Mineral Units Explained: mg, mcg, mEq, mmol, Elemental vs Compound Weight and How to Convert Them

Milligrams and micrograms

The NHS says a microgram is 1,000 times smaller than a milligram, and that it is sometimes written with the Greek symbol "μg". [1] So 1 mg equals 1,000 mcg (or μg). Mixing these up is a thousand-fold error.

The same mineral can appear in either unit depending on the country. For copper, the NHS gives 1.2 mg a day for UK adults aged 19 to 64, [1] while MedlinePlus lists 900 mcg a day as the US figure for adults 19 and older. [2] Those are different national figures, not a typing difference, and 900 mcg is 0.9 mg. For iodine, the NHS says taking 0.5 mg or less a day of iodine supplements is unlikely to cause harm, [3] which is 500 mcg; EFSA lists an adult upper level of 600 µg a day. [4]

Elemental weight versus compound weight

A supplement rarely contains a bare mineral. It contains a compound, such as a carbonate, oxide or citrate, and only part of the compound's weight is the mineral. The mineral's own weight is called the elemental amount.

The US FDA labeling guide says that if calcium carbonate supplies calcium, the Supplement Facts panel must list the weight of calcium, not the weight of the calcium carbonate. [5] (That guide is dated April 2005 and is non-binding guidance.) You can work out the share yourself: elemental share = (atomic weight of the mineral × number of atoms) ÷ the compound's molecular weight. The table below does this with molecular weights from PubChem. The percentages are our own calculation, rounded.

Magnesium oxide (MgO) [6]about 603 mg magnesium

Magnesium glycinate, also called bisglycinate (C4H8MgN2O4) [6]about 141 mg magnesium

Trimagnesium dicitrate (anhydrous magnesium citrate) [6]about 162 mg magnesium

Magnesium sulfate heptahydrate [6]about 99 mg magnesium

Calcium carbonate (CaCO3) [6]about 400 mg calcium

Tricalcium citrate, anhydrous [6]about 241 mg calcium

Ferrous sulfate, anhydrous [6]about 368 mg iron

Ferrous sulfate heptahydrate [6]about 201 mg iron

Ferrous fumarate [6]about 329 mg iron

Ferrous gluconate [6]about 125 mg iron

Zinc oxide [6]about 803 mg zinc

Zinc gluconate [6]about 144 mg zinc

Two cautions. First, water of crystallization changes the answer a lot: the same iron salt is about 37% iron without water and about 20% with seven water molecules, so you need to know which version is meant. Second, these are calculations for a pure compound; real products may differ, and the label is the legal statement of what is in a serving. How the forms behave in the body is covered on the comparison pages for magnesium, calcium, iron and zinc; a higher elemental share does not by itself mean better or worse.

Salt versus sodium

Salt (sodium chloride) is only partly sodium. The World Health Organization's converter says 1 gram of salt is equivalent to 400 mg of sodium, and 5 grams of salt to 2,000 mg of sodium. [7] MedlinePlus says table salt is 40% sodium. [8] Our molecular-weight check agrees: sodium is about 39% of sodium chloride by weight. [6]

To go from sodium to salt, divide by 400 mg per gram, which is the same as multiplying the sodium in grams by 2.5. For example, 2,300 mg of sodium (the sodium Daily Value on US labels [9]) is 2.3 g of sodium, or about 5.75 g of salt. The NHS salt limit for adults is written in salt, not sodium: no more than 6 g of salt a day. [10] Always check whether a number is "salt" or "sodium" before comparing it with a guideline. The NHS also warns that some dissolvable tablets, such as painkillers, vitamins or prescription medicines, may be high in salt, and to speak to a GP before changing any medicines. [10]

Potassium chloride, used in some lower-sodium salt substitutes, is about 52% potassium by weight (our calculation from PubChem). [6] For people who use table salt, WHO suggests replacing it with lower-sodium salt substitutes that contain potassium. [7] People with kidney disease or on certain medicines need individual advice about potassium; see potassium.

Millimoles and milliequivalents

Blood tests and some official reports use moles instead of weight. A millimole (mmol) counts particles rather than grams, so to convert milligrams to millimoles you divide by the mineral's atomic weight. A milliequivalent (mEq) also counts electrical charge: a LibreTexts biology text says mEq/L is the concentration in millimoles multiplied by the number of charges on the ion, so for sodium (one charge) one mEq equals one mmol, and for calcium (two charges) one mEq equals 0.5 mmol. [11]

Two published checks agree with these weights. EFSA writes an adult potassium Adequate Intake as 3,500 mg (90 mmol) a day, [12] and 3,500 ÷ 39.1 is about 89.5. EFSA also defines high blood calcium as above 2.75 mmol/L, which it writes as 11 mg/dL, [12] and 2.75 × 40.08 is about 110 mg per litre, which is 11 mg/dL. Different countries report lab results in different units, so ask the lab or clinician for the reference range printed on your own report rather than converting by hand.

Worked examples (arithmetic only)

• Compound versus element. A label or web page that says "1,000 mg magnesium oxide" holds about 603 mg of magnesium by the table above. Both numbers can appear on packaging, which is why they must be kept apart. EFSA's adult upper level for supplemental magnesium (readily dissociable salts and oxide) is 250 mg a day, [4] and that figure is for the mineral, not the compound weight.
• Percent Daily Value. If a serving has 130 mg of calcium and the calcium Daily Value is 1,300 mg, [9] the serving gives 10% of the Daily Value (130 ÷ 1,300). The FDA's example works the same way. [9]
• Potassium in different units. 2,600 mg of potassium (the US adult female Adequate Intake [13]) is about 66.5 mmol, or 66.5 mEq, since potassium has one charge.
• Sodium in different units. 2,000 mg of sodium (the level below which WHO recommends adults keep intake [7]) is about 87 mmol and about 5 g of salt.