How to use the roof pitch calculator
- Choose what you know. Most people measure rise and run. If you have a digital level or a phone app reading, use Angle. If the plans already give a pitch like 7/12, use Pitch.
- Read the results. You get the pitch as x/12, the angle in degrees, the slope as a percentage and the rafter multiplier (how much longer the sloped roof is than its flat footprint).
- Add the building size to turn the pitch into real roof area, including the eaves, and the number of roofing squares to order.
Roof pitch formula
Roof pitch is the vertical rise for every 12 units of horizontal run. A roof that climbs 6 inches over 12 inches of run is a 6/12 pitch (say "six in twelve").
Angle (°) = arctan(Rise ÷ Run)
Slope (%) = Rise ÷ Run × 100
Rafter multiplier = √(1 + (Pitch ÷ 12)²)
Roof area = (Width + 2 × overhang) × (Length + 2 × overhang) × multiplier
One roofing square is 100 sq ft of roof surface. Shingles, metal panels and underlayment are all sold against squares, so this is the number your supplier will ask for.
How to measure roof pitch
- On the roof: hold a 2 ft level flat against the shingles, mark 12 in from the end, and measure straight down from that mark to the roof surface. That distance in inches is your pitch.
- From the attic: do the same against the underside of a rafter. It avoids climbing on the roof.
- From the ground: phone level apps and digital angle finders read the angle off a rake board or gable end. Enter it with the Angle tab.
Roof pitch chart
Common pitches with their angle, slope and rafter multiplier. Multiply the flat footprint of a roof by the multiplier to get its true surface area.
| Pitch | Angle | Slope | Multiplier |
|---|---|---|---|
| 1/12 | 4.8° | 8.3% | 1.003 |
| 2/12 | 9.5° | 16.7% | 1.014 |
| 3/12 | 14.0° | 25.0% | 1.031 |
| 4/12 | 18.4° | 33.3% | 1.054 |
| 5/12 | 22.6° | 41.7% | 1.083 |
| 6/12 | 26.6° | 50.0% | 1.118 |
| 7/12 | 30.3° | 58.3% | 1.158 |
| 8/12 | 33.7° | 66.7% | 1.202 |
| 9/12 | 36.9° | 75.0% | 1.250 |
| 10/12 | 39.8° | 83.3% | 1.302 |
| 11/12 | 42.5° | 91.7% | 1.357 |
| 12/12 | 45.0° | 100.0% | 1.414 |
| 14/12 | 49.4° | 116.7% | 1.537 |
| 16/12 | 53.1° | 133.3% | 1.667 |
| 18/12 | 56.3° | 150.0% | 1.803 |
Minimum roof pitch by roofing material
The International Residential Code (IRC) sets a minimum slope for each roof covering. Below it, water can back up under the laps. Manufacturers' instructions and your local code can be stricter.
| Roofing material | Minimum pitch |
|---|---|
| Asphalt shingles | 2/12 (double underlayment below 4/12) |
| Clay or concrete tile | 2½/12 (double underlayment below 4/12) |
| Wood shingles and shakes | 3/12 |
| Slate | 4/12 |
| Standing seam metal | ¼/12 |
| Lapped (non-soldered) metal panels | 3/12 |
| Single-ply membrane (EPDM, TPO) | ¼/12 |
Worked examples
You measure 8 in of rise over a 12 in run. Pitch = 8/12, angle = arctan(8 ÷ 12) = 33.7°, slope = 66.7%, multiplier = 1.202.
A phone app reads 22.6° on the rake board. tan(22.6°) × 12 = 5.0, so the roof is a 5/12 pitch.
A 30 × 40 ft house with 12 in overhangs and a 6/12 pitch: (32 × 42) × 1.118 = 1,503 sq ft. Add 10% waste and order 17 squares.
Frequently asked questions
What is the most common roof pitch?
Most houses in North America fall between 4/12 and 9/12, with 6/12 the single most common. That range sheds water well, takes standard shingles and is still walkable for roofers.
What is the difference between pitch and slope?
They describe the same thing in different ways. Pitch is rise per 12 units of run (6/12), slope is usually written as a percentage (50%) or a ratio (1:2), and angle is in degrees (26.6°).
Is a 4/12 pitch roof walkable?
Yes. Pitches up to about 6/12 are generally considered walkable with care. Above 8/12 roofers normally use roof jacks, harnesses and toe boards, which adds to labor cost.
Does a steeper roof need more shingles?
Yes. The same house footprint has more roof surface as the pitch rises. A 12/12 roof has about 41% more area than a flat roof over the same footprint, compared with 12% more at 6/12.
What is a low-slope roof?
Roofs below about 3/12 are usually treated as low slope. Standard shingles need extra underlayment between 2/12 and 4/12 and aren't allowed below 2/12, so low-slope roofs typically use membranes or standing seam metal.