U-100 is the global insulin concentration standard: 100 units per 1 mL. All US insulin syringes are U-100 calibrated. So a 0.5mL syringe holds 50 units, a 1mL syringe holds 100 units. The unit is a volume measurement, not a dose — to convert a unit count to mcg or mg of peptide, you need to know the concentration of the reconstituted solution. U-40 syringes still exist in some countries and for veterinary insulin; never mix them up with U-100 syringes (the math is different).
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