Rafter Length Calculator

Estimate common-rafter length from horizontal run, roof pitch and optional eave overhang

Your result will appear here.

About the Rafter Length Calculator

Rafter Length Calculator estimates the sloping length from horizontal run and roof rise or pitch. It is useful for preliminary roof material planning and checking common-rafter geometry.

Actual cut length can differ because ridge-board thickness, overhang, birdsmouth cuts and connection details are not all represented by the basic right-triangle calculation.

How to Use the Rafter Length Calculator

  1. Enter the horizontal run from wall plate to ridge centreline or the run defined by the calculator.
  2. Enter roof rise, pitch or angle.
  3. Calculate the geometric rafter length.
  4. Add overhang and project-specific cut allowances according to the framing detail.

How the Calculation Works

With horizontal run R and vertical rise H, geometric rafter length L = √(R²+H²).

When pitch is given as rise per 12 units of run, H = R×pitch/12 before applying the Pythagorean relationship.

Worked example

Rafter Length Calculator Worked Example

12 ft run with 6:12 pitch

Rise = 12×6/12 = 6 ft. Rafter length = √(12²+6²)=√180≈13.42 ft before overhang and cuts.

Result guide

Understanding Your Rafter Length Calculator Result

The result is the straight geometric slope length between the defined endpoints. It is not automatically the final stock-board cut length. If rafters sit on a ridge board rather than meeting at the roof centreline, the horizontal run used for the cut geometry may need adjustment for half the ridge thickness or according to the framing detail.

Before you rely on the answer

Assumptions and Limitations

  • Run and rise describe a right-triangle roof section.
  • Pitch is entered in the correct convention.
  • Overhang, ridge adjustments and cut details are handled separately.

Practical tip

Using This Result in Practice

Confirm whether your measured run stops at the ridge centreline or ridge face. Small endpoint differences can affect the final cut length and should follow the framing plan.

Related calculations

Useful Calculations to Continue With

If roof pitch itself is unknown, use the Roof Pitch Calculator first.

If you want the underlying right-triangle relationship without roof-specific context, use the Pythagorean Theorem Calculator.

Common questions

Rafter Length Calculator FAQ

Is rafter length the same as roof run?

No. Run is horizontal; rafter length follows the sloping roof line.

Does the calculator include overhang?

Only if an overhang input is explicitly included. The base geometric result usually does not.

What does 6:12 pitch mean?

The roof rises 6 units for every 12 units of horizontal run.