RafterLengthCalculator.com
Free roof framing tool

Rafter Length Calculator

This rafter length calculator helps you estimate the length of a common rafter using building span, roof pitch, and optional overhang. It is designed to be quick, practical, and easy to use whether you are framing a roof, planning a shed, or checking a rough material estimate.

Use the calculator below to get the rafter length without overhang, the rafter length with overhang, the roof angle in degrees, and the pitch shown as rise over run.

Fast results Get the basic roof rafter numbers in one step.
Simple inputs Enter span, pitch, and optional overhang.
Practical output Clear values for contractors, DIYers, and homeowners.

What This Rafter Calculator Does

A common rafter forms the sloped side of a simple gable roof. To estimate its length, you need the horizontal run and the vertical rise. This page works as a practical rafter calculator by converting your span and pitch into a right triangle, then calculating the sloped side of that triangle.

The tool is helpful for quick estimating, planning, and double-checking rough dimensions. It does not replace a detailed framing plan, layout mark calculations, or any project-specific engineering.

How to Calculate Rafter Length

If you want to know how to calculate rafter length by hand, the basic process is straightforward for a standard common rafter:

If you want to see the math behind this, check the rafter length formula.

1. Start with the building span

The building span is the full width of the structure. For a simple centered gable roof, the common rafter run is half of that span.

2. Convert roof pitch into slope

Roof pitch is usually written as rise over run, such as 6/12. That means the roof rises 6 units vertically for every 12 units horizontally.

3. Find the rise over the common run

Multiply the common rafter run by the pitch ratio. For a 6/12 pitch, the ratio is 6 ÷ 12, or 0.5.

4. Use the right-triangle relationship

Once you know the run and rise, the rafter length is the hypotenuse of the triangle. That is the core idea behind almost every roof rafter calculator.

5. Add overhang if needed

If the rafter extends past the wall, add the horizontal overhang to the run and calculate the new sloped length using the same roof pitch.

Rafter Length Formula

The basic rafter length formula comes from the Pythagorean theorem.

Run = Span ÷ 2 Slope ratio = Rise ÷ RunPitch Rise over common run = CommonRun × (PitchRise ÷ PitchRun) Rafter length without overhang = √(CommonRun² + Rise²) Rafter length with overhang = √((CommonRun + Overhang)² + (Slope ratio × (CommonRun + Overhang))²) Roof angle in degrees = arctangent(PitchRise ÷ PitchRun)

Keep the units consistent. If you enter span and overhang in feet, keep them in feet. If you enter them in inches, keep them in inches.

Worked Example

Example: A building has a 24 foot span, a 6/12 roof pitch, and a 1 foot overhang.

Step 1: Common run = 24 ÷ 2 = 12 feet.

Step 2: Pitch ratio = 6 ÷ 12 = 0.5.

Step 3: Rise over common run = 12 × 0.5 = 6 feet.

Step 4: Rafter length without overhang = √(12² + 6²) = √180 = 13.42 feet.

Step 5: Add overhang: total horizontal distance = 12 + 1 = 13 feet.

Step 6: Rise at outer edge = 13 × 0.5 = 6.5 feet.

Step 7: Rafter length with overhang = √(13² + 6.5²) = √211.25 = 14.53 feet.

That is the same logic used by the calculator above. If you are checking a rough roof framing layout, this approach gives you a quick starting point.

Common Roof Pitch Reference Table

This quick reference table shows common roof pitches, approximate roof angle, and the rafter length factor per 12 units of horizontal run. The factor is the sloped length for each 12 units of run.

Roof pitch Angle in degrees Rafter factor per 12 run
3/12 14.04° 12.37
4/12 18.43° 12.65
5/12 22.62° 13.00
6/12 26.57° 13.42
7/12 30.26° 13.89
8/12 33.69° 14.42
9/12 36.87° 15.00
10/12 39.81° 15.62
12/12 45.00° 16.97

FAQ

What does a rafter length calculator do?
It estimates the sloped length of a common rafter based on the roof span and pitch. A good rafter calculator can also show angle and account for overhang.
Is building span the same as the rafter run?
No. For a typical gable roof, the common rafter run is usually half of the total building span before overhang is added.
Can I use this as a roof rafter calculator for sheds and small buildings?
Yes, this type of roof rafter calculator is useful for rough estimating on sheds, garages, additions, and other simple roofs with common rafters.
How is overhang handled?
The tool adds the horizontal overhang to the common run and then applies the same roof pitch to calculate the extra sloped length.
Why does the page ask for rise and run separately?
Roof pitch is a ratio. Entering rise and run separately lets you use any pitch format, such as 4/12, 6/12, or 9/12.
Does this calculator replace a framing plan?
No. It is meant for quick estimating and general planning. Final construction details still depend on your design, materials, layout, cuts, and project-specific requirements.

Simple, Practical Rafter Length Estimates

If you were searching for a simple rafter length calculator, this page is built to give you the core numbers quickly without extra clutter. Use it to estimate common rafter length, compare roof pitches, and understand the basic rafter length formula in plain English.

Bookmark the page if you regularly need a straightforward rafter calculator for planning roof framing jobs, DIY projects, or quick homeowner checks.

Related Tools and Guides

Use these tools to better understand roof framing and rafter calculations: