Walter Montego: From a practical standpoint a third of a meter is 30cm. Or 33. depends on the application how many digits you need.
I agree that the metric system is not the best for these kinds of calculation, but then no system is. A better system would be based on 12 (but you would need to change the number system as well to make it consitent. Powers of 12 have lots of zeros), exactly for the reason you state. Guess why the babylonians used 60