Good question - and here is an article by superchicken that I found very useful about IU to mg conversions:
---------- What is an iu? ----------
written by superchicken anabolic-alchemy
An IU stands for international unit. It is an internationally accepted amount of a specific substance. Its typically used for hormones, biologicals, enzymes, and certain fat soluble vitamins such as A, D, and E. Look on the back of your Centrum multi vitamin bottle, some stuff is listed in IU not mg or mcg like the other things.
Here’s a quick definition I pulled of the web:
1. The quantity of a biologically active substance, such as a hormone or vitamin, required to produce a specific response.
2. A unit of potency for similarly active substances based on this quantity and accepted as an international standard.
You can see they aren’t concerned with weight(mg) or volume(ml) much when it comes to IU’s. Rather, it is used to express how much biological effect it will have. There is a correlation between weight and iu’s, and to a much lesser degree, volume as well. But this is specific to each substance, as each substance is different.
So you can almost think of IU’s as weight(mg’s), just realize that the weight will be different for each compound.
A few examples of the correlation between iu’s and weight:
-1 IU represents 45mcg of a standard preparation of insulin (u-100 type insulin, there are other types and other types will not be 45.5mcg per iu!)
-1 IU represents 0.6mcg of a standard preparation of penicillin
-1 IU represents 0.33mg(333mcg) of human growth hormone(hgh)
-1 IU represents 0.3 microgram (0.0003 mg) for vitamin A
-1 IU represents 50 micrograms (0.05 mg) for vitamin C
-1 IU represents 25 nanograms (0.000025 mg) for vitamin D
So you can see, an IU does not mean a certain amount of weight if being used to compare 2 different substances.
Now this will probably come shockingly to many, but an iu also DOES NOT correlate to volume(ml). Here’s some examples of common preparations:
-when measuring U-100 type insulin(there is more than 1 type of insulin!), and measuring it with a U-100 type insulin syringe(theres more than 1 type of insulin syringe! And im not talking about bigger and smaller needles, or a syringe which holds more or less volume ie; 30/50/100iu syringes). But for U-100 1ml will equal 100iu. Again, if using diff types this will no longer hold true. But lets just say 100iu represents 1ml.
-HCG is typically presented with 1 amp of powder, and 1 amp of solvent. There is usually 5,000iu or 10,000iu of HCG in the kit. Lets use the 5,000iu kit as our example:
Every iu is in the powder amp. So adding more or less solvent to the powder will NOT change the total amount of iu of hcg that you have. If you add 1ml of solvent to the powder, you have 5,000iu/ml. If you add 5ml of solvent, you now have 1,000iu/ml, but have 5ml of it. You still have a total of 5,000iu all together.
Now lets say you prepared your hcg at 5,000iu/ml. if you draw that ml of hcg into a u-100 insulin syringe, it will be full, and the liquid level will read as 100iu on the graduated markings on the barrel of the syringe. But remember, were using a INSULIN syringe, but were drawing up HCG. So even though your syringe says 100iu, you in fact have 5,000iu in it.
Same goes for HGH. A typical vial has 18iu of powder in it. If you add 3ml of solvent, you now have 6iu per ml. So now if put into a U-100 insulin syringe, 1ml of your HGH will read as 100iu, but you in fact have only 6iu in there.
To figure out how many iu per ml you have, is simple math. Lets say we have 5,000iu. We add 5ml of solvent. Divide the iu by the amount (in ml) of solvent.