Roof area is hard to measure directly and risky to measure on top of the roof. Fortunately you can get it from the ground. Take the building’s footprint, add the overhang on every side, and multiply by a pitch factor that converts horizontal area into sloped area. This calculator does exactly that and then turns the area into roofing squares, shingle bundles and the trim materials that go along the edges.
How to use the roofing calculator
- Choose US or Metric units.
- Pick the roof shape: gable, hip or shed.
- Enter the building length and width, measured outside wall to outside wall.
- Enter the overhang at the eaves and rakes, measured level.
- Enter the pitch as inches of rise per 12 inches of run (in metric, the slope angle in degrees). The roof pitch calculator converts between the two.
- Adjust the waste, bundles per square and underlayment roll coverage to match your products.
Roofing formulas
where o is the overhang. Hips are found with the hip factor √(pitch² + 288) ÷ 12, applied to half the building width.
Worked example: a 40 × 30 ft gable roof
Footprint with 1 ft overhang: 42 × 32 = 1,344 ft².
Pitch factor for 6/12: √(1 + 0.5²) = 1.1180, so the roof is 1,344 × 1.1180 = 1,502.6 ft² = 15.03 squares.
With 10% waste: 16.53 squares × 3 bundles = 49.6 → 50 bundles.
Edges: 84 ft of eaves, 71.6 ft of rakes (156 ft of drip edge, 16 ten-foot pieces) and a 42 ft ridge.
The same house with a hip roof and 15% waste needs 52 bundles, and its four hips plus a 10 ft ridge add up to about 106 ft of hip and ridge cap.
Pitch factors for common slopes
| Pitch | Angle | Pitch factor |
|---|---|---|
| 3/12 | 14.0° | 1.0308 |
| 4/12 | 18.4° | 1.0541 |
| 5/12 | 22.6° | 1.0833 |
| 6/12 | 26.6° | 1.1180 |
| 7/12 | 30.3° | 1.1577 |
| 8/12 | 33.7° | 1.2019 |
| 9/12 | 36.9° | 1.2500 |
| 10/12 | 39.8° | 1.3017 |
| 12/12 | 45.0° | 1.4142 |
Asphalt shingles need a slope of at least 2/12 under the International Residential Code, and slopes from 2/12 up to 4/12 require two layers of underlayment. Flatter roofs need a membrane system instead.
What else to order
- Underlayment: synthetic rolls commonly cover about 1,000 ft²; #15 felt about 400 ft² and #30 felt about 200 ft² per roll.
- Ice barrier: in cold climates, codes require a self-adhering membrane from the eave edge to at least 24 inches inside the exterior wall where ice dams are a risk.
- Drip edge: along eaves and rakes, sold in 10 ft pieces.
- Starter strip: along the eaves (and the rakes, if the shingle maker recommends it).
- Hip and ridge cap: coverage varies by product, often 20–35 linear feet per bundle.
- Flashing and vents: count chimneys, skylights, wall junctions and roof vents separately.
When tearing off, ask about disposal: a square of asphalt shingles weighs roughly 200–350 lb, and dumpster fees are usually by weight.
Measure the attic while you are at it: the insulation calculator shows whether it meets the recommended R-value for your climate zone, and re-roofing is a good time to fix ventilation.
Frequently asked questions
How many bundles of shingles do I need for a 2,000 square foot roof?
A 2,000 ft² roof is 20 squares. Adding 10% for waste makes 22 squares, and at three bundles per square that is 66 bundles. Use about 15% waste for a hip roof and more for roofs with valleys and dormers.
What is a square in roofing?
A roofing square is 100 square feet of roof surface. Shingles are priced and packaged by the square; most asphalt shingles come three bundles to the square, so one bundle covers about 33⅓ ft².
Do I use the house's square footage to estimate a roof?
No. Use the outside dimensions of the building plus the overhang on every side, then multiply by the pitch factor for the slope. The living area of a two-story house, for example, has little to do with its roof area.
Does the roof shape change the area?
If every plane has the same pitch, gable, hip and shed roofs all have the same area: footprint times pitch factor. The shape changes the waste, the edge lengths and the amount of ridge and hip cap, which this calculator reports separately.
How many roofing nails per square?
Most shingles are fastened with four nails each, about 300–320 nails per square for standard three-tab shingles. High-wind installations and some laminated shingles call for six nails per shingle, so check the wrapper.