Stew Mac Fret Calculator

Precise Fret Scaling & Bridge Positioning for Luthiers

Fret #	Distance From Nut	Fret-to-Fret Spacing

What is the Stew Mac Fret Calculator?

The stew mac fret calculator is an essential tool for luthiers, guitar technicians, and hobbyist instrument builders designed to calculate the precise placement of frets on a fingerboard. Accuracy in fret placement is critical for the instrument to play in tune (intonation) across the entire neck.

While often associated with the Stewart-MacDonald brand due to their popularity in the luthier community, the term “Stew Mac fret calculator” typically refers to the industry-standard mathematical method used to determine these positions based on a chosen scale length.

This tool is suitable for anyone building or repairing:

• Electric and Acoustic Guitars
• Bass Guitars
• Ukuleles and Mandolins
• Banjos and Cigar Box Guitars

Stew Mac Fret Calculator Formula Explained

The underlying mathematics of modern fret spacing relies on the “Rule of 18” (historically) or the more precise 17.817 constant used today. This constant is derived from the 12th root of 2, which divides the octave into 12 semitones.

The Core Formula

To find the position of the next fret, you divide the remaining scale length by the constant 17.817.

Distance to next fret = (Remaining Scale Length) / 17.817154

Alternatively, to calculate the distance from the nut to any specific fret (n), we use the exponential formula:

Distance from Nut = Scale Length × (1 – (1 / (2 ^ (n/12))))

Variable	Meaning	Typical Unit	Typical Range
S	Scale Length	Inches / mm	13″ – 34″
n	Fret Number	Integer	1 – 24
K	Constant	Ratio	17.817

Practical Examples

Example 1: Fender Stratocaster Style

For a standard electric guitar with a scale length of 25.5 inches:

• Scale Length: 25.5″
• 1st Fret Position: 25.5 ÷ 17.817 = 1.431″ from the nut.
• 12th Fret Position: Exactly half the scale length (12.75″).
• Financial Note: A pre-slotted fretboard for this scale costs roughly $30-$50, while slotting it yourself requires a saw (~$40) and this calculation.

Example 2: Gibson Les Paul Style

For a shorter scale guitar utilizing 24.75 inches:

• Scale Length: 24.75″
• 1st Fret Position: 24.75 ÷ 17.817 = 1.389″ from the nut.
• Impact: The shorter distance between frets makes chords easier to reach but reduces string tension compared to the 25.5″ scale.

How to Use This Stew Mac Fret Calculator

1. Select Instrument Preset: Choose a standard template (like “Gibson Les Paul”) to auto-fill the scale length, or choose “Custom”.
2. Enter Scale Length: Input the exact distance you want from the nut to the bridge saddle (before compensation). Ensure you select the correct unit (Inches or Millimeters).
3. Set Number of Frets: Enter the total frets required (usually 21, 22, or 24).
4. Review the Chart: The calculator generates a table showing exactly where to cut every fret slot.
5. Apply Compensation: Remember that the bridge saddle must be moved back slightly further than the calculated scale length to account for string stretching (intonation).

Key Factors That Affect Stew Mac Fret Calculator Results

When designing a fretboard, several factors influence the final playability and cost:

1. Scale Length Choice: Longer scales (25.5″+) provide snappier tone and higher tension. Shorter scales (24.75″ or less) offer warmer tone and easier bending.
2. Intonation Compensation: The calculator gives the theoretical position. In reality, you must add ~1-2mm (or more for bass strings) to the bridge position.
3. Blade Thickness (Kerf): When cutting slots, the saw blade thickness (usually 0.023″) removes wood. Measure from the nut to the center of the slot.
4. Material Stability: Wood expands and contracts. Cutting slots in high humidity can result in loose frets later (fret sprout).
5. Metric vs. Imperial: While the math is the same, verify your ruler/calipers match your calculator settings to avoid costly conversion errors.
6. Compound Radius: While this doesn’t change fret spacing, it affects the fretboard geometry and how difficult it is to slot the board accurately.

Frequently Asked Questions (FAQ)

Does this calculator account for bridge compensation?
No. The stew mac fret calculator provides the mathematical fret slot positions. Bridge compensation (moving the saddle back) is an adjustment added after the scale length is determined to correct for sharp notes caused by fretting string tension.

Why is the 17.817 constant important?
Before this constant was standardized, luthiers used the “Rule of 18,” which resulted in the 12th fret being slightly flat. The 17.817 constant ensures the 12th fret is mathematically exactly half of the scale length, ensuring accurate octaves.

Can I use this for a multi-scale (fanned fret) guitar?
This specific calculator computes a single scale. For fanned frets, you must run this calculation twice: once for the treble scale and once for the bass scale, then interpolate the positions between the two sides of the neck.

How accurate do I need to be?
Extremely accurate. An error of just 0.01 inches can be audible to a trained ear. Use digital calipers and a quality straightedge when marking your board based on these results.

Is the scale length measured from the nut or the zero fret?
If your guitar has a zero fret, the scale length is measured from the center of the zero fret to the bridge. If it has a traditional nut, measure from the face of the nut (where the string leaves) to the bridge.

Does string gauge affect fret placement?
No. Fret placement is fixed by geometry. However, string gauge affects bridge compensation. Heavier strings require the bridge saddle to be moved further back.

What is the “Scale Length”?
It is the vibrating length of the open string. Mathematically, it is 2x the distance from the nut to the 12th fret.

Can I print templates from this?
This tool provides the data. To print a template, you would need to transfer these measurements to a CAD program or use a ruler to mark your wood manually, which is the traditional luthier method.

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