mg, mcg, IU, DFE, CFU: the units on a supplement label

A vitamin D bottle says 5,000 IU on the front and 125 mcg on the back. Same amount, two units. Supplement labels use at least six of them – and you can only compare prices once both products use the same one.

g, mg, mcg: three weights

Most supplements are measured by weight. The only thing that changes is the size of the step: a gram has 1,000 milligrams, a milligram has 1,000 micrograms. So 25 mcg of vitamin D is 0.025 mg – a speck you couldn’t see.

US labels write mcg, our price lists write µg – the Greek letter µ (“micro”). It’s the same unit. The trap is a label that switches between mg and mcg: 1 mg of B12 is 1,000 mcg, not 1 mcg.

IU: the old unit on the front of the bottle

IU (international units) measured how much a vitamin does in a lab test, not how much it weighs. Since the label rules changed, the Supplement Facts have to show vitamin D in mcg, vitamin A in mcg RAE and vitamin E in mg. For vitamin D, IU may still follow in brackets (21 CFR 101.36 (opens in a new tab)). The front of the bottle often still leads with the bigger IU number.

Try it with the number from your own bottle:

One thing stands out: the same “400 IU” of vitamin E is 268 mg if it’s natural and 180 mg if it’s synthetic. So two bottles with the same IU on the front can hold different amounts – the mg on the back tells you which. More on vitamin D in our vitamin D guide.

mcg DFE: why folate gets a bigger number

Folate is counted in mcg DFE – dietary folate equivalents. Folic acid, the form in most supplements, counts 1.7 times: mcg DFE = mcg folate from food + 1.7 × mcg folic acid. When a supplement has folic acid, the label shows the DFE amount and the folic acid itself in brackets (FDA (opens in a new tab)):

Both numbers describe the same tablet. When you compare two folate products, make sure you take the same one from each label – DFE with DFE.

CFU: counting live bacteria

Probiotics are living microbes, so their weight says little. That’s why many labels add CFU – colony-forming units, the number of microbes that can grow. Labels have to show the weight; the FDA lets makers add the CFU count next to it (FDA draft guidance (opens in a new tab)). Counts are big, so they’re written in billions: “10 billion CFU”.

We compare probiotics per 10 billion CFU. That only makes sense between products with the same strains – a billion of one strain isn’t a billion of another.

Enzymes: activity instead of weight

Some enzymes are labeled by what they can do, not by what they weigh: bromelain in GDU, nattokinase in FU, lactase in FCC units. If two labels both show the activity, that’s the better number to compare. If one shows only mg, you can’t convert between them.