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Rafter CalculatorEstimate Rafter Length and Angle
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Metric & Imperial
Pitch table

Use this free rafter calculator to find the common rafter length for any roof. Enter the horizontal run and roof pitch to instantly calculate rafter length, roof rise, rise angle, and slope percentage - all the values you need before cutting a single board.

Roof Details
Unit system
Roof pitch (rise per 12 in / 12)
Common rafter length
13.42 ft
Run: 12 ft - Pitch: 6/12
6 ft
Roof rise
26.57°
Roof angle
50%
Slope
For a run of 12 ft at 6/12 pitch, the common rafter length is 13.42 ft (roof angle 26.57 deg).
Getting started
How to use this rafter calculator
1
Select unit system
Choose Imperial (feet) or Metric (meters) based on your measurements.
2
Enter horizontal run
The horizontal distance from the outside wall face to the center of the ridge, measured flat.
3
Select pitch
For imperial roofs, tap a preset or type a value. For metric, enter the slope percentage.
4
Read results
Read the rafter length, rise, angle, and slope percentage from the result card.
The calculation
Step-by-step: how your rafter length was calculated
1
Calculate roof rise from pitch
Rise = Run x (Pitch / 12) = 12 x (6 / 12)
= 6 ft
2
Apply the Pythagorean theorem
Rafter² = Run² + Rise² = 12² + 6² = 144 + 36
Rafter = sqrt(180) = 13.42 ft
3
Calculate roof angle
Angle = arctan(Rise / Run) = arctan(6 / 12)
= 26.57 degrees (pitch 6/12, slope 50%)
Formulas
Rafter length formula

Common rafter length is derived from the Pythagorean theorem. The horizontal run and vertical rise form the two legs of a right triangle, and the rafter is the hypotenuse:

Rise = Run x (Pitch / 12) [imperial]
Rise = Run x (Pitch / 100) [metric]
Rafter Length = sqrt(Run² + Rise²)
Roof Angle = arctan(Rise / Run) x (180 / PI)

For example, a roof with a 6/12 pitch and 12-ft run has a rise of 6 ft (12 x 6/12 = 6). Rafter length = sqrt(144 + 36) = sqrt(180) = 13.42 ft.

Reference Table
Common roof pitch reference table
Pitch (X/12)Angle (deg)Slope (%)Typical use
2/129.516.7Shallow shed
3/1214.025.0Low ranch
4/1218.433.3Contemporary home
5/1222.641.7Common residential
6/1226.650.0Traditional
8/1233.766.7Steep residential
9/1236.975.0Cape Cod
12/1245.0100Very steep
Background Information
Understanding roof pitch and rafter length

Roof pitch is one of the most critical measurements in residential construction. It determines not only the rafter length but also the roof's drainage capacity, snow load performance, visual appearance, and the type of roofing materials that can be used.

When framing a gable roof, every common rafter must be cut to an identical length (assuming a structurally level ridge and plates). Calculating rafter length precisely before cutting eliminates waste and ensures a tight fit.

Context
What the results mean for framing
Rafter length (hypotenuse)
This is the sloped length of the rafter from the top of the outside wall plate to the center of the ridge board. Mark this length on the rafter stock before cutting.
Roof angle
The angle in degrees is the bevel setting for your circular saw when making the plumb cut at the ridge. The complementary angle (90 - angle) is the bevel for the level cut at the seat.
Slope percentage
Slope % expresses the pitch as a fraction of 100. A 6/12 pitch = 50% slope. A 12/12 pitch = 100% slope.
Mistakes
Common rafter calculation mistakes
Confusing run with span

The run is half the total building span for a symmetric gable roof. Using the full building width as the run doubles the rise and produces a rafter that is far too long.

Forgetting the ridge board deduction

This calculator gives the theoretical rafter length to the center of the ridge. In practice, you must subtract half the ridge board thickness from the top end of the rafter before marking the plumb cut.

Not adding the rafter tail

The rafter length shown here does not include the overhang (rafter tail). You need to add it separately.

FAQ
Frequently asked questions
Q
What is a rafter and how is its length calculated?
A rafter is a structural beam that runs from the ridge of a roof down to the wall plate. Its length is calculated using the Pythagorean theorem: rafter squared = run squared + rise squared. The rise is determined from the pitch (slope ratio) and horizontal run.
Q
What is roof pitch and how does it work?
Roof pitch is expressed as rise over run. In imperial measure, pitch is stated as X/12 - meaning the roof rises X inches for every 12 inches of horizontal run. A 4/12 pitch rises 4 inches per foot of run. In metric, pitch can be expressed as a percentage or as cm rise per 100 cm run.
Q
How do I measure the run of a rafter?
The horizontal run is the horizontal distance from the outside edge of the wall plate to the center of the ridge. For a symmetrical gable roof, this equals half the building width. Always measure horizontally, not along the slope.
Q
What is the difference between a common rafter and a hip rafter?
A common rafter runs perpendicular from the wall plate to the ridge. A hip rafter runs at a 45-degree corner angle and is longer than a common rafter. This calculator computes common rafter length. Hip rafter length requires a different formula involving a diagonal run.
Q
Do I need to add extra length for the rafter tail?
Yes. This calculator gives the structural rafter length (from wall plate to ridge). If your roof design includes an overhang (eave), you need to add the rafter tail length. Rafter tails are typically 12 to 24 inches on residential homes. Use the Rafter Length Calculator for overhang-inclusive totals.
Q
How do I convert rafter length to a cut angle?
The plumb cut angle at the ridge equals the roof angle returned by this calculator. If the angle is 26.6 degrees (6/12 pitch), set your circular saw bevel to 26.6 degrees for the plumb cut at the top of the rafter. The level cut at the seat (bird's mouth) is the complementary angle (90 - 26.6 = 63.4 degrees).
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