How to calculate roofing materials
Roof area is larger than the footprint of the house because of the slope. Multiply the footprint by a pitch factor, then divide by 100 to get roofing squares.
The formula
Roof area = footprint × pitch factor; squares = roof area ÷ 100
Worked example
A 1,500 square foot footprint with a 6/12 pitch (factor 1.118) has 1,677 square feet of roof. With 10% waste that is about 19 squares, or 57 bundles of shingles.
| Pitch | Factor |
|---|---|
| 4/12 | 1.054 |
| 6/12 | 1.118 |
| 8/12 | 1.202 |
| 12/12 | 1.414 |
How to use this calculator
Enter the house length and width, choose the roof pitch, then enter the eave overhang and a waste allowance. The roofing calculator works out the roof area, the roofing squares with waste, the shingle bundles and the underlayment rolls to buy. Add an installed cost per square to see an estimated cost. Measure the house at the walls, because the overhang is entered separately.
Where the pitch factor comes from
Pitch is written as rise over run. A 6/12 roof rises 6 inches for every 12 inches of horizontal distance. The pitch factor is the length of the slope compared with that horizontal distance:
Pitch factor = √(1 + (rise ÷ 12)²)
For 6/12, rise ÷ 12 is 0.5, so the factor is √1.25 = 1.118. The same formula gives the factor for pitches that are not in the table above.
| Pitch | Factor |
|---|---|
| 3/12 | 1.031 |
| 5/12 | 1.083 |
| 7/12 | 1.158 |
| 9/12 | 1.250 |
| 10/12 | 1.302 |
More examples
The table shows how pitch changes the order for the same 1,500 square foot footprint, with 10% waste and three bundles per square.
| Pitch | Roof area | Squares to buy | Bundles |
|---|---|---|---|
| 4/12 | 1,581 sq ft | 18 | 54 |
| 6/12 | 1,677 sq ft | 19 | 57 |
| 8/12 | 1,803 sq ft | 20 | 60 |
| 12/12 | 2,121 sq ft | 24 | 72 |
A larger, more complex roof: a 2,000 square foot footprint at 8/12 has 2,000 × 1.202 = 2,404 square feet of roof. With 15% waste that is 2,765 square feet, so you would buy 28 squares, or 84 bundles.
Estimating cost
Multiply the squares by the price per square. For example, if shingles cost $120 per square, the 19 squares in the worked example come to $2,280 for shingles alone. Underlayment, flashing, ridge caps, nails and labor are extra.
What changes the result
- Pitch. A steeper roof has more surface over the same footprint. Going from 4/12 to 12/12 adds about a third.
- Footprint. Overhangs, attached garages and covered porches all add roof area.
- Waste. Hips, valleys and dormers mean more cut shingles.
- Rounding. Shingles are sold in whole bundles, so the order is always rounded up.
Common mistakes
- Entering the home’s total living area. A two-story house has about half as much roof as its floor area suggests, so use the footprint.
- Leaving out the overhangs at the eaves.
- Skipping the pitch factor and ordering for the flat footprint.
- Rounding down to the nearest square.
- Applying one pitch to a roof that has sections at different slopes. Calculate each section and add them.
Related calculators
Use the square footage calculator to work out the footprint of an L-shaped or irregular house. To plan for a future roof replacement alongside other upkeep, see the home maintenance cost calculator, and for a wider project budget use the renovation cost calculator. The quantities here are estimates. A roofer who measures the roof itself may arrive at a different figure.
Frequently asked questions
What is a roofing square?
100 square feet of roof.
How many bundles are in a square?
Usually three for standard asphalt shingles.
How much waste should I add?
10% for a simple roof and 15% for one with many hips and valleys.
How do I find my roof pitch?
Hold a level flat against the roof or a rafter, mark 12 inches along it, and measure straight down from that mark to the roof surface. That distance in inches is the rise, so 6 inches means a 6/12 pitch.
How many squares do I need for a 2,000 square foot roof?
20 squares, or 22 with 10% waste. At three bundles per square that is 66 bundles.
Does a two-story house need more roofing than a one-story house?
Not for the same footprint. The roof covers the footprint once, however many floors are below it.
How long does an asphalt shingle roof last?
About 20 to 30 years, depending on the shingle, the climate and how well the roof is ventilated and maintained.