What roof pitch means
Roof pitch is the vertical rise per 12 in of horizontal run, written as "x/12" or "x in 12". A 6/12 roof rises 6 in for every foot it runs. The same steepness can be written as an angle (26.6° for 6/12) or as a percentage (50%). Roofers and building codes in the US mostly use x/12. Engineers and many non-US plans use degrees or percent.
How to measure pitch
- From the attic: hold a level horizontally against the underside of a rafter. Mark 12 in along the level, then measure straight up from that mark to the rafter. That measurement is the rise.
- On a gable end or rake: hold a level against the edge with one end touching the roof line, and measure the same way.
- On the roof: a level and tape on the shingles works the same way, as do pitch-gauge or phone apps. Only do this if you can work safely up there.
- From plans: the pitch symbol is a small triangle with 12 on the horizontal leg.
If you measured over a different run (for example 24 in), enter that run. The calculator scales it to x/12.
Pitch chart: angle, slope and multiplier
| Pitch | Angle | Slope | Multiplier |
|---|---|---|---|
| 1/12 | 4.76° | 8.33% | 1.003 |
| 2/12 | 9.46° | 16.67% | 1.014 |
| 3/12 | 14.04° | 25% | 1.031 |
| 4/12 | 18.43° | 33.33% | 1.054 |
| 5/12 | 22.62° | 41.67% | 1.083 |
| 6/12 | 26.57° | 50% | 1.118 |
| 7/12 | 30.26° | 58.33% | 1.158 |
| 8/12 | 33.69° | 66.67% | 1.202 |
| 9/12 | 36.87° | 75% | 1.25 |
| 10/12 | 39.81° | 83.33% | 1.302 |
| 12/12 | 45° | 100% | 1.414 |
| 14/12 | 49.4° | 116.67% | 1.537 |
| 16/12 | 53.13° | 133.33% | 1.667 |
| 18/12 | 56.31° | 150% | 1.803 |
The multiplier converts a flat footprint into sloped surface area: roof area = footprint × multiplier. It's what the shingle calculator uses.
The formulas
- Pitch (x/12) = 12 × rise ÷ run
- Angle = arctan(rise ÷ run)
- Slope % = 100 × rise ÷ run
- Multiplier = √(1 + (rise ÷ run)²)
- Rafter length (wall plate to ridge, no overhang) = half the span × multiplier
- Ridge height above the wall plate = half the span × rise ÷ run
Rafter length here is the sloped line length. Real rafter cutting also accounts for the ridge board thickness, the birdsmouth cut and the overhang (tail), so treat it as a planning number.
Pitch and roofing materials
Pitch decides which roofing you can use. Asphalt shingles require at least 2:12 under the IRC (R905.2.2), and from 2:12 up to 4:12 they need double underlayment or a self-adhering membrane (ARMA's lower-slope bulletin covers the details). Flatter roofs use membrane systems. Metal, tile and slate each have their own minimum slopes in code and in the manufacturer's instructions. Steeper roofs shed water faster, but they're harder and slower to work on safely.
Worked examples
Measured 4 in rise over 12 in, 28 ft spanMeasured 10.2 cm rise over 30.5 cm, 8.53 m span
Rise 4 in · Run 12 in · Building span 28 ftRise 10.2 cm · Run 30.5 cm · Building span 8.53 m
- Pitch: 4 / 12
- Angle: 18.43°
- Slope: 33.33%
- Rafter length (no overhang): 14.8 ftRafter length (no overhang): 4.5 m
- Ridge height above wall plate: 4.7 ftRidge height above wall plate: 1.4 m
Measured 15 in rise over a 24 in levelMeasured 38.1 cm rise over a 61 cm level
Rise 15 in · Run 24 in · Building span 30 ftRise 38.1 cm · Run 61 cm · Building span 9.14 m
- Pitch: 7.5 / 12
- Angle: 32.01°
- Pitch multiplier: 1.18 ×
- Rafter length (no overhang): 17.7 ftRafter length (no overhang): 5.4 m
Steep 12/12 roof, 24 ft spanSteep 12/12 roof, 7.32 m span
Rise 12 in · Run 12 in · Building span 24 ftRise 30.5 cm · Run 30.5 cm · Building span 7.32 m
- Angle: 45°
- Slope: 100%
- Pitch multiplier: 1.41 ×
- Rafter length (no overhang): 17 ftRafter length (no overhang): 5.2 m
- Ridge height above wall plate: 12 ftRidge height above wall plate: 3.7 m
Converting degrees back to pitch
If plans give an angle, rise per 12 in = 12 × tan(angle). For example, 30° → 12 × 0.577 = 6.93/12, and 45° → 12/12. Enter the rise you get with a run of 12 to see the rest of the numbers.
| Angle | Pitch (x/12) |
|---|---|
| 15° | 3.22/12 |
| 20° | 4.37/12 |
| 22.5° | 4.97/12 |
| 25° | 5.60/12 |
| 30° | 6.93/12 |
| 35° | 8.40/12 |
| 40° | 10.07/12 |
| 45° | 12.00/12 |
Matching an existing roof
When you add a porch roof, dormer or addition, matching the existing pitch makes the new roof look like it belongs, and it simplifies flashing where the roofs meet. Measure the existing pitch first. A shed or lean-to roof can be lower, but it still has to stay above the minimum slope for the roofing you plan to use. For a lean-to against a wall, the rise available (from the top of the new wall to just below the window sills or eaves above) and the depth of the roof set the pitch: pitch = 12 × available rise ÷ roof depth.
Common mistakes
- Measuring the run along the slope. Run is horizontal. Use a level.
- Confusing pitch with "roof pitch ratio". Older texts sometimes define pitch as rise ÷ span, so a "1/4 pitch" is 6/12. Check which convention the plans use.
- Using the multiplier on the wall footprint without the overhangs. Include the eaves and rakes when estimating roof area.
- Mixing up slope percent and degrees. 50% slope is 26.6°, not 50°.
From pitch to rafter length
The pitch multiplier also gives an approximate rafter length. Multiply the horizontal run (half the building span for a simple gable) by the multiplier, then do the same for the horizontal overhang. A 24 ft wide gable at 6/12 has a 12 ft run: 12 × 1.118 ≈ 13.4 ft of rafter to the wall line, plus 1.1 ft for each foot of horizontal overhang. That's an estimate for buying lumber. The actual cut length subtracts half the ridge board thickness and depends on the birdsmouth and tail cuts, so lay out the first rafter and use it as a pattern.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 6 in rise per 12 in run: angle | 26.57° | Hand calculation | Match |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 15.2 cm rise per 30.5 cm run: angle | 26.57° | Hand calculation | Match |
Frequently asked questions
What angle is a 6/12 roof?
About 26.6°.
What is a 4/12 roof in degrees?
About 18.4°.
What is the pitch multiplier for 8/12?
1.202. Multiply the footprint area by it to get the sloped roof area.
What is the minimum pitch for shingles?
2:12 under the IRC, with double underlayment required from 2:12 up to 4:12.
Is a 12/12 roof 45 degrees?
Yes. Rise equals run, so the angle is 45° and the slope is 100%.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- International Residential Code Chapter 9, Section R905.2 (asphalt shingles)
- ARMA technical bulletin: Installation of Asphalt Shingles on Lower Sloped Roofs
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.