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Rafter Angle Calculator

Converts a roof pitch given as rise per 12 of run into the rafter angle in degrees and the corresponding plumb cut angle, which is 90 degrees minus the rafter angle.

What this calculator does

The Rafter Angle Calculator estimates your result instantly from the values you enter, using standard construction & home formulas. Converts a roof pitch given as rise per 12 of run into the rafter angle in degrees and the corresponding plumb cut angle, which is 90 degrees minus the rafter angle. Calculations run in your browser, so nothing you type is sent to a server, and the output is an estimate rather than a certified or official figure.

Rafter Angle Calculator

Result

—
Rafter Angle
—
Plumb Cut Angle

About Rafter Angle Calculator

This rafter angle calculator converts a roof pitch into angles you can set on a saw or speed square. It takes the rise per 12 of run and computes the rafter angle as the arctangent of rise over 12, then gives the plumb cut angle as 90 degrees minus that rafter angle.

Formula & Mathematical Calculation

Rafter Angle = f(Rise (in per 12 run))

Calculated according to standard construction formulas implemented in the calculator core.

Calculation Example

For a 6:12 pitch (6 in rise per 12 in run):

  • Rafter angle = arctan(6 ÷ 12) ≈ 26.57°
  • Plumb cut angle = 90 − 26.57 ≈ 63.43°

Key Benefits

  • Turns a rise-per-12 pitch into a precise rafter angle in degrees
  • Also gives the plumb cut angle for marking rafter ends
  • Needs only one input, the rise per 12 of run
  • Reports both angles to two decimal places for accurate layout

How to Use this Calculator

  1. Enter the rise
    Type the roof rise in inches per 12 inches of run into the Rise (in per 12 run) field, for example 6 for a 6:12 pitch. The run is fixed at 12.
  2. Calculate
    Press Calculate to compute the rafter angle from atan2(rise, 12).
  3. Read the results
    Read the Rafter Angle box, the Plumb Cut Angle box (90 degrees minus the rafter angle), and the plain-language interpretation below them.

Assumptions & Limitations

What the calculation assumes

  • Every value you enter is treated as an exact, known amount.
  • Any rate or interval you enter is assumed to hold for the whole period unless the form asks for it separately.
  • Results use standard published formulas and account only for the values entered above.
  • Rounding is applied for display; the underlying calculation uses your full precision.
  • Results are returned in the same units you enter, unless the tool states the output unit.

What it does not cover

  • Rafter Angle Calculator produces a calculated result for reference and learning, not a certified or official measurement.
  • Real-world outcomes differ when tolerances, rounding rules or local standards apply.
  • Definitions and standards vary by region and change over time; verify the convention that applies to your case.
  • Figures are only as accurate as the inputs you supply — check them before acting on the result.
  • For work where the result carries real consequences, confirm it against an authoritative source.

Frequently Asked Questions

The rafter angle is the slope measured from horizontal. The plumb cut angle is measured from vertical and equals 90 degrees minus the rafter angle, which is the angle for a vertical cut at the ridge.

It is the vertical rise in inches for every 12 inches of horizontal run. A 6 in rise per 12 in run is a 6:12 pitch.

Yes. A rise of 0 gives a rafter angle of 0 degrees and a plumb cut of 90 degrees, describing a perfectly flat run.

Free to use, no account needed. Calculations run in your browser, so nothing you type is sent to a server. Results are estimates — how we build and check our calculators.