Tonal languages, such as Mandarin, convey information using both phonemes and tones. Using a recently proposed framework for measuring the functional load of a phonological contrast (i.e. how much use a language makes of the contrast), we carry out several computations to estimate how much use Mandarin makes of tones. The most interesting result is that identifying the tone of a syllable is at least as important as identifying the vowels in the syllable. Another computation suggests that the contrast between low and neutral tone carries relatively little information.