QuantityBench Roof Pitch Calculator
Enter rise and run to convert roof pitch into degrees, slope factor, and rafter length. Building width and rafter length support metric.
Estimated pitch
How this is calculated
Roof pitch describes how steep a roof is, expressed as rise over run — almost always rise per 12 inches of run.
1. Pitch ratio: rise ÷ (run ÷ 12), expressed as "x/12".
2. Angle: arctangent(rise ÷ run), converted from radians to
degrees.
3. Slope factor: square root(rise² + run²) ÷ run — the
multiplier between horizontal distance and actual roof-surface distance.
4. Rafter length: half the building width × slope factor, when a
building width is supplied.
Assumptions
- Assumes a simple gable or shed roof geometry — hips, valleys, and dormers change the actual rafter and material count.
- Rafter length here is the raw slope length; it doesn't add an overhang or fascia allowance, which a roofer typically adds on top.
- Rise and run are always in inches, matching the standard "x/12" convention, even when building width is shown in meters.
Worked example
A 4/12 roof (rise 4", run 12") on a 24 ft wide building:
| Pitch ratio | 4/12 |
|---|---|
| Angle | 18.43° |
| Slope factor | 1.054 |
| Rafter length (full span) | 12.65 ft |
Rafter length calculator
Rafter length is always longer than the horizontal distance it covers, because it follows the slope of the roof rather than running flat. The steeper the pitch, the bigger that difference gets. For a 6/12 pitch (a common medium-slope residential roof) on a 30 ft wide building:
| Pitch ratio | 6/12 |
|---|---|
| Slope factor | 1.118 |
| Half-span (flat distance) | 15 ft |
| Rafter length (full span) | 16.77 ft |
Notice the rafter (16.77 ft) is longer than the flat half-span it covers (15 ft) — that gap is exactly what the slope factor accounts for, and it's why steeper roofs need more actual material than their flat footprint would suggest. Use the calculator above with your own rise, run, and building width for an exact figure, and add your own overhang/fascia allowance on top — this is the raw slope length only.
How to measure roof pitch without going on the roof
You don't need to climb up there to get an accurate rise-and-run measurement — both of these methods give you the "rise" number to type into the calculator above (with run left at the standard 12).
From inside the attic (most accurate): hold a torpedo level flat against the underside of a rafter so it's perfectly horizontal. Mark a point exactly 12 inches out along the level. From that mark, measure straight down to the rafter — that vertical distance in inches is your rise. A 5 inch drop over a 12 inch level means a 5/12 pitch.
From the ground or a ladder, outside: use the same technique against the gable end or fascia board, holding a level (or a straight board with a level taped to it) out 12 inches and measuring the vertical drop. A smartphone level app clipped to a straight board works well if you don't have a torpedo level handy.
Once you have that rise measurement, enter it above with the run left at 12 to get the exact pitch ratio, angle in degrees, and — if you add your building width — full rafter length.
Frequently asked questions
What does a "4/12" roof pitch mean?
A 4/12 pitch means the roof rises 4 inches vertically for every 12 inches (1 foot) of horizontal run. The second number is almost always 12 by convention, so roofers usually just say "a 4 pitch" or "4/12."
How do I convert roof pitch to degrees?
Take the arctangent of rise divided by run: angle = atan(rise / run). For a 4/12 pitch, that's atan(4/12), which comes out to about 18.43 degrees.
How do I calculate rafter length from roof pitch?
Multiply the horizontal run (or half the building width, for a full rafter) by the slope factor, which is the hypotenuse-to-run ratio: sqrt(rise² + run²) / run. A 4/12 pitch has a slope factor of about 1.054.
What's a typical roof pitch for a house?
Most residential roofs fall between 4/12 and 9/12. Pitches below 4/12 are considered low-slope and often need special roofing materials and installation methods to shed water properly.
How does roof pitch affect material needed?
A steeper roof has more actual surface area than its flat footprint suggests, because the roof plane is longer than the horizontal span it covers. The slope factor is exactly this multiplier — apply it to your footprint area to estimate actual roofing material needed.
Why can't I enter rise and run in metric units?
The "x/12" pitch ratio is defined around 12 inches of run, so rise and run are kept in inches even in metric mode — converting them to centimeters wouldn't change the angle, but it would break the familiar "x/12" figure roofers use. Building width and rafter length, which are plain linear measurements, do switch to meters when you choose Metric.