How to use the roofing calculator
You don't need to climb onto the roof to get a good estimate. The calculator works from the building's footprint, which you can measure on the ground, and the roof pitch.
- Measure the footprint from the ground. Run a tape along the outside walls to get the building's length and width. Then estimate the overhang: how far the eaves and gable rakes stick out past the walls, measured flat. 12–24 in is typical. Another option is to measure along the drip edge or gutter line and enter 0 for the overhang.
- Find the pitch. Pitch is the inches of rise per 12 in of horizontal run. At a gable end or in the attic, hold a level against the underside of a rafter, mark 12 in along the level, and measure straight up to the rafter. Our roof pitch calculator converts other measurements (rise and run, or an angle) to pitch.
- Choose a waste allowance. Use 10% for a simple gable roof and 15% for hip roofs or roofs with valleys. Every hip and valley means cutting shingles at an angle, and the offcuts are usually too small to reuse.
- Check the bundle count. Most 3-tab and architectural shingles are packed 3 bundles to a square. Some heavyweight designer shingles come 4 or 5 bundles per square, so read the wrapper before you order.
If you already have a roof area, for example from a contractor's measurement report, switch to "I know the roof area". Keep the pitch set, because it decides how much underlayment you need.
The roofing formula
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × pitch multiplier
Squares = roof area × (1 + waste %) ÷ 100
Bundles = squares × bundles per square, rounded up
Underlayment rolls = roof area × layers ÷ coverage per roll, rounded up
A pitched roof has more surface than the flat footprint under it. The multiplier is the length of the sloped rafter line for every 12 in of horizontal run. It works for gable roofs and for hip roofs, as long as every plane has the same pitch, because each plane's area is its flat footprint times the same factor. For roofs with mixed pitches, dormers or several levels, work out each section separately and add the results.
A "square" is the roofing trade's unit: 100 sq ft of roof surface. Shingles are priced and packed by the square.
Worked example: 40 × 30 ft house, 6/12 gable roof
With a 12 in overhang all around, the footprint becomes 42 × 32 = 1,344 sq ft. The 6/12 multiplier is √(36 + 144) ÷ 12 = 1.118, so the roof area is 1,344 × 1.118 = 1,502.6 sq ft. Adding 10% waste gives 1,652.9 sq ft, or 16.5 squares. At 3 bundles per square, 16.53 × 3 = 49.6, which rounds up to 50 bundles. A 6/12 roof needs one layer of underlayment, so rolls covering 1,000 sq ft each give 1,502.6 ÷ 1,000 = 1.5, or 2 rolls.
Pitch multipliers and underlayment
| Pitch | Angle | Multiplier | Underlayment (IRC) |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | Two layers |
| 3/12 | 14.0° | 1.031 | Two layers |
| 4/12 | 18.4° | 1.054 | One layer |
| 5/12 | 22.6° | 1.083 | One layer |
| 6/12 | 26.6° | 1.118 | One layer |
| 7/12 | 30.3° | 1.158 | One layer |
| 8/12 | 33.7° | 1.202 | One layer |
| 9/12 | 36.9° | 1.250 | One layer |
| 10/12 | 39.8° | 1.302 | One layer |
| 12/12 | 45.0° | 1.414 | One layer |
Typical packaging
These are common figures, but packaging varies by brand and product line. Use the coverage printed on the label, and enter it in the calculator if it differs.
| Product | Typical coverage |
|---|---|
| 3-tab and architectural shingles | 3 bundles per square |
| Heavyweight or designer shingles | Often 4–5 bundles per square |
| Synthetic underlayment | Widely varies; 10-square (≈1,000 sq ft) and 4-square rolls are common |
| #15 asphalt felt | About 4 squares per roll |
| #30 asphalt felt | About 2 squares per roll |
Tips for ordering roofing materials
- Starter strip and ridge cap are extra. They aren't included in the bundle count. For starter, measure the eaves (and the rakes, if your shingle maker calls for it there). For hip and ridge cap, measure every hip and ridge. Both are sold by the bundle, with linear-foot coverage printed on the wrapper.
- Don't add waste twice. If a roofer's report already includes a waste factor, set waste to 0% here.
- Remember the flashings. Drip edge goes along eaves and rakes, and walls, chimneys and skylights need step or counter flashing. Measure those lengths separately.
- Check tear-off rules. Codes limit how many layers of roofing a roof may carry. If there is already a layer of shingles, ask your building department whether you can re-cover or must tear off.
- Ask about returns. Many suppliers take back unopened bundles. Ordering one or two extra can save a second delivery.
Planning a deck or porch roof as well? Get the floor area from the square footage calculator and roof it as a separate section.
Frequently asked questions
How many bundles of shingles are in a square?
Most 3-tab and architectural asphalt shingles come 3 bundles to a square (100 sq ft of roof). Some heavier designer shingles come 4 or 5 bundles per square, so check the wrapper.
How many squares of shingles for a 2,000 sq ft house?
It depends on the roof, not the floor area. A single-story house with a 2,000 sq ft footprint and a 6/12 roof (no overhang) has about 2,236 sq ft of roof, or 24.6 squares with 10% waste. That is 74 bundles. A two-story house with the same floor area has roughly half the footprint, so it needs far less roofing.
What is a roof pitch multiplier?
It is the factor that converts flat footprint area into sloped roof area: √(rise² + 144) ÷ 12. A 6/12 roof has a multiplier of 1.118, so each 100 sq ft of footprint becomes about 112 sq ft of roof.
How much waste should I add for shingles?
About 10% for a simple gable roof and 15% for hip roofs or roofs with valleys and dormers. Very cut-up roofs can need more.
Do low-slope roofs need extra underlayment?
Yes. Under the IRC, asphalt shingles on slopes from 2/12 up to 4/12 need two layers of underlayment, and shingles are not allowed below 2/12. Local code and the shingle maker's instructions govern.