hi,
 
 Can I save any Autocad file in metric units?
 

yes u can, just go to format menu, click units and select decimal. Now you can save ur file and it will be in metric units. If u select this unit at start of drawings then it will make ur drawing according to ur dimentions, but if u want to change the already made drawing from architectural to metric then it will be little trouble for u. The trouble is that if u made a line in architectural units of length of 48 inches, now you change it's unit in metric and Autocad does, but the length will make problem. Because it will show you the length of line of 48mm and this can't be rite, because autocad changed the unit not the value of unit. So 48 inches cannot be 48 mm. Just make ur unit decision before starting the drawing. ok Bye!

Hi. AutoCAD starts out essentially unitless -- that is, it draws in "units" and it's up to you to apply a value to those units. You draw in a 1:1 scale.
 
If you select "architectural" in a drawing that used an English template (as opposed to a Metric one), it recognizes one unit as being one inch. If you simply change the units to be decimal, then one unit is one unit. The problem with this is that 1 inch is roughly equal to 2.54 cm, so you would have to scale your entire drawing up by 2.54 in order to get the "unit" to be worth 2.54 "something", in this case centimeters. Or scale it up by 25.4 if you want the value of one "unit" to be in millimeters. You would have to create a new dimension style using the suffix "cm" or "mm", depending on what you want one unit to represent and how you normally measure in metric.
 
An alternative would be to leave the drawing as is in English (inches) without scaling into metric units. In that case you could start with the normal feet-and-inches dimension style, and define an alternative to show the metric equivalent below the dimension line, using the same multiplier and suffix for either cm or mm...