Roof Pitch Calculator
This roof pitch calculator converts between rise-and-run, roof angle, and slope percentage, and estimates rafter length for a given horizontal run — all from either a pitch or an angle input. Whether you’re measuring roof pitch on an existing house, using it as a roofing pitch calculator to check a shingle spec, or you just need to figure out roof pitch before ordering trusses, this roof pitch estimator gets you there in one step.
Also known as: roof slope calculator, rafter length calculator, roof angle calculator, pitch to angle converter, roof pitch estimator, measuring roof pitch calculator.
Roof pitch is rise divided by a 12-inch run, written as rise/12. To figure out roof pitch from an angle: rise/12 = tan(angle) × 12. Enter either a rise-and-run pair or a measured angle below to get pitch, angle, slope percentage, and rafter length together.
Angle = atan(rise ÷ run)How to Use This Roofing Pitch Calculator
Two ways in, four answers out — pick whichever measurement you already have.
Pick rise & run, or angle
If you’re measuring roof pitch directly off a roof surface, use rise & run. If you already measured the angle with a digital angle finder or protractor, switch to the angle mode instead.
Enter your numbers
For rise & run, most framers measure over a fixed 12-inch run — that’s what “6/12” means. Add a horizontal run to the ridge if you also want an estimated rafter length.
Read all four figures
Pitch notation, angle in degrees, slope percentage, and the rafter-length multiplier update together — along with a diagram showing the triangle they describe.
Why Use a Roof Pitch Calculator
Roof pitch, roof angle, and slope percentage all describe the same incline, but they use different units — and framers, roofers, and homeowners each tend to reach for a different one depending on the task at hand. A roof pitch calculator saves you from converting between them by hand: enter a rise-and-run pair or a raw angle, and it instantly shows the equivalent pitch notation, angle in degrees, slope percentage, and the rafter-length multiplier for that incline.
This kind of conversion comes up constantly on a job site. A framer working from a “6/12” spec needs the actual angle to set a saw. A roofer estimating material coverage needs the slope percentage, since some manufacturer span tables and drainage codes are written in that unit instead of the rise/12 convention. A homeowner comparing a contractor’s quote against a shingle manufacturer’s minimum-pitch requirement needs to figure out roof pitch from their roof’s measured rise and run and translate it into the notation everyone else is using.
Because rafter length depends on both the pitch and the horizontal distance being covered, this roof pitch estimator also multiplies your entered run by the rafter factor — the trigonometric relationship √(1 + (rise÷run)²) — so you get a straight-line estimate of the sloped rafter length in the same step, rather than working the Pythagorean theorem out separately.
How This Roof Pitch Calculator Works
Roof pitch describes how steep a roof is, and this calculator expresses it four interchangeable ways from a single rise-and-run or angle input.
Pitch notation
Written as rise/12 — a “6/12” roof rises 6 inches for every 12 inches of horizontal run, regardless of the roof’s actual size.
Angle
Angle = atan(rise ÷ run), converted to degrees — the actual angle the roof surface makes with a level plane.
Slope percentage
Slope % = (rise ÷ run) × 100 — used more in civil and drainage contexts than in residential framing.
Rafter factor
√(1 + (rise÷run)²) — multiply this by the horizontal run to get the actual sloped rafter length.
Common Roof Pitches at a Glance
| Pitch | Angle | Slope % | Typical use |
|---|---|---|---|
| 2/12 | 9.5° | 16.7% | Low-slope, needs special roofing |
| 4/12 | 18.4° | 33.3% | Common minimum for asphalt shingles |
| 6/12 | 26.6° | 50% | Standard residential pitch |
| 8/12 | 33.7° | 66.7% | Steep — strong drainage, more attic space |
| 12/12 | 45° | 100% | Very steep, classic “A-frame” look |
Roof Pitch Calculator: Worked Examples
Three common framing scenarios worked end-to-end using the same formulas as the calculator above.
Finding rafter length for a known pitch and run
A gable roof has a 6/12 pitch and a horizontal run of 15 feet from the wall plate to the ridge. How long should the common rafter be?
Given inputs
- Rise: 6 in
- Run: 12 in
- Horizontal run: 15 ft
Computed outputs
- Rafter factor: √(1 + (6/12)²) ≈ 1.118
- Rafter length: 15 × 1.118 ≈ 16.77 ft
- Result: plan for roughly a 16 ft 9 in rafter before overhang and cuts
Converting a measured angle back to pitch notation
A homeowner measures their existing roof at 30° from horizontal using an angle finder. What’s the equivalent rise/12 pitch?
Given inputs
- Angle: 30°
Computed outputs
- Rise ÷ run: tan(30°) ≈ 0.577
- Pitch: 0.577 × 12 ≈ 6.93/12, roughly a 7/12 pitch
- Result: closer to a 7/12 than a 6/12 — worth rounding up when sourcing trusses
Checking whether a low-slope roof meets a shingle minimum
A porch addition has a rise of 3 inches over a 12-inch run. Does that meet a typical asphalt shingle manufacturer’s 2/12 minimum?
Given inputs
- Rise: 3 in
- Run: 12 in
Computed outputs
- Pitch: 3/12
- Angle: ≈ 14°
- Result: 3/12 clears a typical 2/12 shingle minimum, though it’s still worth checking the specific manufacturer’s spec sheet
Roof Pitch Calculator: Mistakes to Avoid
Confusing pitch with slope percentage
A “6/12” pitch and a “50% slope” describe the same incline, but mixing up the two units when reading a spec sheet or building code can lead to ordering the wrong trusses or misjudging a drainage requirement.
Forgetting the run isn’t always 12 inches
Standard pitch notation uses a 12-inch run as the reference, but if you’re measuring an actual section of roof, your run might be a different length — this calculator’s rise/run mode handles that automatically.
Treating rafter length as the finished cut length
The rafter factor gives the theoretical straight-line slope length for the horizontal run entered — it doesn’t include overhang past the wall, ridge board thickness, or the birdsmouth notch cut at the wall plate.
Measuring rise and run inconsistently
Rise and run both need to be measured in the same units over the same horizontal distance — mixing feet and inches, or measuring rise along the slope instead of vertically, throws off every downstream result.
Assuming every roof plane shares one pitch
Roofs with dormers, hips, or mixed rooflines often have more than one pitch. Calculate each distinct roof plane separately rather than applying one measurement to the whole structure.