Acoustic guitar action height
Martin's service guide for its SC models gives the action of a steel-string acoustic at the 12th fret: 0.080–0.090″ (2.03–2.29 mm) on the low E and 0.055–0.065″ (1.40–1.65 mm) on the high E, with 0.003–0.005″ (0.08–0.13 mm) of neck relief. That leaves room for hard strumming on a .012–.053 set without making barre chords a fight.
Steel-string acoustic action, Martin
Low and high E at the 12th fret and neck relief, from Martin's service guide.
| Instrument | Low (mm) | High (mm) | Relief (mm) | Fret |
|---|---|---|---|---|
| Martin acoustic | 2.0–2.3 | 1.4–1.7 | 0.1–0.1 | 12 |
Martin — SC Service Kit 2024 (Setup & Adjustment of Sure Align Neck System for SC Models, PDF)
How to use it
- Tune to pitch and check relief first: with a capo at the 1st fret and the low E held down where the neck meets the body, the gap around the 7th or 8th fret should fall within Martin's 0.003–0.005″.
- Measure the low and high E at the 12th fret, from the top of the fret to the underside of the string, and enter both. The 25.4″ scale is already filled in.
- Slacken the strings, lift the saddle out and sand the bottom by the amount shown, more on one end if the tool says so. Restring, tune and measure again.
Help and explanations
On an acoustic the saddle has no screws. Action comes down by sanding the bottom of the bone or plastic saddle, and goes back up with a shim or a new, taller saddle. The calculator tells you how much to take off each end.
The saddle moves twice the 12th-fret change
The 12th fret sits halfway along the scale, so action there changes by half of what the saddle moves. An acoustic with 2.6 mm (0.102″) on the low E and 1.9 mm (0.075″) on the high E needs 0.8 mm (about 1/32″) off the saddle on both ends to reach 2.2 mm and 1.5 mm, inside Martin's range.
Mark the line on the saddle with a pencil, sand on paper laid on a flat surface, and keep the bottom square so the saddle sits flat in its slot. A saddle that rocks loses volume, and with an under-saddle pickup it makes some strings louder than others.
Relief first, then saddle, then the neck
Check relief before you sand anything: a neck with too much bow can add a good part of a millimeter at the 12th by itself. On many steel-string acoustics the truss rod nut is reached through the soundhole, at the end of the fretboard; on others it's under a cover on the headstock. Tighten to reduce relief, loosen to add it, a quarter turn at most per step.
If the saddle is already low and the action is still high, the neck angle has shifted and the guitar needs a neck reset. Sanding further only cuts the string angle over the saddle and the drive on the top.
Tips
- A lower saddle changes how far each string stretches when fretted; compare every 12th-fret note with its harmonic using the intonation checker.
- Moving from lights to mediums adds pull on the neck: compare sets in the string tension calculator.
- Fitting a new saddle or nut? Acoustic guitar string spacing shows where each string goes.
Frequently asked questions
What is normal action on an acoustic guitar at the 12th fret?
Martin's spec is 0.080–0.090″ (2.03–2.29 mm) on the low E and 0.055–0.065″ (1.40–1.65 mm) on the high E, measured from the top of the 12th fret to the underside of the string.
What is that in 64ths of an inch?
Just over 5/64″ on the low E (5/64″ = 1.98 mm) and close to 4/64″ on the high E (4/64″ = 1.59 mm).
How much should I sand off my acoustic saddle?
Twice what you want to lose at the 12th fret. To drop 0.4 mm there, take 0.8 mm off the bottom of the saddle.
How much neck relief should an acoustic guitar have?
Martin gives 0.003–0.005″ (0.08–0.13 mm), about the thickness of one or two sheets of printer paper. Use a feeler gauge: a gap that small can't be judged by eye.
Can I lower the action with the truss rod instead of the saddle?
Only if relief is above spec. Tightening the rod past that point flattens the neck and makes the strings buzz on the first frets; the rest has to come from the saddle.
What if the saddle is already too low?
Add a thin shim or make a taller saddle if the action went too low. If it's low and the action is still high, the neck angle has changed and needs a neck reset.