Roofing Calculator
Turn the footprint of your building and the roof pitch into the real roof area — then into squares, shingle bundles, underlayment and cost.
- Area from pitch
- Squares and bundles
- US and metric
- Private
Measure the footprint on the ground and include the roof overhang (eaves) on every side.
10 % suits a simple gable roof; use 15 % for hips, valleys and dormers.
Materials and cost
Optional: pieces needed per unit of area, from the manufacturer. 0 = skip.
Leave at 0 to skip the cost.
How to calculate roofing materials
Footprint, pitch, waste.
- 1
Measure the footprint
Enter the length and width of the building at ground level, including the roof overhang on each side.
- 2
Enter the pitch
As rise over 12 (for example 6/12) or as an angle in degrees.
- 3
Add waste and read the list
You get the roof area, roofing squares, bundles of shingles and rolls of underlayment — plus the cost if you enter a price.
From tape measure to shopping list
No climbing on the roof required for a first estimate.
Area from the ground
The slope factor converts the flat footprint into the true sloped area.
Pitch your way
Rise over 12 as used in North America, or degrees as used in Europe; each is converted to the other.
Squares and bundles
Roofing squares of 100 ft² and the bundles needed, with adjustable coverage per bundle.
Tiles and slates
Enter the pieces per m² or ft² from the manufacturer to count clay tiles, concrete tiles or slates.
Cost estimate
Price per bundle in, material cost out.
Link to your calculation
Copy a link that reopens the calculator with your numbers filled in — handy for sharing or saving a scenario.
Made for phones
Large fields, the right keyboard for numbers and a layout that fits small screens.
Private by design
Everything is calculated in your browser. Your numbers are not sent to a server.
How roof area is calculated
A sloped roof is larger than the floor area beneath it. The slope factor tells you by how much: for a pitch of x/12 it is √(1 + (x ÷ 12)²), and for an angle it is 1 ÷ cos(angle). A common 6/12 roof (26.6°) has a factor of 1.118, so a 40 × 30 ft footprint of 1,200 ft² carries about 1,342 ft² of roof.
Roofers work in squares: one square is 100 ft² of roof. Asphalt shingles are sold in bundles, normally three bundles to the square, so the example needs 13.4 squares before waste. Add 10 % waste for a simple gable roof and about 15 % for hips, valleys and dormers, where many shingles are cut.
This method is exact for gable, shed and hip roofs with a single pitch, because all planes share the same slope. For roofs with several pitches, dormers or flat sections, calculate each section separately and add the results. Starter strips, ridge caps, flashing and drip edge are not included — order them by the linear foot.
Slope factor by roof pitch
| Pitch | Angle | Slope factor |
|---|---|---|
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Tips
- Painting fascia, siding or walls as well? Estimate it with the Paint Calculator.
- New floors inside? The Flooring Calculator adds up rooms and boxes.
- Pouring a footing or pad for the job? Use the Concrete Calculator.
- Measure pitch safely from inside the attic: hold a level against a rafter and measure the rise over 12 inches of run.
Frequently asked questions
Can’t find your answer? Contact us — we reply quickly.
How do I calculate the area of a roof?
Multiply the footprint (length × width including overhangs) by the slope factor for your pitch. A 1,200 ft² footprint at 6/12 is 1,200 × 1.118 ≈ 1,342 ft² of roof.
What is a roofing square?
A unit of 100 square feet of roof surface. A 1,500 ft² roof is 15 squares.
How many bundles of shingles are in a square?
Usually three for standard three-tab and architectural shingles, each covering about 33.3 ft². Some heavy shingles need four or five; check the wrapper and adjust the coverage per bundle.
How much waste should I add?
About 10 % for a plain gable roof and 15 % for roofs with hips, valleys or dormers. Very cut-up roofs can need more.
How do I convert roof pitch to degrees?
Angle = arctan(rise ÷ 12). A 4/12 pitch is 18.4°, 6/12 is 26.6°, 8/12 is 33.7° and 12/12 is 45°.
Are my numbers stored or sent anywhere?
No. The calculation runs in your browser. Your last inputs are remembered on your own device only, so the form is still filled in when you come back; use Reset to clear them.
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