Rafter Length Calculator
Calculate Rafter Length Quickly and Accurately
Our free rafter length calculator helps DIYers, carpenters, and roofers determine the exact length of rafters needed for roofing projects. Simply enter your roof dimensions and get instant results with supporting calculations.
Rafter Calculator
Calculation Results
Calculation History
Roof Diagram
How to Use the Rafter Length Calculator
Follow these simple steps to calculate your rafter length:
- Select your preferred unit system (Imperial or Metric)
- Choose your calculation mode:
- Rise + Run: Use this if you know the vertical rise and horizontal run
- Run + Pitch: Use this if you know the horizontal run and roof pitch
- Enter your measurements in the appropriate fields
- Click “Calculate Rafter Length” to see your results
- Use the advanced options if you need to account for overhang or beam width
Understanding Roof Measurements
Run
The horizontal distance from the outside of the wall plate to the center of the roof ridge. This is half of the total span of the roof.
Rise
The vertical height from the top of the wall plate to the highest point of the roof (the ridge).
Pitch
The slope of the roof expressed as a ratio of vertical rise to horizontal run. For example, a 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run.
Rafter Length
The diagonal length of the rafter from the outside of the wall plate to the center of the ridge board.
Formulas Used
The calculator uses the Pythagorean theorem to determine rafter length:
Rafter Length = √(Run² + Rise²)
When using pitch instead of rise:
Rise = Run × (Pitch/12)
Rafter Length = √(Run² + (Run × Pitch/12)²)
To calculate roof angle:
Angle = arctan(Rise / Run)
Slope factor is calculated as:
Slope Factor = Rafter Length / Run
Example Calculations
Example 1: Using Rise and Run
If your roof has a run of 10 feet and a rise of 6 feet:
Rafter Length = √(10² + 6²) = √(100 + 36) = √136 ≈ 11.66 feet
Example 2: Using Run and Pitch
If your roof has a run of 12 feet and a pitch of 6/12:
Rise = 12 × (6/12) = 6 feet
Rafter Length = √(12² + 6²) = √(144 + 36) = √180 ≈ 13.42 feet
Frequently Asked Questions
Use the “Run + Pitch” mode. Enter your roof run and the pitch ratio (e.g., 6 for a 6/12 pitch). The calculator will automatically compute the rise and then the rafter length.
Run is the horizontal distance from the outside of the wall plate to the center of the roof ridge (half the span). Span is the total horizontal distance between the outside edges of the wall plates.
Overhang extends beyond the wall plate. When you include overhang in your calculation, the rafter length increases to account for this extension.
This calculator is designed for common rafters. Hip and valley rafters require different calculations due to their compound angles.
The calculator provides precise mathematical calculations. However, always add appropriate safety margins and consult building codes for your specific project.
Embed This Calculator
Want to add this calculator to your website? Use the code below: