Quick read: Nut slot depth has a correct value and it is measurable. Hold the string down at the third fret, slide a feeler gauge into the gap over the first fret, and cut until that gap reaches your target — roughly 0.005 in on the high E, climbing to about 0.012 in on the low E. Plain strings want less, wound strings want more, lower tunings want a hair more again. Too shallow and the chords fight you and play sharp; too deep and the open string buzzes and goes sharp. Sneak up on the number, check often, and write it down so the next setup starts from data.
The nut sets the action for exactly one fret — the open string — and everybody ignores it because that is only one note out of however many your neck has. But that one note is the reference the rest of the guitar is tuned against, and a nut cut by feel is the single most common reason a setup that looks right still plays sharp in the first position. The fix is to stop guessing the depth and start measuring it.
There is a companion piece to this one on nut slot geometry — the back-angle, the bottom radius, the witness point. That post argues geometry beats material. This post is the procedure for the one geometric element you can put a number on: depth.
Why Depth Is the Number You Can Actually Measure
Slot width, back-angle, and bottom radius are shapes. You can describe them, but you cannot easily hand someone a target value for them. Depth is different. Depth is a vertical distance, and a vertical distance is something a feeler gauge reads directly.
The measurement that matters is not how far the slot sits below the top of the nut. It is the clearance between the bottom of the string and the top of the first fret. That clearance is what your fretting hand feels in the first three frets, and it is what determines whether an open chord plays in tune.
Here is the part most people get backwards. The string does not sit on the bottom of the slot at rest in a way that touches the first fret — there is air between them, and that air gap is the whole game. Too much air and the string is high. Too little and it buzzes. The slot depth controls the air gap. So you measure the air gap, not the slot.
The Fret-Rock Method: Take Everything Else Out of the Equation
The problem with measuring nut height on an assembled guitar is that three things all affect the action down at the first fret: the nut, the truss rod relief, and the saddle height. If you measure the first-fret clearance straight, you are reading all three at once and you cannot tell which one is wrong.
The fret-rock method removes two of them.
- Hold the string down at the third fret. A capo at the third works and frees both your hands.
- With the string pressed at the third, the first and second frets become the reference plane. The string is now a straightedge resting on those frets.
- Look at the gap over the first fret. With the string held at the third, that gap is set by one thing only — the nut slot depth. Truss rod relief and saddle height no longer touch it, because the string is fretted past them.
- Slide a feeler gauge into that gap. The blade that just fits is your measurement.
That is the entire method. Capo on the third, feeler over the first, read the number. It isolates the nut so completely that you can dial it without ever touching the truss rod.
The Per-Gauge Clearance Chart
Different strings want different gaps, and the reason is mechanical, not magic. A thicker string vibrates in a wider arc for the same picking force, so it needs more room before it slaps the first fret. A wound low string in a dropped tuning is the widest arc of all.
This is a starting chart for standard tuning, 10–46 on an electric, measured fretted at the third:
| String | Gauge | First-fret gap (fretted at 3rd) |
|---|---|---|
| High E | .010 | 0.005 in |
| B | .013 | 0.006 in |
| G | .017 | 0.007 in |
| D | .026w | 0.008 in |
| A | .036w | 0.010 in |
| Low E | .046w | 0.012 in |
Treat these as the high side of "low as it goes without buzzing," then add clearance if you play hard. A picker who attacks like they are trying to break the string needs more gap than someone with a light touch — pick attack drives string excursion, and excursion is what closes the gap. If you tune to drop D or lower, add about 0.002 in to the low strings before you start.
I expected the bass strings to want only a little more than the treble strings — maybe one gauge step of feeler. They wanted more than double. The low E sitting at 0.005 in like the high E buzzed on every open chug, and the gap had to nearly triple before the rattle cleared. The arc width scales faster than the string diameter does, and the chart reflects that.
Cutting to the Number
You cut down to a target, never up, so the rule is to approach slowly and measure constantly.
- Measure the current gap with the fret-rock method. Note how far you are from target.
- Make two or three passes with the correct nut file for that string width. Keep the file angled down toward the tuners so the front edge of the slot stays the witness point.
- Re-seat the string, tune to pitch, capo the third, measure again.
- Repeat until the feeler gauge that fits the gap matches the chart. Stop the moment it does.
The failure modes are symmetric, which is what makes the number worth trusting. Cut too shallow and the first-position chords feel stiff and play sharp — the extra height means you stretch the string sharp just by fretting it near the nut. Cut too deep and the open string buzzes against the first fret and also plays sharp, because the string is resting low enough that the first fret is partly fretting it. Same symptom, opposite cause. The feeler gauge is what tells the two apart before you have ruined the slot.
Write It Down
The reason to use a gauge instead of your eye is not precision for its own sake. It is repeatability. Once you have a number that works for your touch, your gauge, and your tuning, it is the number forever. Record it per string. The next restring in the same gauge starts from a known target. A new guitar starts from a known target. A switch to 11s means you adjust the chart up a couple thousandths and you still start from data, not from a fresh afternoon of trial and error.
A nut cut by feel is a nut you have to relearn every time. A nut cut to a measured chart is a setup you can reproduce on any instrument you own. If a string still pings as you tune after the depth is right, that is a width-and-shape problem, not a depth problem — different fix, covered in the geometry post. Get the depth on the number first. Everything else in the first position depends on it.
