What the Nut Slot Helper checks
The nut is the small slotted piece at the top of the neck, and it sets how high your open strings sit above the first few frets. When those slots are cut too high, first-position chords feel stiff, notes play slightly sharp near the nut, and your hand works harder than it should. When they're cut too low, open strings buzz against the first fret.
This tool walks you through the check without asking you to own a set of feeler gauges. Pick your instrument type, choose whether you're going by eye or by measurement, and it tells you where you've landed — too high, about right, or too low.
The 3rd-fret test, in plain terms
Tune the guitar to pitch first. Nut height only means anything under normal string tension, so checking a slack guitar will mislead you.
Then fret any string at the third fret and look at the gap between the string and the top of the first fret. You're looking for a very small clearance — enough to see light through, not enough to be obvious. If the string is resting on the first fret, the slot is likely too low. If there's a visible gap you could slide something into, it's probably too high.
Do this string by string. It's normal for one or two slots to be off while the rest are fine.
Reading your result
Too high is the common one, and it's the reason a lot of guitars feel harder to play than they should near the headstock. It's fixable, but it's fixed by removing material — and material doesn't go back on. Filing a slot too deep means a new nut.
About right means leave it alone and look elsewhere. If first position still feels stiff, neck relief or action are the more likely culprits.
Too low can't be filed away, because the problem is that too much has already been removed. Options are shimming the slot, filling and recutting it, or replacing the nut — all of which are reasonable jobs for a tech.
Where this fits in a setup
Nut height is one link in a chain. Neck relief comes first, because the shape of the neck changes where every string sits. Nut height comes next, then action at the bridge, then intonation. Checking the nut on a neck that hasn't been sorted out usually means checking it twice.
Nut height questions, answered
How high should my nut slots be?
Low enough that first-position chords don't fight you, high enough that open strings don't buzz. The exact figure varies by instrument and by playing style — classical guitars sit higher than electrics, and a heavy strummer wants more clearance than a light one. That's why the tool asks what you're playing before giving you a target.
Do I need feeler gauges?
No. The 3rd-fret test gets you a useful read with nothing but your eyes and a tuned guitar. Feeler gauges give you a number you can write down and compare later, which is nice if you like tracking your setups, but they're not required to find out whether something's wrong.
Can I file the slots myself?
You can, but go slowly and know that it's one-way. Nut files are shaped for specific string gauges, and a slot cut too deep means a new nut rather than a do-over. If you've never done it, a tech will usually cut a nut for less than the cost of the file set.
My guitar plays sharp on the first few frets. Is the nut the reason?
It might be. A high slot stretches the string further to reach the fret, which pulls the pitch up — and it shows up worst in first position, where the stretch is proportionally largest. If your intonation is fine higher up the neck but off near the nut, that's a strong hint.
Should I check the nut before or after adjusting neck relief?
After. Relief changes the neck's curve, which moves every string. Setting the nut first means measuring against something that's about to shift.