String Height Gauge Instructions

Quick guide for the McGrath Guitars Digital String Height Gauge (based on the LMI design). It measures nut slot height, neck relief and action. The reading is the gap between the top of the fret and the bottom of the string.

Download the PDF instructions (printable, includes the setup log).

1. Get ready

Fit one CR2032 3V coin cell battery. Batteries are not included with the gauge.
Press ON. If your display has an in/mm button, choose your unit now.
Tune the guitar to pitch and hold it in playing position, or lay it flat on a neck rest. Measure the same way every time.
Wipe the fret tops clean so the base and string sit on bare metal.

2. Zero and read

Rest the base flat on the fret you want to measure, with the plunger over the string. Then use one of these two ways to zero:

Press-down zero (most accurate). Gently push the plunger down until the string just touches the fret. Press ZERO. Let go: the string lifts the plunger and the display shows the gap.
Fret zero. Rest the plunger tip on the fret right beside the string. Press ZERO. Slide the gauge over so the tip rests lightly on top of the string, and read.

The display may show the number as negative, for example -0.078. Read the number and ignore the minus sign.

3. Tips

Keep both ends of the base flat on the fret. A tilted base gives a false reading.
Touch the string lightly. If the plunger pushes the string down, the number will read low.
On the outer E strings the base can tip. Steady it with a fingertip on the base, not on the string.
The open base fits basses, 7 and 8 strings, 12-strings and mandolins. On a 12-string, measure each string of a course.
Zero again whenever you move to a new fret, or if the display turns off.
When filing a nut slot, zero once with the string pressed to the 1st fret. You can then file and check without zeroing again.
Take each reading twice. If the two differ, zero again.
Don't leave the gauge sitting on the guitar unattended.

4. What to measure

Check	Where the gauge sits	How
Nut height	1st fret	Strings open. Measure each string. Adjust by filing the nut slot.
Neck relief	7th or 8th fret	Capo at the 1st fret. Hold the low E down where the neck meets the body (about the 15th to 17th fret). Measure the low E. Adjust with the truss rod.
Action	12th fret	Strings open. Measure the low E and the high E. Adjust at the saddle.

Set them in this order: relief, then action, then nut. Each one changes the next. Check relief again after a truss rod change has settled.

5. Typical starting points

Measurement	Treble side	Bass side
Nut, over 1st fret	0.010–0.012″ (0.25–0.30 mm)	0.018–0.022″ (0.45–0.55 mm)
Relief, electric	0.005–0.010″ (0.13–0.25 mm)
Relief, acoustic	0.008–0.012″ (0.20–0.30 mm)
Relief, bass	0.010–0.015″ (0.25–0.38 mm)
Action 12th, electric	0.060″ (1.5 mm)	0.080″ (2.0 mm)
Action 12th, acoustic	0.070″ (1.8 mm)	0.095″ (2.4 mm)
Action 12th, bass	0.080″ (2.0 mm)	0.100″ (2.5 mm)
Action 12th, classical	0.110″ (2.8 mm)	0.150″ (3.8 mm)