[boost] Using units prefix dynamically to get the appropriate prefix for the size of the value?

I'd like to use the units library prefix (k, M, G ...) system, but I'd really like to determine the prefix dynamically (automatically) depending on the value. For example, if I have a length of 12300 m I'd like an output of "12.3 km" but if my length is 0.0123 m, I'd like an output of "12.3 mm". I can't see how to do this (and including the correct length unit of m or meter of course). Suggestions? Paul --- Paul A. Bristow Prizet Farmhouse Kendal, UK LA8 8AB +44 1539 561830, mobile +44 7714330204 pbristow@hetp.u-net.com