Any time I’m talking about measuring action on an electric guitar or bass (we’ll get to acoustics in a little while), I always recommend fretting or capo-ing at the 1st fret while you measure the action at the 17th fret.
When measuring action, you check the distance between the top of a fret and the bottom of the string. That’s fine but why the 17th fret? You’ve probably read others telling you to measure somewhere else — probably at the 12th fret.
Well, there’s a very good reason for my 17th fret measuring and it’s not just because Dan Erlewine says so (although he does).
When you fret/capo the strings at the 1st fret, you immediately take the nut out of the equation.
This is good.
Measuring at the 12th fret with open strings, as many recommend, means that the height of the strings in the nut slots could throw out your action measurement.
If the nut slots are too high, you’re not getting an accurate picture of your action.
Fret/capo the string, though, and it doesn’t matter what your nut is doing. You’ll have an unbiased measurement for your action.
Here’s the thing. If your guitar or bass is perfectly set up, with a well cut nut with string-slots at the right height, measuring the open string at fret 12 will give you pretty much the same measurement as the capo-ed string at 17.
If you're working with guitars, however, you can’t just assume the instrument has a perfect nut so I recommend always fretting at 1 and measuring at 17. Doing this makes things consistent and reduces the potential for error.
Grrr. Acoustics ruin this beautiful system.
What happens beyond the neck/body join on many acoustics is a world of mystery and unpredictability. If you’re lucky, what you’ll have is a fall-away where the frets/fingerboard get slightly lower towards the end. If you’re unlucky, you might have a hump or ‘rising tongue’ — where the fingerboard extension ramps up. Or anything in between.
Long story, short, you can’t rely on consistency past the 14th fret on many acoustics.
So… I have to tell you to measure at the 12th fret.
Yes, all that stuff about the nut still applies so you have to be very aware of this. Keep an eye on things down at the nut end and don’t consider your action measurement ‘final’ until things are good there.
What About Measuring in Other Locations?
I’ve read a number of different ideas about where to measure action and the truth is, if you’re doing it for yourself — just to keep an eye on your own guitars’ action — it doesn’t really matter. Just be consistent in where you measure; if it’s always in the same place you’ll be fine.
If you’re checking against specs in books or on the net, see where the authors recommend measuring and go with that.
For consistency, (and as a reasonably standard practice) I recommend measuring the capo-ed string at 17 for electrics and (carefully) measuring the open string at 12 for acoustics.