253mm is read 253 Millimeter.
'Milli" is a sub-multiple that has a value of:
[tex]10^{-3}[/tex]Thus, 253mm is
[tex]253\times10^{-3}m[/tex]Expressing this in standard form, it becomes:
[tex]2.53\times10^{-1}m[/tex]To Decimal places, it is;
[tex]0.253[/tex]