Rafter Calculator
Find rafter length, rise, or run. Geometry only, not structural design.
Solve rafter geometry
Find length from rise and run, or work backwards from a known rafter.
Choose the rafter values you know
The calculator solves one right triangle. Pick inputs that describe the same triangle and keep every length in the same unit.
A roof rafter calculator needs two sides of that triangle. Enter rise and run to calculate rafter length, or enter the known length with either leg to work backward. The three modes use the same equation in different directions, so choose the measurements you can verify most reliably.
Rise and run to length
Enter the vertical rise and horizontal run to solve the sloped rafter length. You can also enter pitch as rise per 12 and a real horizontal run; the calculator derives the corresponding rise first.
Length and run to rise
Use the inverse mode when rafter length and horizontal run are known. The rafter must be at least as long as the run. Equal values describe a flat triangle with zero rise.
Length and rise to run
Use the other inverse mode when length and vertical rise are known. The rafter must be longer than the rise so a positive horizontal run exists.
Rafter calculator formulas for a right triangle
Rise and run are the perpendicular legs; rafter length is the hypotenuse. The three solve modes rearrange the same Pythagorean relationship.
Rafter length = √(run² + rise²)
Use this when both perpendicular legs are known.
Rise = √(length² − run²)
Use this when the hypotenuse and horizontal leg are known.
Run = √(length² − rise²)
Use this when the hypotenuse and vertical leg are known.
Worked 6/12 rafter calculator example
A 6/12 pitch means the roof rises 6 units for every 12 units of horizontal run. Use matching real units when the result needs to be a real length.
1. Define the legs
Use a 6 ft rise and a 12 ft run for this triangle.
2. Apply the formula
√(12² + 6²) = √180 = 13.4164 ft before display rounding.
3. Read the result
The calculator displays a 13.4 ft rafter length, a 26.6° angle, and a 50% slope.
Use the roof pitch calculator for angle, percent, and pitch conversions. Formula precision and validation are documented in the methodology.
This rafter calculator can also check a drawing. If a plan lists rafter length and run, solve for rise and confirm that the resulting pitch matches the plan before you calculate rafter length for an overhang extension. Consistent units matter more than extra decimal places: mixing feet and inches can make a correct formula produce the wrong length.
Measure run, rise, and overhang consistently
Most bad results come from mixing units or measuring a different triangle than the one being solved.
Keep units consistent
If rise is in feet, run and overhang must also be in feet. An X/12 pitch is a ratio, not permission to mix a 6-inch rise with a 12-foot run.
Use horizontal run, not total span
Run is the horizontal distance represented by one right triangle, commonly from a ridge centerline to an outside wall. Total building span may contain two runs and should not be substituted automatically.
Overhang extends the run
In length-solving modes, the overhang is added to horizontal run and extended along the same pitch. It is not part of the structural run used to locate the ridge, and it does not model fascia details or tail cuts.
Rafter length from pitch is a useful route when pitch is already known: multiply horizontal run by the pitch ratio to get rise, then solve the hypotenuse. In practice, a rafter length calculator is most dependable when you measure the horizontal run directly—commonly half the span on a symmetrical gable—and enter any overhang separately. Recheck the pitch, too; even a small reading error grows across a long run.
Geometry is not structural or cut design
This rafter calculator solves triangle dimensions. It does not determine whether a roof member or connection is safe for a real building.
No lumber or load sizing
The result does not size lumber, check species or grade, calculate dead, live, snow, or wind loads, set spacing, or design trusses and connections.
No fabrication details
The number is geometric line length. It does not locate seat cuts, birdsmouth depth, ridge deductions, plumb cuts, fascia allowances, or hardware. Those details change the actual cut list.
Confirm the design locally
A qualified designer, engineer, or building official must confirm member sizing, connections, spans, loads, and building-code compliance. Use the roofing calculator only when you need material-area estimates from the resulting pitch.
Rafter calculator FAQ
This rafter calculator solves the same triangle three ways: length, rise, or run.
How do I calculate rafter length?
Rafter length is the hypotenuse of rise and run: √(run² + rise²). For a 6/12 pitch, every 12 inches of run needs about 13.4 inches of rafter.
Can I solve for rise or run instead?
Yes. If you know rafter length and run, rise = √(length² − run²). If you know length and rise, run = √(length² − rise²). Length equal to run is a flat roof (rise 0). Length equal to rise would be run 0, which this tool rejects.
Is this a truss or structural calculator?
No. This page is roof geometry only. It does not size lumber, check loads, design trusses or connections, detail cuts, or verify building-code compliance.
Should rise, run, and rafter length use the same unit?
Yes. This calculator labels real lengths in feet, so convert inch measurements to decimal feet before entering them. Pitch such as 6/12 is a ratio, but every rise, run, length, and overhang input must use a consistent real-length unit.
How does overhang change rafter length?
In a length-solving mode, the calculator adds overhang to the horizontal run and extends the rafter along the same pitch. Inverse rise/run modes do not use overhang. The result remains geometric and excludes fascia details, ridge deductions, and tail cuts.
Does the result include birdsmouth or seat cuts?
No. The result is a triangle line length. Seat cuts, birdsmouth depth, plumb cuts, ridge details, connections, and local code requirements need a separate construction design.