Simply put:
cents = ln(R)/ln(2) x 1200
where R is the ratio of the frequency in question over
the frequency of equal temperament AND ln is the
natural log function (ln is a function button on most
calculators).
To find the frequency from a cents value:
frequency = 440 x 2^(n/12) x 2^(cents/1200)
where n is the number of half steps the note is away from
440 ( n is negative if it is below 440 and positive
otherwise).
^ means to the power of.
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