Roof Pitch Calculator
Convert roof pitch between every form — rise/run (x / 12), angle in degrees, slope percentage, and the pitch factor — plus rafter length and ridge height for your span, with a diagram and downloadable PDF.
U.S. roof pitch is written as rise over a 12-inch run (for example 6 / 12). Rafter length here excludes ridge-board thickness and birdsmouth cuts. For planning purposes only.
| Pitch | Angle | Grade | Factor |
|---|
The highlighted row is closest to your pitch. Pitch factor multiplies flat area to get sloped roof area.
How to use the roof pitch calculator
Four quick steps convert your roof slope into every form you need — rise over run, degrees, percent grade, the slope multiplier, and rafter length — with a pitch comparison and a downloadable PDF.
Enter what you know
Type a pitch like 6/12, a rise and run, an angle in degrees, or a percent grade — any one works.
Add the span (optional)
Enter the building span or run if you also want the common rafter length worked out.
Read all four forms
See the pitch as rise/run, degrees, grade, and multiplier instantly, plus the slope category.
Save a PDF
Get the conversions, rafter length, a pitch comparison, and a summary you can download.
How roof pitch is calculated
Roof pitch is one slope described four ways. Everything comes from the rise over a 12-inch run — a little trigonometry turns it into degrees, grade, the slope multiplier, and the rafter length. Here is the exact math.
Worked example — a 6/12 roof pitch, 30 ft span
Angle: arctan(6 ÷ 12) ≈ 26.57°
Grade: (6 ÷ 12) × 100 = 50% | Multiplier: √(6²+12²) ÷ 12 ≈ 1.118
Run & rise: 30 ÷ 2 = 15 ft run, 15 × (6÷12) = 7.5 ft rise
Rafter: √(15² + 7.5²) ≈ 16.77 ft
6/12 = 26.6° = 50% grade = ×1.118, and a 30 ft span gives a ~16.8 ft common rafter (before overhang).
Roof pitch conversion charts
Handy lookups for the questions people ask most — pitch to degrees, grade, and multiplier, rafter lengths, and which pitch suits which roof. Everything follows from the rise over a 12-inch run.
| Pitch | Angle | Grade | 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 |
| 6/12 | 26.6° | 50.0% | ×1.118 |
| 8/12 | 33.7° | 66.7% | ×1.202 |
| 9/12 | 36.9° | 75.0% | ×1.250 |
| 10/12 | 39.8° | 83.3% | ×1.302 |
| 12/12 | 45.0° | 100.0% | ×1.414 |
The multiplier is what converts flat footprint to true roof area — a 12/12 roof has 41% more surface than the plan below it. A 12/12 is exactly 45° because the rise equals the run.
| Span | 4/12 | 6/12 | 8/12 | 12/12 |
|---|---|---|---|---|
| 20 ft | 10.5 | 11.2 | 12.0 | 14.1 |
| 24 ft | 12.6 | 13.4 | 14.4 | 17.0 |
| 28 ft | 14.8 | 15.7 | 16.8 | 19.8 |
| 32 ft | 16.9 | 17.9 | 19.2 | 22.6 |
Common rafter = (span ÷ 2) × the pitch multiplier. Add your overhang and subtract for the ridge board to get the real cut length.
| Category | Pitch range | Angle | Notes |
|---|---|---|---|
| Flat / low-slope | Up to 3/12 | 0–14° | Needs membrane roofing; slow drainage |
| Conventional | 4/12 – 8/12 | 18–34° | Sheds water well; walkable with care |
| Steep | 9/12 – 12/12 | 37–45° | Dramatic look; needs roof jacks / staging |
| Very steep | Over 12/12 | Over 45° | Specialist work; highest labor cost |
Most homes sit in the conventional band. Steeper roofs shed snow and look striking but cost more to build and re-roof because of the added area and the safety staging.
| Material | Minimum pitch | Notes |
|---|---|---|
| Membrane (EPDM / TPO / built-up) | ~1/4:12 | The choice for flat and low-slope roofs |
| Standing-seam metal | ~1/2:12 – 3/12 | Handles low slopes with sealed seams |
| Asphalt shingles | 2/12 (4/12 standard) | Below 4/12 needs double underlayment |
| Clay / concrete tile | ~2.5/12 – 4/12 | Underlayment detail matters at the low end |
| Wood shakes / slate | 4/12+ | Steeper is better for shedding water |
These are general minimums — always follow the manufacturer’s instructions, since going below the rated slope can void the warranty. Steeper roofs shed water faster and last longer for a given material.
18°
26.6°
45°
Pitch is a right triangle: the run is the flat 12-inch base, the rise is how far it climbs, and the roof surface is the sloped hypotenuse. Steepen the rise and every number grows together.
- Rise / run — the builder’s shorthand, always over a 12-inch run, like 6/12.
- Degrees & grade — the angle from arctan(rise/12), and the grade as a percent of run.
- Multiplier & rafter — the slope factor sizes roof area, and the hypotenuse is the rafter.
Everything the calculator works out
One slope, entered any way you like, comes back in every form you need — for ordering materials, cutting rafters, or checking a plan.
Key figures behind roof pitch
Built for anyone working with roof slope
From reading a plan to cutting a rafter, the same calculator gives the slope in whatever form you need.
Framing a shed, porch, or addition and needs the angle and rafter length from a pitch on the plans.
- Enter the pitch, get the angle
- Add the span for rafter length
- Remember overhang and ridge board
Needs the slope multiplier to turn a footprint into true roof area before ordering materials.
- Grab the multiplier for area
- Check the slope category for staging
- Confirm material minimum pitch
Wants to know their roof’s pitch in degrees or grade for solar, gutters, or a design decision.
- Measure with a level and tape
- Convert to degrees or grade
- See if shingles suit the slope
7 tips for working with roof pitch
Small habits keep your slope numbers right, whether you’re framing, roofing, or just reading a plan.
Roof pitch calculator FAQ
The pitch, conversion, measurement, and rafter questions people ask most.
Roof pitch is the slope of a roof, written as the vertical rise in inches for every 12 inches of horizontal run, like 6/12. A 6/12 roof rises 6 inches for each foot it runs.
The same slope can be shown four ways: as rise over run (6/12), as an angle in degrees (about 26.6 degrees), as a percent grade (50%), or as a slope multiplier used to find true roof area.
Measure the vertical rise over a 12-inch horizontal run. Hold a level out horizontally from the roof or a rafter, mark 12 inches along it, then measure straight down from that mark to the roof surface — that drop in inches is the rise over a 12-inch run.
So if it drops 6 inches over 12 inches of level, the pitch is 6/12. The calculator then turns that into degrees, grade, and the slope multiplier for you.
Take the arctangent of rise divided by run. Since the run is always 12, the angle in degrees is the inverse tangent of rise divided by 12.
For example, a 6/12 pitch is arctan(6/12), which is about 26.6 degrees. A 4/12 is about 18.4 degrees, and a 12/12 is exactly 45 degrees because the rise equals the run.
The pitch multiplier, or slope factor, converts a flat plan area into true sloped roof area. It’s the square root of (rise squared plus 144), divided by 12.
A 6/12 pitch has a multiplier of about 1.118, so the roof surface is roughly 12% larger than the footprint below it. A steep 12/12 multiplier is about 1.414, or 41% more area. It’s the key number for estimating roofing materials.
Use a level and a tape. Hold the level horizontally against the underside of a rafter or on top of the roof, measure 12 inches along it from where it touches, then measure straight down from the 12-inch mark to the roof — that vertical distance in inches is your pitch over 12.
You can also use a smartphone pitch or angle app held against a rafter, or measure the rise and run in the attic. Always take safety precautions if you go up on the roof.
Most residential roofs fall between about 4/12 and 9/12, which sheds water and snow well and is safe to walk with care. Anything from 4/12 to 8/12 is considered conventional slope.
Low-slope roofs under about 3/12 usually need membrane roofing rather than shingles, and steep roofs over 9/12 are dramatic but harder and costlier to work on. The best pitch depends on climate, style, and the roofing material.
A common rafter is the hypotenuse of a right triangle whose legs are the run and the rise, so its length is the square root of (run squared plus rise squared). You can also multiply the run by the pitch multiplier.
For a 30 ft span at 6/12, the run is 15 ft and the rise is 7.5 ft, so the rafter is about 16.8 ft before adding any overhang. Add the overhang and account for the ridge board thickness for the actual cut length.
Asphalt shingles can go down to about 2/12 as an absolute minimum, but only with a special double-underlayment or waterproofing detail. A 4/12 pitch or steeper is the standard for a normal shingle installation.
Below 2/12, use a low-slope membrane such as EPDM, TPO, or built-up roofing instead. Always follow the shingle manufacturer’s minimum-slope instructions to keep the warranty valid.
General Estimating Notice: This calculator converts roof pitch between rise/run, degrees, percent grade, and the slope multiplier, and estimates common rafter length from your span, using standard trigonometry. Rafter results are to the wall before overhang and do not account for ridge board thickness, seat cuts, or bird’s-mouth details. Minimum-pitch and material guidance is general — always follow the manufacturer’s instructions and local building code, have structural work reviewed by a professional, and take full safety precautions when measuring on a roof. For planning purposes only.