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Rafter Length Calculator

Enter the horizontal run, rise, and overhang and this rafter length calculator returns the sloped rafter length in feet as the square root of run squared plus rise squared, then adds the overhang.

What this calculator does

The Rafter Length Calculator estimates your result instantly from the values you enter, using standard construction & home formulas. Enter the horizontal run, rise, and overhang and this rafter length calculator returns the sloped rafter length in feet as the square root of run squared plus rise squared, then adds the overhang. 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 Length Calculator

Result

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Rafter Length (ft)

About Rafter Length Calculator

The Rafter Length Calculator works out how long each common rafter needs to be from the roof's horizontal run and vertical rise, then adds your eave overhang. Use it when you know the run and rise of a pitched roof and need the straight-line rafter length to cut.

Formula & Mathematical Calculation

Rafter Length (ft) = f(Horizontal Run (ft), Rise (ft), Overhang (ft))

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

Calculation Example

Enter a Horizontal Run of 12 ft, a Rise of 6 ft, and an Overhang of 1 ft. The tool takes the square root of (12 x 12 + 6 x 6), which is the square root of 180, and adds the 1 ft overhang, so the Rafter Length (ft) box reads 14.416 ft.

Key Benefits

  • Computes the sloped common-rafter length from run and rise using the Pythagorean relationship
  • Adds your eave overhang directly into the reported rafter length
  • Reports the result in feet rounded to three decimals for a cutting reference

How to Use this Calculator

  1. Enter the horizontal run
    Type the roof's horizontal run in feet into the Horizontal Run (ft) field.
  2. Enter the rise and overhang
    Fill in the Rise (ft) and Overhang (ft) fields.
  3. Calculate
    Click Calculate and read the Rafter Length (ft) box, which already includes the overhang.

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 Length 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

Yes. The tool adds your Overhang (ft) value to the sloped length, so the Rafter Length box is the full cut length including the eave overhang.

It takes the square root of the run squared plus the rise squared to get the sloped distance, then adds the overhang. The result is shown in feet to three decimals.

All three inputs (run, rise, overhang) are in feet, and the result is returned in feet.

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.