Roofing Calculator

Calculate roof area, roofing squares and how many bundles of shingles you need from your home's footprint and roof pitch. Includes underlayment and waste.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To find how many shingles you need, multiply the house footprint (including overhangs) by the pitch factor to get roof area, divide by 100 for squares, and multiply by 3 bundles per square plus 10% waste. A 40 x 30 ft house with a 6/12 pitch and 1 ft overhangs needs about 50 bundles of shingles.

Results

Shingle bundles to buy: 50 bundles; 16.5 squares including 10% waste
Roof area: 1,503 sq ft; 15.03 squares (100 sq ft each) before waste
Pitch multiplier: x 1.118; 6/12 pitch, 26.6 degrees
Underlayment: 5 rolls; Synthetic or #15 felt, 4 squares (400 sq ft / 37 m²) per roll
Footprint with overhang: 1,344 sq ft; 42 ft x 32 ft

Roofing formula

Footprint = (Length + 2 x overhang) x (Width + 2 x overhang)
Pitch factor = sqrt(1 + (rise / 12)^2)
Roof area = Footprint x Pitch factor
Bundles = Roof area x (1 + waste %) / 100 x bundles per square

Worked example

A 50 x 28 ft ranch with 1 ft overhangs and a 4/12 pitch: footprint 52 x 30 = 1,560 sq ft. The pitch factor for 4/12 is 1.054, so the roof is 1,644 sq ft, or 16.4 squares. With 10% waste that is 18.1 squares x 3 = 55 bundles, plus 5 rolls of underlayment.

Quick reference

Roof pitch factors (multiply footprint area by the factor)
Pitch	Angle	Factor
2/12	9.5 deg	1.014
3/12	14.0 deg	1.031
4/12	18.4 deg	1.054
5/12	22.6 deg	1.083
6/12	26.6 deg	1.118
7/12	30.3 deg	1.158
8/12	33.7 deg	1.202
9/12	36.9 deg	1.250
10/12	39.8 deg	1.302
12/12	45.0 deg	1.414

How to calculate roof area from the ground

Measure the length and width of the house at the ground, add the overhang on each side, and multiply for the footprint. Then multiply by the pitch factor, which accounts for the slope. A 6/12 pitch has a factor of 1.118, so a 42 by 32 foot footprint becomes about 1,503 square feet of roof.

Roofers measure in squares: one square is 100 square feet. Shingles are sold in bundles, usually three bundles per square for standard shingles. Some heavyweight architectural shingles need four or five bundles per square, so check the wrapper.

Pitch factors and waste

Common pitch factors: 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414. If you do not know the pitch, hold a level against the roof and measure the rise over a 12 inch run.

Add 10 percent waste for a plain gable roof. Hip roofs, valleys, dormers and skylights create many cut shingles, so allow 15 to 20 percent. Remember starter strips, ridge caps, drip edge, flashing and nails, which are sold separately.

Key facts

One roofing square equals 100 sq ft, and standard shingles come 3 bundles per square.
Pitch factors: 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414.
Add 10% waste for a simple gable roof and 15 to 20% for hip roofs with valleys and dormers.
A roll of synthetic underlayment or #15 felt covers about 4 squares (400 sq ft).

Frequently asked questions

How many bundles of shingles do I need for 1,500 square feet?

1,500 square feet is 15 squares. With 10% waste that is 16.5 squares, or 50 bundles at 3 bundles per square.

How many square feet are in a roofing square?

One roofing square equals 100 square feet of roof surface.

How do I find my roof pitch?

Place a level horizontally on the roof surface, measure 12 inches along it, then measure straight down to the roof. That distance in inches is your pitch over 12. A 6 inch drop is a 6/12 pitch.

How many bundles of shingles are in a square?

Three bundles for most 3-tab and architectural shingles. Premium or thick designer shingles can be four or five bundles per square.

Sources and references

Timberline HDZ shingles product page (3 bundles per square) (GAF)
Residential Asphalt Roofing Manual: Design and Application Methods (Asphalt Roofing Manufacturers Association)
Chapter 9 Roof Assemblies, Section R905.2 Asphalt Shingles, 2021 IRC (ICC)

Next steps for this project

The usual order for a roofing & siding project. See all roofing & siding calculators.

Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.