Correct bridge placement determines a guitar's intonation when playing fretted notes. The distance between the guitar nut and bridge is the scale length. Placing the bridge too close to the neck shortens scale length and makes fretted notes sharp. Moving the bridge too far from the neck increases scale length and creates flat fretted notes. No amount of tuning helps a guitar with a poorly positioned bridge. Any fretted note falls high or low of the desired pitch.
Finding Scale Length
Place the end of a tape measure against the fretboard side of the guitar nut. If your guitar has a zero fret past the nut, begin measuring there.
Take a measurement at the 12th fret, which is where the midpoint of the scale length crosses.
Multiply the distance from the nut to the 12th fret by two. The product is the scale length, the proper distance between the fretboard edge of the nut and the fretboard edge of the bridge saddle.
Calculating Bridge Position
Measure from the fretboard side of the saddle, where the high E string crosses, to the fretboard edge of the bridge.
Subtract the above result from the scale length.
Measure the resulting distance from the guitar nut to find proper bridge placement.
Most electric guitar bridges have adjustable saddles.
Acoustic guitar bridges do not adjust. Take your guitar to a professional if you are unsure of you measurements.