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Most of the websites i have looked at, say that the frequency of middle C note is approximately 261.626 Hz. and that middle C on guitar is at the 5th string 3rd fret. but my tuner says the frequency at the 5th string 3rd fret is 130.81 Hz.
Thus, the same note in each octave, say C, is always twice the frequency of the previous one. For example, C3 is 131 Hz and C4 is 262 Hz (any minor deviation from this rule in the table below is simply the result of rounding errors).