Roof Square Footage Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
Roof square footage is the ridge length multiplied by the rafter length, doubled for a two-plane gable. A 40 ft ridge with 18 ft rafters gives 40 × 18 × 2 = 1,440 sq ft, which is 14.4 roofing squares before any waste allowance.
Your measurements
Enter the values you know — results update live.
Length of the roof measured along the ridge, including gable overhangs.
The slope distance from the eave edge up to the ridge, measured along the roof.
Two for a standard gable, four for a simple hip, one for a shed roof.
Analysis
Live results from your measurements.
Show the working
- Area of one plane:
40 × 18= 720 sq ft - All planes:
720 × 2= 1,440 sq ft - Squares:
1,440 ÷ 100= 14.4 - With waste:
1,440 × 1.1= 1,584 sq ft - Bundles:
ceil(15.84 × 3)= 48
What this calculator does
Roof square footage is the measured surface of every roof plane added together. Unlike the footprint method, this approach works directly from measurements taken on the roof itself, so it handles unequal planes, shed roofs and additions without needing a pitch multiplier at all. It is the method to use when you already have a tape on the roof, a drone survey or a set of dimensioned drawings. Roofers then divide by 100 to get squares, which is how every quote, bundle count and labour rate in the trade is expressed.
How to use it
- Measure the ridge length end to end, including any overhang past the gable walls.
- Measure the rafter line from the eave edge up to the ridge, along the roof surface.
- Multiply those two numbers to get the area of one plane.
- Multiply by the number of planes that share those dimensions.
- Repeat for any plane with different dimensions and add the totals.
- Divide the total by 100 to convert to roofing squares.
The formula
Plane area
plane area = ridge length × rafter length
A rectangular roof plane. Triangular hip planes use base × height ÷ 2.
Squares
squares = total square feet / 100
One square is 100 square feet of finished roof.
Bundle count
bundles = squares with waste × 3
Standard three-tab and most architectural shingles run three bundles to the square.
Worked example
A gable roof with a 36 ft ridge and 16.5 ft rafters, two planes, 12% waste.
- One plane: 36 × 16.5 = 594 sq ft
- Two planes: 594 × 2 = 1,188 sq ft
- Squares: 1,188 ÷ 100 = 11.88
- With waste: 1,188 × 1.12 = 1,330.6 sq ft
- Bundles: 13.31 squares × 3 = 40 bundles
The roof is 11.88 squares; order 40 bundles to cover it with a 12% waste allowance.
Reference chart
| Pitch | Multiplier | Area increase | Angle |
|---|---|---|---|
| 2:12 | 1.0138 | 1.4% | 9.46° |
| 3:12 | 1.0308 | 3.1% | 14.04° |
| 4:12 | 1.0541 | 5.4% | 18.43° |
| 5:12 | 1.0833 | 8.3% | 22.62° |
| 6:12 | 1.118 | 11.8% | 26.57° |
| 7:12 | 1.1577 | 15.8% | 30.26° |
| 8:12 | 1.2019 | 20.2% | 33.69° |
| 9:12 | 1.25 | 25% | 36.87° |
| 10:12 | 1.3017 | 30.2% | 39.81° |
| 12:12 | 1.4142 | 41.4% | 45° |
| 14:12 | 1.5366 | 53.7% | 49.4° |
| 16:12 | 1.6667 | 66.7% | 53.13° |
| 18:12 | 1.8028 | 80.3% | 56.31° |
Bundles per square by shingle type
| Shingle type | Bundles per square | Coverage per bundle |
|---|---|---|
| Three-tab asphalt | 3 | 33.3 sq ft |
| Architectural / laminated | 3 | 33.3 sq ft |
| Heavyweight designer | 4 to 5 | 20–25 sq ft |
| Cedar shakes (medium) | 4 to 5 | 20–25 sq ft |
| Slate | n/a | sold by the piece |
Common mistakes
- Measuring the run instead of the rafter length. The rafter line is longer on every pitched roof.
- Missing the gable overhang at each end of the ridge.
- Treating a hip plane as a rectangle. Triangular ends are base × height ÷ 2.
- Counting the ridge twice when two planes meet — the ridge length is shared, not doubled.
- Forgetting porches, dormers and attached garages that share the roof line.
Assumptions and limits
- Each plane is measured as a rectangle. Break irregular planes into rectangles and triangles.
- Bundle counts assume three bundles per square unless your product says otherwise.
- Waste is applied to the whole roof rather than per plane.
Frequently asked questions
How many square feet is a roofing square?
One roofing square is 100 square feet of finished roof surface. It is a trade unit, not an area of ten feet by ten feet on the ground, because it measures the sloped surface.
How many bundles of shingles do I need per square?
Three bundles cover one square for standard three-tab and architectural shingles. Heavyweight designer shingles and cedar shakes need four or five bundles per square.
How do I measure roof square footage on a hip roof?
Split it into two trapezoids and two triangles. Trapezoid area is the average of the two parallel sides times the slope height. Triangle area is base times slope height divided by two.
Should I measure the roof or the ground?
Measuring the roof itself is more accurate because it captures the real plane sizes. Measuring the ground and applying a pitch multiplier is safer and accurate enough for ordering.
Does square footage include valleys and ridges?
The plane area already includes them, but the material for ridge caps, starter and valley flashing is counted separately in linear feet, not square feet.
What if my roof planes are different sizes?
Calculate each plane on its own and add the totals. Use the tool once per plane and set the plane count to one, then sum the results.
Related guides
Last updated 2026-08-05. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.