For ieee754 floating point numbers (the most common) we have abs(f(r) - r) <= c with a rather small c as long as r is in the range of normalised numbers. For tiny numbers (denormalised) it behaves like fixed point.
You get better bounds if you assume that $2^k <= x < 2 \cdot 2^k$ with an error <= $c \ cdot 2^k$$c \cdot 2^k$, especially if x is slightly smaller than some power of two - you can often improve the precision of an algorithm if you can arrange things so intermediate results are slightly smaller than powers of two.