By Ashish · Updated · Formulas checked against the methodology
Quick answer: roof square footage = (length + 2 × overhang) × (width + 2 × overhang) × pitch multiplier. For a 40 × 30 ft house with a 12 inch overhang and a 6/12 pitch: 42 × 32 × 1.118 = 1,503 sq ft, which is 16.53 roofing squares and 50 bundles of shingles with 10% waste.
How to Calculate Roof Square Footage
A roof is bigger than the floor it covers. It hangs past the walls, and it slopes, so the surface is longer than the flat footprint you see from above. Here is the method roofers use, in four steps.
- Measure the footprint. Measure the length and width of the house along the outside walls. For an L-shaped or T-shaped house, split it into rectangles and measure each one.
- Add the overhang. Eaves usually stick out 12 to 18 inches. Add the overhang to both ends of each dimension. A 12-inch overhang adds 2 feet to the length and 2 feet to the width.
- Multiply by the pitch multiplier. Find your pitch (rise per 12 inches of run) and multiply the footprint by its multiplier from the chart below.
- Add waste and convert to squares. Add 10–15% for waste, then divide by 100 to get roofing squares.
Worked example
A ranch house measures 40 ft × 30 ft with a 12-inch overhang and a 6/12 pitch.
- Footprint with overhang: 42 ft × 32 ft = 1,344 sq ft
- Roof area: 1,344 × 1.118 = 1,503 sq ft
- With 10% waste: 1,503 × 1.10 = 1,653 sq ft = 16.53 squares
- Shingle bundles: 16.53 × 3 = 49.6, rounded up to 50 bundles
These are the default values in the calculator above, so you can check the math yourself.
Roof Pitch Multiplier Chart
Pitch is written as rise over run: a 6/12 roof rises 6 inches for every 12 inches it runs horizontally. The
multiplier is √(1 + (rise ÷ 12)²). Multiply your flat footprint by it to get the sloped area.
| Pitch | Angle | Multiplier | 1,000 sq ft footprint becomes |
|---|---|---|---|
| 1/12 | 4.76° | 1.0035 | 1,003 sq ft |
| 2/12 | 9.46° | 1.0138 | 1,014 sq ft |
| 3/12 | 14.04° | 1.0308 | 1,031 sq ft |
| 4/12 | 18.43° | 1.0541 | 1,054 sq ft |
| 5/12 | 22.62° | 1.0833 | 1,083 sq ft |
| 6/12 | 26.57° | 1.1180 | 1,118 sq ft |
| 7/12 | 30.26° | 1.1577 | 1,158 sq ft |
| 8/12 | 33.69° | 1.2019 | 1,202 sq ft |
| 9/12 | 36.87° | 1.2500 | 1,250 sq ft |
| 10/12 | 39.81° | 1.3017 | 1,302 sq ft |
| 11/12 | 42.51° | 1.3566 | 1,357 sq ft |
| 12/12 | 45.00° | 1.4142 | 1,414 sq ft |
Most US homes have a pitch between 4/12 and 9/12. Shingles need at least 2/12, with double underlayment up to 4/12; below 2/12 you need a membrane or roll roofing. Above 9/12 is steep, and roofers often charge more to work on it. See the full roof pitch angle chart (1/12 to 24/12 with degrees) and minimum pitch by material.
How Many Roofing Squares Do I Need?
A roofing square is 100 sq ft of roof surface. It is the unit roofers use to quote jobs and the unit suppliers use to sell shingles. To convert:
squares = roof area × (1 + waste %) ÷ 100
For the example above, 1,503 sq ft with 10% waste is 16.53 squares. Round up to 17 when ordering. Use 10% waste for a simple gable roof, 15% for hip roofs or roofs with valleys and dormers, and about 5% for a flat roof.
If you know your local installed price per square, enter it in the calculator to get a cost estimate. Installed asphalt shingle prices in the US commonly run a few hundred dollars per square and vary a lot by region, material and roof complexity, so always get quotes. Our roof replacement cost guide shows how to check a quote line by line.
How Many Bundles of Shingles?
Standard 3-tab and architectural shingles come 3 bundles per square, so each bundle covers about 33.3 sq ft. Multiply your squares by 3 and round up:
- 10 squares × 3 = 30 bundles
- 16.53 squares × 3 = 49.6, so buy 50 bundles
- 25 squares × 3 = 75 bundles
Heavy designer or luxury shingles may need 4 or 5 bundles per square. Starter strips and ridge cap shingles are often sold separately, so measure your eaves and ridges too. More detail, including pallet counts, is in how many bundles of shingles are in a square.
Average Roof Size by House Size
If you just need a ballpark number, this table shows typical roof areas for US homes, assuming a 4/12 to 8/12 pitch and a 12-inch overhang. A two-story house has a smaller roof than a one-story house with the same living space, because the roof only covers the footprint.
| Living area (sq ft) | 1-story roof (sq ft) | 2-story roof (sq ft) |
|---|---|---|
| 1,000 | 1,150 to 1,350 | 600 to 700 |
| 1,500 | 1,750 to 2,000 | 850 to 1,000 |
| 2,000 | 2,300 to 2,650 | 1,150 to 1,350 |
| 2,500 | 2,900 to 3,300 | 1,450 to 1,650 |
| 3,000 | 3,500 to 4,000 | 1,750 to 2,000 |
These are estimates. Garages, porches and dormers can add a lot, so measure your own roof for a real number.
Measuring Roof Square Footage by Address
You can estimate your roof without a ladder using free satellite images. The trick is that satellite photos show only the flat footprint, so you still need the pitch.
- Open your address in Google Maps or Google Earth and switch to satellite view.
- Right-click the roof corner and choose Measure distance, then trace the length and width of the outer roof edge (this already includes the overhang).
- For complex roofs, measure each rectangle separately and add each as a section in the calculator.
- Set overhang to 0 (it is already in the satellite measurement), choose your pitch, and read the result.
Not sure of the pitch? Look at the gable end from the street. A shallow roof is usually 4/12, a typical suburban roof is 6/12, and a roof that looks close to 45° is 12/12. For an exact number, follow how to measure roof pitch without climbing.
FAQ
How do I calculate roof square footage?
Measure the length and width of the house footprint, add the overhang to each side, and multiply length by width. Then multiply that footprint by the pitch multiplier for your roof slope (for example 1.118 for a 6/12 pitch). The result is the actual sloped roof area in square feet.
Can I calculate roof square footage by address?
Yes, roughly. Open your address in Google Maps or Google Earth satellite view, use the measure tool to trace the outer edges of the roof, and enter that length and width here with your pitch. Satellite photos show the flat footprint only, so the pitch multiplier is what turns it into true roof area.
How many square feet is a roofing square?
One roofing square is 100 square feet of roof surface. Roofers price shingles, underlayment and labor per square, so a 2,000 sq ft roof is 20 squares before waste.
How many bundles of shingles do I need per square?
Most 3-tab and architectural asphalt shingles come 3 bundles per square. Some heavy designer shingles need 4 or 5 bundles per square, so check the wrapper before you order.
How much waste should I add for a roof?
Add about 10% for a simple gable roof, 15% for hip roofs or roofs with valleys and dormers, and around 5% for flat or low-slope roofs. Waste covers cut shingles, starter strips, ridge caps and mistakes.
What is the average roof size for a house?
A typical single-story 1,500 sq ft home in the US has roughly 1,750 to 2,000 sq ft of roof, or about 17 to 20 squares. A two-story home of the same size has about half that, because the roof only covers the footprint.
How do I find my roof pitch without climbing on the roof?
From a ladder at the gable end, hold a level horizontally against the roof edge, measure 12 inches along the level, and measure straight up to the roof surface. That vertical number is your rise, so 6 inches means a 6/12 pitch. Phone apps that measure angles also work; convert the angle to pitch with the calculator above.
How many roofing squares are in 2,000 square feet?
2,000 sq ft of roof surface is 20 roofing squares. Add 10% to 15% waste, so order about 22 to 23 squares. Note that a 2,000 sq ft house is not the same as a 2,000 sq ft roof: pitch and overhang make the roof larger than a one-story footprint.
What degree is a 4/12 pitch roof?
A 4/12 roof pitch is 18.43 degrees, a 33.3% slope. Its area multiplier is 1.054, so the roof is about 5.4% larger than its flat footprint.
How many sheets of plywood do I need for my roof?
Divide the roof area by 32 (a 4 × 8 sheet) and add about 10% for cuts. A 1,503 sq ft roof needs about 47 sheets, or 52 with waste.
Is this roof calculator accurate enough to order materials?
It is accurate for planning and budgeting. For a final order, have a roofer or a measurement report confirm the numbers, especially on complex roofs with many valleys, dormers or different pitches.
