You will most likely purchase either 5000iu or 1500iu both come powdered in a vial with 1ml of sterile water. When mixed a 5000iu vial will yield 5000ius/1ml, this means if you want to shoot 500ius you will have to measure out 0.1ml.
Now most insulin syringes measure in ius,this is where some people become confused, for example a 1ml insulin syringe will have 100 marks down the side usually going 5,10,15,20, etc.. 0.1ml of HCG is measured to the 10 mark on the syringe this provides you with a 500iu shot.
Many people, including myself choose to add more sterile water diluting the HCG and providing more overall volume of solution, if you take the previous example of the 5000iu vial and add a further 4ml of sterile solution you will still have 5000ius of HCG but it will be 5ml of solution hence 500iu of HCG now becomes a 0.5ml shot or the 50 mark on the insulin syringe.
I hope this has helped you