I play acoustic and electric, and I build acoustics as a hobby. I want to build a dreadnought guitar for a friend. He has been playing forever but recent hand surgeries make him unable to play a standard 1-11/16 or 1-3/4" nut. He can play a classical with a 2" nut, but hates it and misses his Guild dreads. I want to make a steel string dreadnought, 25.4 scale, with a 2" nut width. What should the spacing be at the nut and bridge, or how can I calculate it?
I agree with everything Tim said in his excellent answer. I have 11 acoustic guitars in 8 different brands and each brand seems to have a slightly different idea on the string spacing.
The one thing that is consistent among all guitars I have examined however, is that the spacing between strings is consistent. This is important because if there was an inconsistency from string to string it would play tricks with your mind and trip up the guitarist the way an odd step height on a staircase will trip you up every time (no matter how many times you climb the same staircase).
So the variable that determines the overall spacing between strings - is the distance between the 1st and 6th strings on a given nut width. Whatever you start with there you will want to be sure the spacing between strings is exactly the same. The spacing between strings at the nut will be a smaller distance than it will be at the bridge. But the spacing between strings should remain equal at any given point.
Playing style should inform how close you can put the outermost strings to the edge of the fretboard. I have played some guitars where the strings were too close to the edge and I would have a tendency to push them off the edge of the fret in some instances. I don't like too much space though (particularly at the bass end) because I like to be able to reach over the top of the fretboard with my thumb to fret the low E string occasionally - but for me to do that with my short fingers, the string must be close enough to the edge.
Nut width and string spacing can have a big impact on comfort and playability for the individual guitarist. What works for one, will not work for others. Mass produced guitars must have a formula that works for as many guitarist as possible. But your lucky friend has a luthier (you) willing to work out a custom designed spacing scheme that works best for him.
My advice would be to go to a guitar retailer or two and look at different guitars with 2" nuts and let your friend play guitars with different string spacing templates to see if he finds one that is more comfortable for him than others. You said he can play a classical? Is there a particular classical guitar that he is comfortable with? Why not use that as a guide (or a starting point)?
Does he wish a particular guitar that he can play had the strings slightly closer or slightly farther from the edge of the fretboard? If yes - take that into consideration but remember, that if you move the 1st and 6th string farther apart from one another, the spacing between all strings should be adjusted so that the spacing between strings is consistent.
As far as the spacing at the bridge, that's another area where a luthier building a custom guitar can customize the spacing to fit the individual guitarist. Almost all guitars have wider spacing at the bridge than at the nut so strings can be more easily isolated when picking individual strings. And the neck of the guitar will taper so that the space between the outer strings and edge of the fretboard doesn't vary to an extreme degree. The degree of neck taper from nut to bridge will vary among different makes of guitars just as string spacing will.
But the exact spacing at the bridge can be varied or customized to suit the preference of a particular guitarist. Again - have your friend play guitars with different spacing schemes at the bridge (perhaps using only the picking/strumming hand) to see if wider spacing or tighter spacing might be preferable. If you are building a guitar from scratch, you could create a fretboard taper to accomodate either wider or narrower spacing at the bridge.
I would start with the nut. Let your friend determine through playing different classical guitars how close the outbound strings should be to the edge of the fretboard to suit his preference. Note that the low E string and high e string do NOT have to be the same distance from the edge of the fretboard. I have guitars that put the low E closer than the high e and vice versa. This is not as important as insuring that the space between strings is consistent.
Once you determine the optimal spread between top and bottom strings and the ultimate distance between the top string and edge of fretboard and the bottom string and edge of fretboard - then decide how far apart you want the strings to be at the bridge.
To keep the string spacing consistent at both ends of the neck, you will need to be careful to locate the holes in the bridge such that the off axis angle and and overall spacing between strings remains consistent. Be careful with that part if you choose to make the bottom string closer to the edge of the fretboard than the top string (or vice versa).
I'm not a luthier but my engineer brain tells me to start by finding the center point between the two middle strings (3rd and 4th) at the nut and shoot a straight (no angle) line to the bridge location keeping the relative space between the center line and edge of the fret-board consistent from the nut to the sound-hole. That would give you a starting point for your bridge holes. Keep the distance between center-line and the middle bridge holes even on both sides of the center-line and work your way out from there by adding the distance between the two middle holes to each subsequent hole.
The strings themselves will all travel from nut to bridge at an angle, but the centerline will allow you to properly align the nut and bridge and provide a common starting point to keep the string angle of opposing strings consistent from bridge to nut.
Good luck with your project and I hope your friend derives tremendous joy from playing his new custom guitar.
I doubt if there is an industry standard. Each maker will have his own ideas, thus dimensions. Looking at the space between the top or bottom string and the edge of the fretboard is a good start point, as that needs to be enough so the string doesn't disappear over the edge, but not so much that there's a reach to get to it. From there, the strings would be evenly spaced.
If it's for this guy, do a mock up of the lowest 3 or 4 frets, in whatever material, tension some old strings to it, with a nut, of course, and get an idea of what's good from him. That's probably how the first guitars were sorted anyway.