles. A square is an area (100 sq ft), not a package. At 3 bundles per square, a 15-square roof needs 45 bundles, not 15.
• Measuring only one slope. For an L-shaped or multi-plane roof, measure each rectangle separately and add the areas. This calculator assumes one rectangle.
• Using eave length as ridge length. The ridge is the top horizontal peak, usually close to the roof length, not the full perimeter.

How it works

1. Enter the footprint. Measure the length and width of the roof at ground level, in feet, and pick the pitch.
2. Adjust waste and materials if needed. The defaults (10% waste, 3 bundles per square, 400 sq ft rolls) are already in the result. Open Advanced options to change them.
3. Read the material list. Press Calculate My Roofing Materials to update the result and jump to it, then order the rounded-up bundle, roll, and ridge cap counts.

Frequently asked questions

How many shingles do I need for a 1,200 sq ft house?

It depends on pitch. For a 40 ft by 30 ft footprint (1,200 sq ft) at 6/12 pitch with 10% waste, you need about 45 bundles: 1,341.64 sq ft of true roof area, 14.76 squares with waste, times 3 bundles per square, rounded up.

What is a roofing square?

One roofing square is 100 square feet of roof surface. Roofers and suppliers quote and sell by the square because it keeps large roofs in manageable numbers: a 1,341.64 sq ft roof is 13.42 squares.

How many bundles are in a square of shingles?

Usually 3 bundles per square for standard 3-tab and architectural shingles, covering about 33.3 square feet per bundle. Some designer shingles need 4 or 5 bundles per square, so check the product label and update the calculator.

How much waste should I add for roofing?

Use 10% for a typical roof, 5% for a simple rectangle, and 15% for a roof with hips, valleys, dormers, or skylights. Waste covers cuts, starter rows, and damaged pieces, and it is already included in this calculator at the default 10%.

How do I find my roof pitch?

Measure 12 inches horizontally along a rafter or level, then measure how far the roof rises over that run. A 6 inch rise over a 12 inch run is a 6/12 pitch. If you cannot safely access the roof, check the attic rafters or ask your roofer.

Does this calculator include ridge cap and underlayment?

Yes. Underlayment rolls divide the waste-adjusted area by your roll coverage (default 400 sq ft per roll), and ridge cap bundles divide the ridge length by about 33 linear feet per bundle. Starter shingles, drip edge, flashing, and vents are not included.

Disclaimer

This calculator gives planning estimates for a single rectangular roof plane. Real roofs have hips, valleys, dormers, skylights, and local code requirements that change material needs. Confirm final quantities with your roofer or supplier before ordering, and never climb on a roof without proper safety equipment. Prices are estimates only and vary by brand, region, and supplier.