If you are replacing lines measured with an imperial system measurement, it is not a problem!  The general rule of thumb is to convert your measurement in inches to mm (google can do that for you).  Then, if it does not equal an exact vacuum hose size in mm (and it likely won't), just size down one size.  For example, 1/4 inch line is 6.35 mm.   So, size down to 6mm because that is the closest size down.  It is recommended you size down so that your hose stays firmly attached to your fittings - silicone hose stretches laterally rather easily and tends to stay put on the fitting with tension.  When properly fitted, clamps are not needed.  If you are concerned about the fit of hose on your fittings, we have clamp options that are suitable for use with silicone vacuum hose.