Can't believe this is confusing as many people as it is.
As Darren Wilson posted back in 2001, measure the distance from the edge of the nut (or zero fret if your guitar has a zero fret) to the 12th fret and double the distance.
clintham, not sure where you read this as stewmack site says the same thing.
WHAT IS SCALE LENGTH?
A guitar's scale length is calculated by measuring the distance from the front edge of the nut, where it butts against the end of the fingerboard, to the center of the 12th (octave) fret, then doubling that measurement.