A rafter length calculator connects the horizontal run and roof pitch to the sloping line of the roof triangle. Instead of converting feet, inches, and pitch ratios through several separate calculations, you can enter the measurements you already know and review the rafter line together with the related rise, angle, and percent slope.
Construction Calculator is included inside Unit Converter and provides a dedicated Rafter tool for this workflow. The rafter length calculator app works with feet and fractional inches, decimal inches, meters, and centimeters, and Unit Converter is free to download for Android, iPhone, and iPad.
Understand the triangle behind rafter length
In a simple roof triangle, the run is the horizontal leg, the rise is the vertical leg, and the rafter line is the diagonal. A pitch written as 6:12 means the roof rises 6 units for every 12 units of horizontal run.
When the run and pitch are known, the rise follows from the pitch ratio. The rafter line can then be found as the hypotenuse of the right triangle.
How to calculate rafter length using pitch
Start by converting the x:12 pitch into a ratio. For a 6:12 pitch, the ratio is 6 ÷ 12, or 0.5. Multiply the horizontal run by that ratio to find the rise:
Rise = run × (pitch ÷ 12)
Then use the Pythagorean relationship to calculate the diagonal:
Rafter line length = √(run² + rise²)
You can also combine those steps into one formula:
Rafter line length = run × √(1 + (pitch ÷ 12)²)
This rafter length formula uses the horizontal run rather than the full roof span. Once the run and x:12 pitch are entered, the app handles the ratio and diagonal calculation together.
For a 12 ft horizontal run and a 6:12 pitch, the rise is 6 ft. The diagonal is √(12² + 6²), which is approximately 13.4167 ft. In feet-and-inches format, Construction Calculator displays the result as 13 ft 5 in.
Calculate the rafter line in the app
- Open Construction Calculator and select Rafter.
- Enter the horizontal Run as 12 ft.
- Enter 6 in the Pitch field; the interface supplies the :12 part.
- Read the calculated 13 ft 5 in rafter line length.
The result updates from the entered geometry. Press Next to move through the connected values without entering the run and pitch again.
Watch the rafter length calculation
See how a 12 ft horizontal run and 6:12 pitch produce a 13 ft 5 in rafter line, together with the related rise, angle, and slope.
Review rise, angle, and percent slope
The same 12 ft run and 6:12 pitch also produce a 6 ft rise, a 26.57-degree angle, and a 50% slope. These are different expressions of the same triangle, so having them together makes it easier to move between a pitch ratio, an angle, and the measured sides.
If you need to begin with rise and run or calculate one of those values from the other measurements, our roof pitch calculator app guide explains the dedicated Pitch, Rise, and Run workflows.
A simple 3–4–5 roof-triangle example
A second example shows the relationship cleanly. Enter a 4 ft horizontal run and a pitch value of 9, representing a 9:12 pitch. The corresponding rise is 3 ft, and the rafter line is 5 ft. The related result pages show an angle of approximately 36.87 degrees and a 75% slope.
Because 3² + 4² = 5², this familiar right triangle is also an easy way to understand what the Rafter tool is calculating from the entered run and pitch.
Work with feet, inches, fractions, or metric measurements
The Rafter tool accepts FT/IN, decimal inches, meters, and centimeters. A horizontal run can therefore stay in the same format used by a tape, drawing, or specification. If the measurement includes fractional inches, FT/IN provides separate positions for feet, inches, numerator, and denominator.
Compatible result-unit buttons change how a displayed length is presented. This makes it easy to review the rafter line or rise in the measurement format needed for the next calculation. For more detail on fractional entry and metric conversion, see our feet and inches calculator app guide.
Who can use a common rafter length calculator?
Roofers and framers can use the run-and-pitch workflow when reviewing a simple roof triangle. Builders, remodelers, shed builders, estimators, designers, and DIY users can calculate the same geometry for houses, additions, porches, workshops, and other sloped structures.
Because the Rafter tool is part of Unit Converter’s Construction Calculator, the same app also includes dedicated Pitch, Rise, and Run modes, feet-and-inches arithmetic, and tools for stairs, balusters, studs, lumber, miters, areas, volumes, weights, and common shapes.
What makes a useful mobile rafter calculator?
- Direct run and pitch inputs: Start with the two values normally shown on a roof drawing or field note.
- Familiar x:12 notation: Enter the pitch numerator while the interface supplies :12.
- Feet-and-inches support: Keep fractional tape measurements readable.
- Metric options: Work with meters or centimeters when the drawing uses metric units.
- Connected geometry: Review the rafter line, rise, angle, and percent slope from the same inputs.
- Related construction tools: Continue into pitch, layout, stair, lumber, and dimensional calculations within the same app.
Frequently asked questions
Is there a free rafter length calculator app?
Yes. Unit Converter is free to download from Google Play and the App Store, and its built-in Construction Calculator includes the Rafter tool.
What measurements do I enter?
Select Rafter, enter the horizontal run, and enter the x:12 pitch numerator. For example, enter 6 for a 6:12 pitch.
What rafter length does a 12 ft run at 6:12 produce?
The app calculates a 13 ft 5 in rafter line. The related pages also show a 6 ft rise, 26.57-degree angle, and 50% slope.
Can I use feet, inches, and fractions?
Yes. Select FT/IN to enter the horizontal run in feet, inches, and fractional inches. Decimal inches, meters, and centimeters are also available.
Is the Rafter tool a separate download?
No separate download is needed. Rafter is one of the focused tools included in Construction Calculator inside Unit Converter.
Calculate rafter geometry on your phone or tablet
Enter the horizontal run and x:12 roof pitch to calculate the rafter line, then move through the connected rise, angle, and slope results. For the full collection of building and layout features, visit our Construction Calculator app guide.
Download Unit Converter on the App Store for iPhone or iPad, or get Unit Converter on Google Play for Android.