What this rafter calculator measures
If you know the two roof-end heights but not the pitch, start with the shed roof pitch and height calculator. It uses the full horizontal run for a single slope; return here in Run mode to add an overhang.
Calculate theoretical common-rafter length from roof pitch, horizontal run and overhang. Results exclude ridge-board deductions, birdsmouth depth and cutting allowances. Use the numbers to check geometry before preparing a construction-specific cutting layout.
The input fields use feet for span, run and horizontal overhang. Enter 18 inches as 1.5 feet. Pitch can be entered as x:12 or as an angle from horizontal.
Enter the dimensions you know
- Choose Building span for a symmetric roof with a centered ridge, or Run to enter the horizontal wall-to-ridge distance directly. Span mode uses run = span ÷ 2.
- Choose Pitch x:12 or Degrees. For 6:12, enter 6 in the pitch field. If you need to convert a percentage, use the roof pitch calculator.
- Enter the horizontal overhang beyond the wall. Use zero if no overhang is required in this calculation.
- Read the theoretical lengths with and without overhang. Keep those distinct from the eventual board length and end-cut layout.
The default run uses the ridge centerline as the idealized reference point. The tool has no ridge-thickness input and does not decide a deduction for you. Use dimensions consistent with your plans, and do not apply an adjustment twice.
A worked example: 8:12 over a 10-foot run
Choose Run, enter 10 feet, set the pitch to 8:12 and enter a horizontal overhang of 1.5 feet.
- Slope = 8 ÷ 12.
- Rise over the wall-to-ridge run = 10 × 8 ÷ 12 ≈ 6.667 feet.
- Rafter factor = √(1 + (8 ÷ 12)²) ≈ 1.20185.
- Length without overhang = 10 × factor ≈ 12.019 feet.
- Length with overhang = (10 + 1.5) × factor ≈ 13.821 feet.
The interface formats lengths as feet and inches, rounded to the nearest 1/16 inch. The roof pitch chart is useful for comparing factors; the calculator keeps more precision internally.
What the cut-angle labels mean
The displayed plumb and seat angles are measured off a square crosscut. For a 6:12 roof, they are about 26.6° and 63.4° respectively. They are complementary geometric angles, not a complete birdsmouth specification or a universal saw setting.
The tool does not calculate seat width, notch depth, member capacity or required bearing. Those depend on the actual detail. The Johnson Level rafter-square guide shows how measurement and layout reference lines relate.
Keep horizontal and sloped distances separate
A rafter is the diagonal side of the roof triangle. Entering its length as the run produces a different result. Use the run versus rafter length diagram to identify the correct line, or follow the pitch measuring guide if the slope is unknown.
See our calculation methods for assumptions and precision, or report an unclear result.
Use Copy results or Print results to keep your span or run, slope, overhang and theoretical lengths together.
Good questions. Straight answers.
Is this a finished cutting length?
No. Results are theoretical common-rafter lengths. Ridge-board deductions, birdsmouth depth, bearing details and cutting allowances are not included.
Is the overhang measured horizontally?
Yes. Enter the horizontal distance beyond the wall in feet. The tool converts that additional run into length along the same roof slope.
When can I use building span?
Building span mode divides the width by two. Use it only when the roof section has a centered ridge and equal runs; otherwise enter the actual horizontal run directly.
Does it calculate hip or valley rafters?
No. This tool models common rafters in a single vertical plane. Hip and valley rafters need different geometry and are outside its scope.