How to use this roofing calculator
- Measure the house. Run a tape along the outside walls for the length and width. You do not need to go on the roof. If you only know the floor area, switch to Floor area.
- Add the overhang. Look at the eaves from below and estimate how far the roof sticks out past the wall. Most houses are between 1 and 2 feet.
- Enter the pitch. Pitch is how many inches the roof rises for every 12 inches across, written like 6/12. If you do not know it, use our roof pitch calculator first.
- Pick the roof shape and waste. Gable roofs have two slopes and triangle ends. Hip roofs slope down on all four sides. Choose more waste for roofs with valleys and dormers.
- Read your list. The result shows bundles to buy and every other part of the job. Press Add to my project to save it, or Print to take it to the store.
How roof area and squares are worked out
A roof is bigger than the house under it for two reasons: it hangs over the walls, and it slopes. The calculator handles both.
Footprint = (length + 2 × overhang) × (width + 2 × overhang)
Pitch factor = √(1 + (rise ÷ 12)²)
Roof surface = footprint × pitch factor
Squares = roof surface ÷ 100
Bundles = squares × (1 + waste) × bundles per square, rounded up
The pitch factor is how much longer the sloped surface is than the flat distance it covers. It comes straight from the right-angled triangle formed by the rise and the run. The same factor works for gable and hip roofs, as long as every slope has the same pitch.
Roofers buy and price by the square, which is 100 square feet of roof surface. Shingles usually come three bundles to a square.
Worked example
A 40 ft × 30 ft house with a 6/12 gable roof
- Overhang of 1 ft on every side makes the footprint 42 ft × 32 ft = 1,344 sq ft.
- Pitch factor for 6/12 = √(1 + 0.5²) = 1.118.
- Roof surface = 1,344 × 1.118 = 1,503 sq ft, which is 15.03 squares.
- Add 10% waste: 15.03 × 1.10 = 16.53 squares.
- At 3 bundles per square: 16.53 × 3 = 49.6, so buy 50 bundles.
- Drip edge: two 42 ft eaves plus four rakes of 17.9 ft = 155.6 ft, or 16 pieces of 10 ft.
- Ridge: 42 ft, which is 2 bundles of ridge cap at 25 ft each.
These are the numbers the calculator shows when you first open it, so you can check every line.
Roof pitch factor table
Multiply your roof footprint by the factor for your pitch to get the roof surface. This covers every pitch from 1/12 to 24/12.
| Pitch | Angle | Pitch factor | Extra area vs. flat |
|---|---|---|---|
| 1/12 | 4.8° | 1.003 | +0.3% |
| 2/12 | 9.5° | 1.014 | +1.4% |
| 3/12 | 14.0° | 1.031 | +3.1% |
| 4/12 | 18.4° | 1.054 | +5.4% |
| 5/12 | 22.6° | 1.083 | +8.3% |
| 6/12 | 26.6° | 1.118 | +11.8% |
| 7/12 | 30.3° | 1.158 | +15.8% |
| 8/12 | 33.7° | 1.202 | +20.2% |
| 9/12 | 36.9° | 1.250 | +25.0% |
| 10/12 | 39.8° | 1.302 | +30.2% |
| 11/12 | 42.5° | 1.357 | +35.7% |
| 12/12 | 45.0° | 1.414 | +41.4% |
| 13/12 | 47.3° | 1.474 | +47.4% |
| 14/12 | 49.4° | 1.537 | +53.7% |
| 15/12 | 51.3° | 1.601 | +60.1% |
| 16/12 | 53.1° | 1.667 | +66.7% |
| 17/12 | 54.8° | 1.734 | +73.4% |
| 18/12 | 56.3° | 1.803 | +80.3% |
| 19/12 | 57.7° | 1.873 | +87.3% |
| 20/12 | 59.0° | 1.944 | +94.4% |
| 21/12 | 60.3° | 2.016 | +101.6% |
| 22/12 | 61.4° | 2.088 | +108.8% |
| 23/12 | 62.4° | 2.162 | +116.2% |
| 24/12 | 63.4° | 2.236 | +123.6% |
Gable and hip roofs
When every slope has the same pitch, a hip roof covers the same area as a gable roof on the same footprint. The difference is in the edges: a hip roof has drip edge along all four eaves and extra ridge cap along the hips, while a gable roof has drip edge up the rakes. The calculator handles both.
How much waste to allow
Shingles are cut at every edge, hip and valley, and the cut-offs are rarely usable. Waste also gives you a few spares for future repairs, which is useful because shingle colors change between production runs.
- 10% for a plain gable roof with few cuts.
- 15% for hip roofs, roofs with one or two valleys, or a single dormer.
- 20% for cut-up roofs with several valleys, dormers or turrets.
Many stores take back unopened bundles. Ask before you buy, and round up rather than down.
Everything else a shingle roof needs
- Underlayment goes over the roof deck under the shingles. Synthetic rolls commonly cover about 1,000 sq ft; felt rolls cover less. Use the coverage printed on the roll, which already allows for the overlap.
- Ice and water shield is a self-adhesive membrane for eaves, valleys and around chimneys. Cold-climate codes often require it along the eaves. This calculator does not count it, so ask your supplier how much your area needs.
- Drip edge is the metal edge along the eaves and rakes that guides water into the gutter and protects the deck edge.
- Starter strip is the first course along the eaves (and often the rakes). It seals the bottom edge so wind cannot lift the first row.
- Ridge cap covers the ridge and, on hip roofs, the hips. It is sold by the linear foot of coverage per bundle.
- Nails: most shingles need 4 to 6 nails each, which works out at a few hundred nails per square. The wrapper says how many your shingle and wind zone need.
- Vents and flashing depend on the house and are best counted on site.
Roofing materials and how long they last
Typical service life depends on climate, ventilation, installation quality and upkeep. Use these as rough planning ranges, and check the manufacturer’s warranty for your product.
| Material | Typical life | Good to know |
|---|---|---|
| 3-tab asphalt shingles | 15 to 20 years | Lowest cost; flat, uniform look |
| Architectural asphalt shingles | 25 to 30 years | The most common US re-roof choice; thicker and more wind-resistant |
| Metal (standing seam) | 40 to 70 years | Light, sheds snow well; higher upfront cost |
| Wood shakes | 25 to 40 years | Needs upkeep; check local fire rules |
| Clay or concrete tile | 50 to 100 years | Very heavy; the roof structure must be rated for it |
| Natural slate | 75 to 150 years | Very heavy and specialist work |
| Low-slope membrane (EPDM, TPO) | 15 to 30 years | For flat and low-slope roofs below 2/12 |
This calculator counts asphalt shingle materials. For metal, tile or slate, use the roof area and squares from the result and ask your supplier how their product is sold.
Common mistakes to avoid
- Using the floor area of every storey. Only the footprint under the roof matters, not the total living area.
- Forgetting the overhang. A 1 ft overhang on a 40 × 30 house adds about 144 sq ft of roof before the pitch is applied.
- Ignoring the pitch. A footprint is not a roof. Skipping the pitch factor on a 9/12 roof leaves you a quarter short.
- Rounding bundles down. Always round up. Running out halfway through a roof is far more costly than a spare bundle.
- Assuming 3 bundles per square. Most shingles are, but heavy designer shingles can be 4 or 5. Read the wrapper.
Frequently asked questions
How many bundles of shingles are in a square?
Most asphalt shingles, both 3-tab and architectural, come 3 bundles to a square, and one square covers 100 square feet of roof. Some heavy designer shingles need 4 or 5 bundles per square, so check the wrapper and change "Bundles per square" if yours is different.
How do I work out roofing squares?
Multiply the roof footprint (including the overhang) by the pitch factor to get the roof surface in square feet, then divide by 100. A 1,500 sq ft roof surface is 15 squares. Add your waste allowance before you order.
How much waste should I add for shingles?
About 10% for a simple gable roof, 15% for a hip roof or one with valleys, and up to 20% for a cut-up roof with dormers and several valleys. Waste covers cut-offs at the rakes, hips and valleys and a few spare shingles for repairs.
Is roof area the same as the floor area of my house?
No. The roof is bigger than the floor because it overhangs the walls and slopes upward. A 6/12 roof is about 12% larger than its footprint, and a 12/12 roof is about 41% larger.
Do I need to count the second floor?
No. Only the footprint the roof covers matters. A two-storey house with a 1,200 sq ft footprint has roughly the same roof as a one-storey house with the same footprint and pitch.
Can I measure my roof from the ground?
Yes. Measure the outside walls with a tape, add the overhang, and find the pitch from inside the attic or from a gable end with a level. You do not need to climb on the roof to use this calculator.