Use this Rafter Length Calculator to estimate straight-line common-rafter geometry from horizontal run or full building span, roof pitch and optional overhang. Review roof rise, angle, slope factor and sloped length in US or metric units.
Enter roof geometry
Start with one-side horizontal run or full building span. Then enter roof pitch and any horizontal overhang you want included.
Your rafter geometry
Review the common-rafter length to the wall line, optional overhang geometry, rise, angle and slope factor.
Transparent calculation
How rafter length is calculated
Roof run and rise form a right triangle. Pitch establishes the rise-to-run ratio, and the Pythagorean theorem gives the straight-line geometric length.
run = span ÷ 2 (span mode)rise = run × (pitch ÷ 12)√(1 + (pitch ÷ 12)²)run × slope factorhorizontal overhang × slope factor(run + overhang) × slope factorImportant: these formulas describe a straight roof line. They do not calculate lumber size, structural capacity, bearing, connections, birdsmouth dimensions, ridge deductions or a finished cut list.
Step by step
How to use the Rafter Length Calculator
Choose whether you know the one-side horizontal run or the full wall-to-wall span of a symmetrical gable roof. Enter the pitch as rise per 12 units of horizontal run, then add a horizontal overhang only if you want that extra sloped geometry included.
- Choose run or span. Use run when you already know the one-side horizontal distance. Use span when you know the full wall-to-wall width and want the calculator to divide it by two.
- Enter the roof pitch. A 6:12 pitch means 6 units of rise for every 12 units of horizontal run.
- Add horizontal overhang if needed. The calculator converts that horizontal extension to a sloped length using the same pitch.
- Calculate. Review rafter geometry, roof rise, angle, slope factor and total sloped geometry.
- Apply project-specific cut details separately. Ridge, seat/birdsmouth, fascia and bearing details are not inferred by this geometry tool.
Need to establish the pitch first? Use the Roof Pitch Calculator.
Read your result
What each rafter result means
Horizontal run
The one-side horizontal distance used to create the right-triangle roof geometry.
Roof rise
The vertical height created by the selected run and pitch ratio.
Geometric rafter length
The straight sloped distance from the wall-line reference to the ridge reference before project-specific cut deductions or additions.
Slope factor
The multiplier used to convert a horizontal distance into the corresponding sloped distance at the selected pitch.
Overhang sloped length
The sloped distance created by the optional horizontal overhang using the same roof pitch.
Total sloped geometry
The geometric rafter line plus the optional overhang line. It is not automatically a finished stock/cut length.
Worked example
Example: 12 ft span with a 6:12 roof pitch
Suppose a symmetrical gable roof has a 12 ft wall-to-wall span, a 6:12 pitch and a 12 in horizontal overhang. The calculator uses a 6 ft one-side run.
12 ft ÷ 2 = 6 ft6 ft × 6/12 = 3 ft6 × √1.25 ≈ 6.71 ft7 × √1.25 ≈ 7.83 ftThe approximately 7.83 ft total is straight-line geometry through the selected horizontal overhang. A real rafter layout still needs the correct ridge, bearing, birdsmouth/seat, tail/fascia and connection details for the project.
Measurement guidance
Rafter measurement tips
- Use horizontal run—not roof-surface length—as the input dimension.
- When using span mode, confirm that a simple half-span is appropriate for the roof geometry you are modeling.
- Measure overhang horizontally if you want the calculator to convert it to a sloped extension.
- Do not assume the geometric result automatically includes ridge-board thickness, birdsmouth cuts or tail details.
- Use the same reference lines consistently when transferring measurements to a physical layout.
Know the limits
Assumptions and limitations
This calculator models a straight common-rafter roof line using simple right-triangle geometry. Span mode assumes the selected roof geometry can use one-half of the full span as the horizontal run. That is a geometric simplification, not a structural design decision.
The result does not determine lumber species or size, allowable span, loading, structural capacity, ridge-board or ridge-beam requirements, fastening, connections, birdsmouth/seat dimensions, plumb-cut deductions, fascia/tail layout, uplift resistance or local code compliance.
Use project drawings, adopted code requirements, manufacturer information and qualified professional guidance where those details are required.
Roof terminology
Run, rise, pitch and slope factor
A roof pitch such as 6:12 is a ratio: 6 units of vertical rise for every 12 units of horizontal run. The roof angle expresses the same slope in degrees. The slope factor is the ratio of sloped length to horizontal run, so it can convert the measured run into straight-line rafter geometry.
For more background on slope descriptions, read Roof Pitch vs Roof Slope or How to Measure Roof Pitch.
Common questions
Rafter Length Calculator FAQ
Does this calculator give the finished rafter cut length?
No. It gives straight-line geometric length from the selected run/span reference and optional horizontal overhang. Ridge, seat/birdsmouth, tail and other cut details can change the physical layout.
Why is the run half the span?
For a simple symmetrical gable roof, one roof side commonly spans half the wall-to-wall width. That assumption is not appropriate for every roof shape, offset ridge or structural layout, which is why the tool also provides direct run mode.
Does the calculator include roof overhang?
Yes, if you enter a horizontal overhang. It converts that horizontal extension to a sloped geometric length at the selected pitch.
Can I enter fractional pitch values?
Yes. In US-style x:12 notation, values such as 6 1/2 are accepted.
Does this calculator size the rafter lumber?
No. Rafter size and allowable span depend on structural factors that are outside this geometry calculation.
Can I use metric measurements?
Yes. Switch to Metric and enter run/span and overhang in millimeters. Pitch remains a unitless rise-per-12 ratio.
Keep planning
Related roofing calculators
Helpful guides
Learn the measurements behind the result
Sources & methodology
How this page is checked
The calculator uses standard right-triangle geometry and keeps the distinction between geometric slope length and project-specific structural/cut details.