How to Use the Roofing Calculator
- Choose gable or hip. The waste allowance adjusts to suit the roof type.
- Enter the building length, width and the overhang past the walls.
- Enter the roof pitch in 12, or switch the format to degrees.
- Check bundles per square, underlayment roll coverage and the bundle price.
- Read the bundles to buy, plus squares, underlayment, ridge cap and drip edge.
Measure the outside length and width of the building, choose feet or meters, and enter the overhang that the roof extends past the walls on each side. Enter the roof pitch as rise in 12 (for example 6 for a 6/12 roof) or switch the format to degrees. Pick gable or hip: the roof area is the same for both when every slope has the same pitch, but hips need more cutting waste and different ridge and edge lengths.
Set the waste allowance, the number of bundles that make one roofing square (shown on the wrapper), the coverage of your underlayment roll and the price per bundle. The result gives the shingle bundles to buy and a list of accessory quantities that a supplier will ask for.
Roof Area Formula
roof area = (length + 2 × overhang) × (width + 2 × overhang) × pitch factor
squares = roof area × (1 + waste %) ÷ 100
bundles = round up (squares × bundles per square)
hip length = (width ÷ 2) × √(2 + (rise ÷ 12)²)
A roofing square is 100 sq ft (9.29 m²) of roof surface. The pitch factor converts the flat footprint into the sloped surface: a 6/12 roof has a factor of 1.118, so each square foot of footprint holds 1.118 sq ft of shingles. Gable ends get rake edges along the slope, while a hip roof has eaves all around and four hip lines that need cap shingles.
Worked Example: A 40 by 30 Gable Roof
A house is 40 by 30 ft with a 12 in overhang, so the roof footprint is 42 by 32 ft, or 1,344 sq ft. At 6/12 pitch the factor is 1.118, giving a sloped area of 1,503 sq ft (139.6 m²). With 10% waste that is 16.53 squares, and at 3 bundles per square you need 49.6, so buy 50 bundles. At $45 each the shingles cost $2,250.
The same roof needs two 1,000 sq ft rolls of synthetic underlayment, 42 ft of ridge cap, 84 ft of starter strip along the eaves and 155.6 ft of drip edge, counting the four sloped rakes. As a hip roof the cap length becomes 106 ft (10 ft of ridge plus four hips of 24 ft) and the drip edge runs the full 148 ft perimeter.
Pitch Factor Table
| Pitch | Angle | Pitch factor | Hip factor |
|---|---|---|---|
| 3/12 | 14.0° | 1.031 | 1.436 |
| 4/12 | 18.4° | 1.054 | 1.453 |
| 5/12 | 22.6° | 1.083 | 1.474 |
| 6/12 | 26.6° | 1.118 | 1.500 |
| 8/12 | 33.7° | 1.202 | 1.563 |
| 10/12 | 39.8° | 1.302 | 1.641 |
| 12/12 | 45.0° | 1.414 | 1.732 |
Multiply the footprint by the pitch factor for the roof area. Multiply half the building width by the hip factor for the length of each hip on a hip roof with equal pitches.
Tips Before You Order
- Check the bundle wrapper. Most three-tab and architectural shingles come at 3 bundles per square, but some heavy designer products need 4 or 5.
- Add extra waste for valleys, dormers, skylights and short runs. Complex roofs can need 15 to 20%.
- Ridge cap shingles are sold separately and labeled with the linear feet each bundle covers.
- Ice and water shield is often required along eaves and in valleys in cold climates. Your local building code sets the width.
- If you are re-roofing, ask whether the old layers must be removed and include disposal in your budget.
Assumptions and Limits
The calculator models a simple rectangular gable or hip roof with one pitch and an equal overhang on every side. L-shaped homes, valleys, gambrel and mansard roofs should be split into rectangles and added together. Measurements from the ground are estimates: a roofer's measurement or an aerial report is more precise. Costs cover shingles only, not labor, flashing, vents or disposal.
Frequently asked questions
How many bundles of shingles do I need?
Find the sloped roof area, add waste, divide by 100 to get squares, then multiply by the bundles per square, usually 3. A 1,503 sq ft roof with 10% waste needs 50 bundles.
What is a roofing square?
A roofing square is 100 square feet of roof surface, about 9.29 square meters. Shingles, underlayment and labor are often priced per square, so contractors quote roof size in squares.
How do I calculate roof area from the house footprint?
Add the overhang to each side of the footprint, multiply length by width, then multiply by the pitch factor. A 6/12 roof has a pitch factor of 1.118.
How much waste should I add for shingles?
Around 10% is typical for a simple gable roof and about 15% for a hip roof. Roofs with valleys, dormers and skylights may need 15 to 20%.
Is a hip roof more expensive to shingle than a gable roof?
With the same pitch and footprint, the shingle area is the same. A hip roof needs more cutting waste and more ridge cap, so materials and labor usually cost a little more.
How do I convert roof pitch to degrees?
Divide the rise by 12 and take the arctangent of the result. A 6/12 pitch is 26.6 degrees, 4/12 is 18.4 degrees and 12/12 is 45 degrees.