International Units (IU) measure biological activity rather than mass. The conversion to mg is compound-specific: 1 IU of HCG = 0.001 mg, 1 IU of HGH = ~0.33 mg, 1 IU of EPO = 8.4 nanograms. Always check the conversion before doing dose math, and never substitute IU for mcg/mg without converting. Most peptides on this site are dosed in mcg or mg; IU shows up mainly for HCG, HGH, HMG, and EPO.