Question 1

Why does my vitamin conversion look wrong?

Accepted Answer

Because IU conversions are nutrient-specific, not universal. The 2016 label rules moved vitamins A, D and E from IU to metric: vitamin D is declared in micrograms, vitamin E in milligrams of alpha-tocopherol, vitamin A in micrograms RAE. The factor differs per nutrient and, for vitamins A and E, per source form. A single IU number with no nutrient attached cannot be converted.

Question 2

Should I use 4.184 or 4.2 for calories to kilojoules?

Accepted Answer

Use 4.184, the thermochemical factor, and round only at the end. Rounding the factor first introduces error that shows up on higher-energy products. EU and UK labels declare both kJ and kcal, with kJ first, so this conversion is on the critical path for any product crossing the Atlantic.

Question 3

Can I convert cups to grams?

Accepted Answer

Not with a single factor, because it depends entirely on what is in the cup. A cup of flour and a cup of honey differ by more than a factor of two. Use the specific gravity or bulk density of the actual material, which is what the Volume to Weight tool is for. Nutrition panels are calculated on mass for exactly this reason.

Question 4

Does the label use the converted number or the original?

Accepted Answer

Whichever value you declare must then follow the rounding rules for that nutrient, and the %DV must be consistent with the basis you chose. The order matters: convert first, then round to the increment the format requires, rather than rounding and then converting, which can move a value across a threshold.

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