mEq ↔ mmol ↔ mg
Convert electrolyte amounts between milligrams, millimoles, and milliequivalents.
Sodium chloride — NaCl — molar mass 58.443 g/mol, valence 1. One millimole of NaCl provides 1 mEq of sodium and 1 mEq of chloride.
Divide the milligrams by the molar mass to get millimoles, then multiply by the ion’s charge to get milliequivalents. One gram of sodium chloride is 1000 ÷ 58.443 = 17.11 mmol, and because sodium carries one charge that is also 17.11 mEq of sodium.
Three units for the same substance
Electrolyte orders and labels move between three units, and each answers a different question. Milligrams measure mass — how much substance is there. Millimoles measure the number of particles, which is what osmotic effects depend on. Milliequivalents measure charge, which is what matters when the body balances cations against anions.
For a singly charged ion such as sodium or potassium, millimoles and milliequivalents are numerically identical, which is why laboratory panels can report sodium as 140 mmol/L or 140 mEq/L without ambiguity. For calcium and magnesium, each carrying two charges, one millimole is two milliequivalents — and that factor of two is a frequent source of error.
Salts versus elements
A supplement label may state the mass of the salt, the mass of the element, or the milliequivalents, and they are not interchangeable. One gram of potassium chloride is 13.41 mmol, providing 13.41 mEq of potassium — but one gram of elemental potassium is 25.58 mmol, nearly twice as much, because the chloride accounts for over half the mass of the salt.
Valence is the magnitude of the ion’s charge: 1 for Na⁺, K⁺, Cl⁻, and HCO₃⁻; 2 for Ca²⁺ and Mg²⁺. Molar mass is in g/mol, which makes mg and mmol line up directly.
Worked example: 1 g of sodium chloride
Mass to moles first, then moles to charge:
- 1 Identify the substance and its molar mass. Sodium chloride, NaCl, has a molar mass of 58.443 g/mol.
- 2 Convert the mass to millimoles. 1000 mg ÷ 58.443 = 17.11 mmol of NaCl.
- 3 Note the valence. Sodium carries a single positive charge, so the valence is 1.
- 4 Multiply to get milliequivalents. 17.11 mmol × 1 = 17.11 mEq of sodium — and the same 17.11 mEq of chloride.
- 5 Check a divalent ion for contrast. One gram of calcium is 1000 ÷ 40.078 = 24.95 mmol, but 49.90 mEq, because calcium carries two charges.
Molar mass, valence, and what one gram provides
Molar masses from standard atomic weights. Milliequivalents = millimoles × valence.
| Substance | Molar mass (g/mol) | Valence | 1 g = mmol | 1 g = mEq |
|---|---|---|---|---|
| Sodium, Na⁺ | 22.99 | 1 | 43.50 | 43.50 |
| Potassium, K⁺ | 39.098 | 1 | 25.58 | 25.58 |
| Calcium, Ca²⁺ | 40.078 | 2 | 24.95 | 49.90 |
| Magnesium, Mg²⁺ | 24.305 | 2 | 41.14 | 82.29 |
| Chloride, Cl⁻ | 35.453 | 1 | 28.21 | 28.21 |
| Bicarbonate, HCO₃⁻ | 61.016 | 1 | 16.39 | 16.39 |
| Sodium chloride, NaCl | 58.443 | 1 | 17.11 | 17.11 |
| Potassium chloride, KCl | 74.551 | 1 | 13.41 | 13.41 |
Where this comes up in practice
Potassium replacement is the most common case: an order for 20 mEq of potassium chloride corresponds to about 1491 mg of KCl, and the ampoule or tablet may be labelled either way. Sodium is the next — dietary advice given in grams of salt has to be reconciled with intake measured in millimoles of sodium, and the salt is only about 39% sodium by mass.
Two cautions are worth carrying. Anything with a variable charge, phosphate above all, cannot be converted with a single valence, because the proportion of each form shifts with pH — phosphate is conventionally ordered in millimoles for exactly that reason. And a laboratory reporting mmol/L for a divalent ion such as calcium is not reporting mEq/L; the two differ by a factor of two.
This tool is a study aid for practising conversions. Electrolyte replacement must be prescribed and verified by a qualified professional against the product label.