if 0 {[Richard Suchenwirth] 2004-02-11 - Here's a routine for formatting non-negative integers (e.g. file sizes), so that the resulting field width does not exceed 5 characters, of which max. three significant digits, and K(ilo = 1024), M(ega = 1024^2) etc. modifiers are used for scaling. This may be useful on small displays.}
 proc fixform n {
    if {wide($n) < 1000} {return $n}
    foreach unit {K M G T P E} {
        set n [expr {$n/1024.}]
        if {$n < 1000} {
            set n [string range $n 0 3]
            regexp {(.+)\.$} $n -> n
            return $n$unit
        }
    }
    return Inf ;# :)
 }
 % fixform 1
 1
 % fixform 10
 10
 % fixform 100
 100
 % fixform 1000
 0.97K
 % fixform 10000
 9.76K
 % fixform 100000
 97.6K
 % fixform 1000000
 976K
 % fixform 10000000
 9.53M
 % fixform 100000000
 95.3M
 % fixform 1000000000
 953M

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In the [Tcl chatroom] I was informed that according to http://physics.nist.gov/cuu/Units/binary.html , the units based on powers of 2 should be abbreviated 
 Ki Mi Gi Ti Pi Ei
but I haven't seen that used yet... change if you wish.

[DKF]: Some standards are just destined to be ignored...