mEq to mmol to mg Converter
mEq to mmol to mg Converter
Convert milliequivalents to millimoles and to milligrams for any ion, using its atomic or molecular weight and its valence — the two numbers the conversion cannot be done without.
mEq, mmol and mg
mEq → mmol → mg20 mEq of potassium: atomic weight 39.098 g/mol, valence 1
Formula, in the order the conversion runs
mg = mmol × molecular weight
so mg = mEq × molecular weight ÷ valence
- mEq
- milliequivalents — a count of CHARGE. One mEq is one millimole of positive or negative charge, whatever ion carries it
- mmol
- millimoles — a count of PARTICLES. Equal to mEq for a monovalent ion and half of it for a divalent one
- valence
- the magnitude of the ionic charge: 1 for sodium, potassium, chloride and bicarbonate; 2 for calcium and magnesium; 3 for aluminium and for phosphate written as PO₄³⁻
- molecular weight
- g/mol for the species you are actually converting. Use the atomic weight for an element and the formula weight for a salt — they are not interchangeable
Worked example
20 mEq of potassium: atomic weight 39.098 g/mol, valence 1
mmol = 20 ÷ 1 = 20 mmol, because potassium is monovalent and mEq and mmol coincide
mg = 20 × 39.098 = 781.96 mg of elemental potassium
The same 20 mmol supplied as potassium chloride is 20 × 74.55 = 1,491 mg of salt
Repeat it for calcium and the first step changes: 20 mEq ÷ 2 = 10 mmol, and the mass halves
Common electrolytes: weight, valence and what one mEq weighs
| Ion | Atomic or formula weight (g/mol) | Valence | 1 mEq is | 1 mmol is |
|---|---|---|---|---|
| Sodium (Na⁺) | 22.990 | 1 | 1 mmol = 22.99 mg | 1 mEq = 22.99 mg |
| Potassium (K⁺) | 39.098 | 1 | 1 mmol = 39.10 mg | 1 mEq = 39.10 mg |
| Chloride (Cl⁻) | 35.45 | 1 | 1 mmol = 35.45 mg | 1 mEq = 35.45 mg |
| Bicarbonate (HCO₃⁻) | 61.02 | 1 | 1 mmol = 61.02 mg | 1 mEq = 61.02 mg |
| Calcium (Ca²⁺) | 40.078 | 2 | 0.5 mmol = 20.04 mg | 2 mEq = 40.08 mg |
| Magnesium (Mg²⁺) | 24.305 | 2 | 0.5 mmol = 12.15 mg | 2 mEq = 24.31 mg |
Salt on the label, element in the patient
| Preparation | Formula weight of the salt | Ion delivered | In mmol and mEq |
|---|---|---|---|
| Potassium chloride 600 mg | 74.55 | About 315 mg potassium | 8.0 mmol = 8.0 mEq K⁺ |
| Sodium chloride 1 g | 58.44 | About 393 mg sodium | 17.1 mmol = 17.1 mEq Na⁺ |
| Calcium carbonate 1,250 mg | 100.09 | About 500 mg calcium | 12.5 mmol = 25.0 mEq Ca²⁺ |
| Magnesium oxide 400 mg | 40.30 | About 241 mg magnesium | 9.9 mmol = 19.9 mEq Mg²⁺ |
Charge, particles and mass are three different counts
Milliequivalents count charge and millimoles count particles, and the two coincide only when each particle carries a single charge. For sodium, potassium, chloride and bicarbonate the numbers are therefore identical, and a generation of laboratory scientists has been able to treat mEq/L and mmol/L as interchangeable without coming to harm. For calcium and magnesium they differ by a factor of two, because one millimole of a divalent ion supplies two milliequivalents of charge. That factor of two is where most conversion errors live, and it is invisible in the arithmetic unless the valence is written down.
Mass is a third count again, and it is the one the label usually gives. A supplement is formulated as a salt, so the milligrams printed on the packet refer to the whole compound rather than the ion of interest: potassium chloride 600 mg supplies about 315 mg of potassium, because chloride accounts for nearly half the formula weight. Calcium carbonate 1,250 mg supplies about 500 mg of calcium for the same reason. Converting from mEq or mmol to milligrams therefore needs a decision before it needs a calculation: the atomic weight if you want the element, the formula weight if you want the salt.
Which unit you meet depends on where you are standing. SI reporting in mmol/L has largely displaced mEq/L on laboratory reports across most of the world, because the mole is the SI unit of amount of substance and reporting particles rather than charge keeps results comparable between analytes. Prescribing has moved more slowly, and electrolyte replacement in the United States and several other countries is still written in mEq. A result in mmol/L and an order in mEq describing the same potassium are common, and for a monovalent ion they agree, which conceals the problem until a divalent ion is involved.
Phosphate is the case that defeats the equivalent altogether. Its charge depends on how many protons it has lost, and at physiological pH it exists as a mixture of the divalent and monovalent forms, so the valence to divide by is neither a whole number nor a constant — it shifts with pH. There is no stable milliequivalent for phosphate, which is why phosphate is prescribed, reported and replaced in millimoles. When an equivalent figure for phosphate does appear, it belongs to a stated pH assumption, and it should not be carried across to a patient whose acid-base state is abnormal.
Frequently asked questions
How do I convert mEq to mmol?
Divide by the valence. For a monovalent ion such as sodium, potassium or chloride the two are numerically equal; for calcium and magnesium 2 mEq is 1 mmol, because each divalent ion carries two charges.
How do I convert mEq to mg?
Divide the mEq by the valence to get millimoles, then multiply by the molecular or atomic weight. For 20 mEq of potassium that is 20 ÷ 1 × 39.098, or about 782 mg of elemental potassium.
Is 600 mg of potassium chloride 600 mg of potassium?
No. The formula weight of potassium chloride is 74.55 and potassium accounts for 39.10 of that, so a 600 mg tablet supplies about 315 mg of potassium, which is 8 mmol or 8 mEq. Supplement labels state the salt, not the element.
Why are laboratory results reported in mmol/L rather than mEq/L?
Because the mole is the SI unit of amount of substance, and counting particles rather than charge keeps results comparable across analytes. Prescribing in several countries still uses mEq, so both units remain in daily use.
Why is phosphate given in mmol rather than mEq?
Because its valence depends on pH. At physiological pH phosphate is a mixture of monovalent and divalent forms, so there is no fixed number of equivalents per millimole. Reporting and prescribing in millimoles avoids the ambiguity.
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References
- Bureau International des Poids et Mesures. The International System of Units (SI Brochure). 9th ed. — definition of the mole and of amount of substance.
- Cohen ER, Cvitaš T, Frey JG, et al., eds. Quantities, Units and Symbols in Physical Chemistry (the IUPAC Green Book). 3rd ed. RSC Publishing — on the equivalent and its deprecation.
- Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier — electrolytes and units of measurement.
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