I recently did some work on a six string cbg. The bridge was free floating, and I forgot to mark it’s sweet spot beforehand. Now, I want to put it back, but have no idea where to place it. Does anyone have a trick to find the scale length from spacing of the frets? I’m hoping for a reverse fret calculator or something. Nut to 1st fret is 33 mm (1 1/4ish inch) . nut to 12th is 30 cm (11 7/8 inch)
Forget about the first fret distance for a moment. Instead, carefully measure from the edge of the nut to the 12th fret. Twice that measurement is your scale length (where the bridge should go).
If your 30 Cm measurement is correct, that translates to 11.811". Double that and you have 23.622" (or 23 5/8") total scale length.
Once you have the bridge positioned and the strings tuned, you can check to see that the note is exactly one octave above the open note. Move the bridge slightly until it is and you're good to go.
The 12th fret is always the key to determining scale length.
I think the important thing here is to remeasure, too many conflicting measurements
I think Richard hit things on the head.
If the nut to 12th is 11.7/8" then the scale would be 23 3/4" for that scale length, the first fret would be 33.86 mm from the nut (call it 34 mm). A careful re-check and you may find that the actual nut to 1st fret is close to 34mm.
In any case you are close enough that you can put the bridge at 23 3/4" then tune and check intonation at the 12th to find the final position of the bridge.
You're almost home.
Hi, the easy way as mentioned by Tom, remember to measure from the inside edge of the nut to the string contact point on the saddle, or from the centre of a zero nut to the same point on the saddle.
Also the amount of compensation varies depending on string gauge and/or string height.