Stew Mac Fret Calculator

The professional tool for accurately calculating guitar, bass, and ukulele fret spacing for perfect intonation.

What is a Stew Mac Fret Calculator?

A stew mac fret calculator is a specialized tool designed for luthiers, guitar builders, and repair technicians to precisely determine the placement of frets on the neck of a stringed instrument like a guitar, bass, or ukulele. The term is synonymous with high-accuracy fret placement, a critical factor for achieving correct intonation—meaning the instrument plays in tune across the entire fingerboard. Without precise fret spacing, a guitar can sound out of tune when playing chords or notes higher up the neck, even if the open strings are tuned perfectly.

This calculator removes the complex manual math involved in the “Rule of 18” or the more accurate twelfth root of two formula, providing a quick, reliable reference chart for any custom scale length. It’s an indispensable digital version of the traditional rulers and templates used in lutherie. If you are building a custom guitar, a precise scale length calculator is your first step toward a professional instrument.

The Fret Placement Formula and Explanation

The modern, scientifically accurate method for fret placement relies on the principle of the twelve-tone equal temperament scale. The constant used to achieve this is the twelfth root of two (approximately 1.0594635). The formula to find the distance of any fret from the bridge is:

Distance_from_Bridge(n) = Scale_Length / (2(n/12))

To get the measurement we actually use—the distance from the nut—we subtract this value from the total scale length:

Distance_from_Nut(n) = Scale_Length - (Scale_Length / (2(n/12)))

This ensures that each fret divides the remaining string length in the correct proportion to raise the pitch by one semitone. While historically the “Rule of 18” was used, our stew mac fret calculator uses this more precise formula for professional results.

Variables Table

Variable	Meaning	Unit (Auto-Inferred)	Typical Range
Scale_Length	The total vibrating length of the string, from nut to bridge.	inches or mm	24″ – 26″ (Guitar), 30″ – 35″ (Bass)
n	The fret number you are calculating for (e.g., 1, 2, 3…).	Unitless	1 – 24+
Distance_from_Nut	The calculated measurement from the nut to the center of the fret slot.	inches or mm	Varies by fret

Practical Examples

Example 1: Fender-style Guitar (25.5″ Scale)

A luthier is building a classic T-style guitar and needs to cut slots for 22 frets. They use a professional stew mac fret calculator for this task.

• Inputs:
  • Scale Length: 25.5
  • Units: Inches
  • Number of Frets: 22
• Results (Partial):
  • Fret 1: 1.431″ from the nut
  • Fret 12: 12.750″ from the nut (exactly half the scale length)
  • Fret 22: 17.042″ from the nut

This ensures the guitar will have perfect intonation, a hallmark of a well-made instrument. For more info on building, see our guide on how to build a guitar.

Example 2: Gibson-style Guitar (24.75″ Scale in Millimeters)

Another builder is working on an LP-style guitar with a shorter 24.75-inch scale length, but prefers to work in metric for higher precision.

• Inputs:
  • Scale Length: 628.65 (which is 24.75 inches)
  • Units: Millimeters
  • Number of Frets: 22
• Results (Partial):
  • Fret 1: 35.34 mm from the nut
  • Fret 12: 314.33 mm from the nut
  • Fret 22: 421.11 mm from the nut

Using a guitar fret calculator allows for easy switching between units without introducing errors.

How to Use This Stew Mac Fret Calculator

1. Enter Scale Length: Input the total vibrating string length of your instrument. This is the primary measurement that determines all fret positions. Common scale lengths are 25.5″ for Fender, 24.75″ for Gibson, and 25″ for PRS.
2. Select Units: Choose whether the scale length you entered is in inches or millimeters. The calculator will automatically display all results in your chosen unit.
3. Set Number of Frets: Enter how many frets your fingerboard will have. The calculator will generate a complete table up to this number.
4. Interpret the Results: The primary result is the ‘Distance from Nut’ column. This is the crucial measurement for marking your fretboard. Always measure from the nut for each fret to avoid compounding small errors. The ‘Inter-Fret Distance’ is provided as a secondary check.
5. Analyze the Chart: The dynamic chart provides a visual representation of how fret spacing decreases as you move up the neck. This is characteristic of any equal-tempered instrument.

Key Factors That Affect Fret Placement

• Scale Length Accuracy: The most critical factor. An incorrect scale length measurement will make all subsequent calculations wrong. Measure from the nut to the 12th fret and double it for the most accurate result.
• Nut Position: All measurements are taken from the face of the nut (the side closest to the fretboard). A poorly placed or shaped nut will throw off the entire scale. A good nut slotting calculator can be helpful here.
• Saddle Compensation: While the calculator provides the theoretical scale length, the actual saddle position is often adjusted slightly (compensated) to account for string stretching during fretting. This calculator provides the ideal fret positions for a given theoretical scale.
• Measurement and Cutting Tools: The precision of your ruler and saw blade are paramount. Using fine-lined rules and ultra-thin saws available from luthier suppliers like StewMac is recommended.
• Environmental Factors: Wood is a natural material that can expand or contract with changes in temperature and humidity. Always work in a stable environment.
• Fretwire Size: The calculations give the position for the center of the fret slot. Consistent cutting is key, regardless of the fretwire size you intend to use.

Frequently Asked Questions (FAQ)

1. What is the “Rule of 18”?

The “Rule of 18” is a historical approximation for calculating fret positions. It states that the distance from the nut to the first fret is the scale length divided by 17.817 (often rounded to 18). While close, it’s less accurate than the twelfth root of two formula used by this modern stew mac fret calculator.

2. Why do my results show 3 decimal places?

High precision is vital in lutherie. While working to a thousandth of an inch or hundredth of a millimeter may seem excessive, this accuracy helps minimize cumulative error, ensuring the best possible intonation. Modern luthier tools allow for this level of precision.

3. Can I use this for a bass or ukulele?

Yes. The calculator is based on universal mathematical principles for equal temperament. Simply enter the correct scale length for your instrument (e.g., 34″ for a standard bass, or 17″ for a soprano ukulele) to get accurate fret positions. A dedicated bass fret calculator uses the exact same logic.

4. Should I measure from the nut or fret-to-fret?

ALWAYS measure from the nut to each individual fret. Measuring from one fret to the next (using the “Inter-Fret Distance” column) is a recipe for disaster, as tiny errors will accumulate and can be significant by the 12th fret.

5. What is scale length?

Scale length is the distance from the nut to the bridge, which defines the vibrating length of an open string. It is the single most important measurement in guitar design, affecting tone, feel, and fret spacing.

6. Does the calculator account for saddle compensation?

This calculator determines the precise, theoretical positions of the frets based on the entered scale length. Saddle compensation is a separate, physical adjustment made at the bridge to correct for string stretching and is not part of the fretboard layout itself.

7. Why is the 12th fret exactly half the scale length?

The 12th fret represents the octave, which is exactly half the vibrating string length. This is a fundamental property of Western music theory and a great way to double-check your measurements.

8. What’s the difference between a 24.75″ and 25.5″ scale?

These are the two most common scale lengths. The 24.75″ (Gibson standard) has frets that are slightly closer together and lower string tension, often described as feeling “slinkier” and warmer in tone. The 25.5″ (Fender standard) has wider fret spacing and higher tension, contributing to a “snappier,” brighter tone.