What this calculator does
This tool turns your measurements into a consistent planning estimate you can review before pricing or starting work. It is useful during early takeoffs, option comparisons, and on-site checks where transparent math matters.
How the calculation works
Formula
Rafter length = horizontal run × √(1 + (pitch ÷ 12)²).Worked example
A 12-foot run at 6:12 has a basic rafter length of about 13.42 feet.
How to measure
- 01Enter the horizontal distance from wall plate to ridge—not the full building span.
- 02If you know full span, divide it by two before entering it.
Common mistakes to avoid
- Entering full span instead of half-span
- Treating overhang as sloped length
- Ignoring field cuts and ridge details
Limits of this estimate
Results exclude ridge-board deductions, birdsmouth cuts, lumber defects, loads, and structural requirements.
Frequently asked questions
Does overhang follow the slope?
Enter its horizontal projection; the calculator converts it to sloped length.
Is this a cut list?
No. It is a geometric estimate for planning.