What do you do if the intonation at the 12th fret is not correct on an instrument you are repairing or on a instrument you've just built? If 12th fret intonation for a string is flat then you'll need to move the point where the string actually contacts the saddle back towards the nut a little. If it is sharp then you'll have to move the contact point away from the nut. But how much do you move the contact point? Here is a calculator that will tell you.
Looking for a way to calculate compensation for a new instrument? Try the Fretted Instrument Bridge Saddle Compensation Calculator.
Initial appearance: September 22, 2018
Last updated: September 22, 2018
When adjusting intonation of an acoustic guitar it is useful to be able to calculate how much the string contact point on the saddle will have to be moved to compensate for an intonation error at the 12th fret. This is easily done using the following calculator which is a modification of the simple fret location calculator found on the Calculating Fret Positions page of this site.
n = 12 + (e / 100)d = ((s – (s / (2 ^ (n / 12)))) - (s / 2)) * 2
To use the calculator, use a digital tuner to accurately tune the open string. Then use the tuner to measure the pitch error in cents when fretting at the 12th fret. Put this value in the calculator field for e. If the error is flat, express this as a negative value e.g. if the pitch is 5 cents flat put -5 in calculator field. Fill in the scale length field. This value can be in any units (inches, mm, etc.). The results field shows how much the saddle must be moved away from the nut in the same units you used to specify the scale length (if the result is a negative value, then it shows how much the saddle must be moved toward the nut).