Here is a table giving the frequencies in Hz of musical pitches, covering the full range of all normal musical instruments I know of and then some. It uses an even tempered scale with A = 440 Hz.
The octave number is in the left column so to find the frequency of middle C which is C4, look down the "C" column til you get to the "4" row : so middle C is 261.6 Hz.
Here is a utility courtesy of Colin Crawley which will calculate the frequencies of notes and can handle tunings other than A = 440Hz.
It can also play the notes, so is useful as a tuning note reference.
It works on Windows and Linux. Playing notes may not work on Safari on the Mac, though Firefox or Chrome on the Mac is ok.
Middle C is C4=261.6Hz
Standard tuning fork A is A4=440Hz
Piano range is A0=27.50Hz to C8=4186Hz
Guitar strings are E2=82.41Hz, A2=110Hz, D3=146.8Hz, G3=196Hz, B3=246.9Hz, E4=329.6Hz
Bass strings are (5th string) B0=30.87Hz, (4th string) E1=41.20Hz, A1=55Hz, D2=73.42Hz, G2=98Hz
Mandolin & violin strings are G3=196Hz, D4=293.7Hz, A4=440Hz, E5=659.3Hz
Viola & tenor banjo strings are C3=130.8Hz, G3=196Hz, D4=293.7Hz, A4=440Hz
Cello strings are C2=65.41Hz, G2=98Hz, D3=146.8Hz, A3=220Hz
Bear in mind that everything here is in relation to the even tempered (aka equal tempered) scale, where an octave is a frequency ratio of exactly two and a semitone is a frequency ratio of exactly the twelfth root of two. In the real world however many different temperaments may be used - see en.wikipedia.org/wiki/Musical_temperament - and octaves too can vary in size, see en.wikipedia.org/wiki/Stretched_octave.
Also we call middle C "C4" : this is the commonest octave numbering but some people call middle C "C3" or even "C5".
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