iu is international units.
For your purpose this is all about dilution. Totally forget about mg; it's not relevant to HcG; if someone says you need 200mg or 1000mg, etc then they're (almost) certainly making a misstatement and either meant iu's, or else they don't know what talking about.
If you put 5000 units in 5 ml, then each 1 ml has 1000 units. 1/10 of a ml has 100 units. If you do a different dilution, your math will be different.
I can think of some reasons for doing different dilutions, if you do so and need to run the math by someone just ask.