I have been asked where are the reference points for the guitar nut. Nut length is based on the guitar neck width. Hopefully, the following information can help eliminate some confusion about the nut length.
I have found that most novice players do not consider nut length measurement to be important while more advanced players have a specific preference which is based on their preference for finger picking or plectrum style picking. The player's picking style is the dominant factor in nut length choice.
Common neck width are as follows:
2.0" (51 mm) - Classical or nylon string guitars.
1 7/8" (48 mm) - Some steel string, finger style guitars.
1 13/16" (45 mm) - Steel string guitars.
1 3/4" (44 mm) - Most common length/width.
1 11/16" (43 mm) - Many electric guitars use this length/width.
1 5/8 (41 mm) - Smaller electric guitar similar to Fender® Duo-Sonic®.
This image will give you a good idea for comparison.
One important note about measurement is the fact the US standard measurement is used. I prefer the metric system but it seems that the guitar world isn't about to change this anytime soon so I have listed 3 measurement reference in my details; inch/decimals, inch (fractions), and metric equivalence. Save your time and don't send me mail telling me that the US Standard measurement system sucks. I AGREE!
This is a Telecaster nut with a 9.5 º radius
According to the calipers: this neck is 1.6205" or 41.16mm in length
(NOTE: This is not a Fender Telecaster - It's a BILL NASH TELECASTER)
Usually Telecaster nut length is 1 11/16 inch or 43.0mm in length. Nevertheless, the critical points for dimensions of nut length will be end to end of the current nut on the guitar or the width of the neck if there isn't a nut available to measure.
Below are random examples of guitar nuts being measured
This is a Taylor 514 with a length of 1.75" or 44.66mm - measured end to end.
This would round up to what is called a 1 3/4 nut
Below is a Fender Stratocaster 50th anniversary measured at 1.6915 " or 42.96mm.
This would round up to what is called a 1 11/16 nut.
Below is a Paul Reed Smith Custom 24 nut measurement.
This nut measured 1.6935" or 43.0 mm. This is also a 1 11/16 inch neck
If you need to replace your nut and you are buying one online; often you will be asked to provide the E to E measurement. Below is an example of a Taylor 514ce E to E (Center of the Low E to Center of the High C) measurement being recorded.
String Spacing: I use a tool that I purchased from Stewart MacDonald to determine and replicate string spacing. Below are some images of the tool in use.
More of the same .....
The tool is about 12" or 304mm measured length
In summary, nut length is measured end to end of the nut. If you are making a nut from stock it is always better to start out with a little more than you need then file the blank down to match the neck width. Measure twice and cut once! For further information I recommend Frank Fords site Frets.com.
Other valuable Links:
A closer look at Nuts (LOL!)
Frank Ford is one of the owners and Luthiers at Gryphon Stringed Instruments.