Why electrolytes are measured in charge, not mass
An equivalent is one mole of charge. A milliequivalent is a thousandth of that. The unit exists because what the body responds to is charge, not mass: sodium and potassium balance chloride and bicarbonate ion for ion, not milligram for milligram, and a cell membrane does not care that potassium weighs nearly twice as much as sodium.
That matters clinically because two salts with the same mass can supply very different numbers of equivalents. One gram of sodium chloride supplies 17.1 mEq of sodium. One gram of potassium chloride supplies 13.4 mEq of potassium. The difference is entirely the molecular weight, and if you replaced one with the other by mass you would be short on ions by a fifth.
Valence multiplies the effect. Calcium carries two charges, so one millimole of calcium is two milliequivalents. That single factor of two is the source of most milliequivalent errors, because it is easy to divide by the molecular weight, get a millimole figure, and stop there.
Three units are in daily use and each has its own home. Milligrams appear on the product label and the invoice. Milliequivalents appear on the order and on serum chemistry panels for sodium, potassium and chloride. Millimoles appear in SI laboratory results and in phosphate ordering, where valence is not fixed. Converting between them requires exactly two facts: the molecular weight and the valence.
The conversion, term by term
Start with moles. Millimoles = milligrams ÷ molecular weight. This is just the definition of a mole and needs no chemistry beyond the periodic table. Use the molecular weight of the salt as it is supplied, including any water of hydration: magnesium sulfate heptahydrate weighs 246.47, not the 120.37 of the anhydrous salt, and using the wrong one is a factor-of-two error.
Then multiply by the valence. mEq = mmol × z. A monovalent ion supplies one equivalent per mole, so mEq and mmol are numerically the same and the two units get used interchangeably for sodium, potassium and chloride. A divalent ion supplies two, so a millimole of calcium is 2 mEq. Trivalent ions supply three.
The equivalent weight is the mass that supplies one equivalent: MW ÷ z. It is a convenient single number to memorise per salt — 74.55 mg per mEq for KCl, 58.44 for NaCl, 73.5 for calcium chloride dihydrate. Multiply it by the milliequivalents you want and you have the milligrams to weigh.
For solutions, apply the same conversion per millilitre: mEq/mL = (mg/mL × z) ÷ MW. Potassium chloride injection at 2 mEq/mL is 2 × 74.55 = 149.1 mg/mL, which is 14.91% w/v. Multiply by the container volume for the total: a 10 mL vial holds 20 mEq, a 20 mL vial holds 40 mEq.
Percent w/v converts by a factor of ten: a 1% w/v solution is 1 g in 100 mL, that is 10 mg/mL. So 10% calcium chloride is 100 mg/mL, and at MW 147.01 and valence 2 that is 100 × 2 ÷ 147.01 = 1.36 mEq/mL — which is why a 10 mL ampoule of 10% calcium chloride is labelled 13.6 mEq.
Which valence do you want? A salt has a cation and an anion, and they supply equal numbers of equivalents by electroneutrality but different numbers of millimoles. One millimole of calcium chloride supplies 2 mEq of Ca²⁺ and 2 mEq of Cl⁻, but 1 mmol of calcium and 2 mmol of chloride. Enter the valence of the ion you are counting.
Worked example: how many mEq of potassium are in a 20 mEq/L bag, and how much KCl is that?
An order reads: 1 litre of 0.9% sodium chloride with 20 mEq of potassium chloride, over 8 hours. The pharmacy stocks KCl injection at 2 mEq/mL in 10 mL vials. You need the volume to draw and the mass of KCl involved.
- Equivalent weight of KCl. MW 74.55 ÷ valence 1 = 74.55 mg per mEq.
- Mass of KCl in 20 mEq. 20 × 74.55 = 1,491 mg, that is 1.491 g.
- Millimoles. 1,491 ÷ 74.55 = 20 mmol — the same number as the milliequivalents, because potassium is monovalent.
- Volume of the 2 mEq/mL concentrate. 20 ÷ 2 = 10 mL, exactly one vial.
- Check the concentrate. 2 mEq/mL × 74.55 mg/mEq = 149.1 mg/mL, which is 14.91% w/v. ✓
- Final bag concentration. 20 mEq in about 1,010 mL is 19.8 mEq/L, within the 40 mEq/L ceiling normally applied to peripheral infusion.
A divalent case for contrast. An order for 2 g of magnesium sulfate uses the heptahydrate, MW 246.47, valence 2. The equivalent weight is 246.47 ÷ 2 = 123.24 mg/mEq, so 2,000 mg supplies 2,000 ÷ 123.24 = 16.23 mEq of magnesium, or 2,000 ÷ 246.47 = 8.11 mmol. Note how the millimole figure is exactly half the milliequivalent figure — that halving is the valence, and it is the number people drop. If you had used the anhydrous molecular weight of 120.37 instead, you would have calculated 33.2 mEq, more than double the truth.
Reading the result against clinical limits
Milliequivalents are how orders and limits are written, so convert into them before comparing. Peripheral potassium infusion is usually limited to about 10 mEq/hour and 40 mEq/L; central lines allow more under monitoring. A product label in milligrams tells you nothing about those limits until you have converted it.
The salt is not the ion. A gram of sodium chloride is not a gram of sodium. NaCl is 39.3% sodium by mass, so 1 g of the salt is 393 mg of sodium — and 17.1 mEq of each, because the equivalents are what balance. Dietary sodium labels are in milligrams of the element; pharmacy labels are in milligrams of the salt. Read which one you have.
Check the hydration state. Magnesium sulfate is supplied as the heptahydrate, calcium chloride as the dihydrate, sodium acetate anhydrous. The water is part of the weighed mass and part of the molecular weight, and mismatching them produces errors of tens of percent.
Phosphate does not have a fixed valence. At physiological pH, phosphate exists as a mixture of H₂PO₄⁻ and HPO₄²⁻, so its average charge is between 1 and 2 and shifts with pH. That is exactly why phosphate products are labelled in millimoles of phosphorus rather than in milliequivalents, and why potassium phosphate injection states both the mmol of phosphate and the mEq of potassium separately. Do not compute a milliequivalent figure for phosphate.
Serum results and dose orders use different reference frames. A serum potassium of 3.2 mEq/L is a concentration in the patient; a 20 mEq order is an absolute amount being given. The units look alike and the quantities are unrelated.
Molecular weight, valence and mg per mEq for common electrolyte salts
| Salt | Formula | MW (g/mol) | Valence of the cation | mg per mEq | mEq per gram |
|---|---|---|---|---|---|
| Sodium chloride | NaCl | 58.44 | 1 | 58.44 | 17.11 |
| Potassium chloride | KCl | 74.55 | 1 | 74.55 | 13.41 |
| Sodium bicarbonate | NaHCO₃ | 84.01 | 1 | 84.01 | 11.90 |
| Sodium acetate (anhydrous) | C₂H₃NaO₂ | 82.03 | 1 | 82.03 | 12.19 |
| Potassium acetate | C₂H₃KO₂ | 98.14 | 1 | 98.14 | 10.19 |
| Ammonium chloride | NH₄Cl | 53.49 | 1 | 53.49 | 18.70 |
| Calcium chloride dihydrate | CaCl₂·2H₂O | 147.01 | 2 | 73.51 | 13.60 |
| Calcium gluconate | C₁₂H₂₂CaO₁₄ | 430.37 | 2 | 215.19 | 4.65 |
| Magnesium sulfate heptahydrate | MgSO₄·7H₂O | 246.47 | 2 | 123.24 | 8.11 |
| Potassium phosphate monobasic | KH₂PO₄ | 136.09 | 1 (potassium) | 136.09 | 7.35 |
The last row gives the potassium equivalents only. The phosphate itself is quantified in millimoles of phosphorus because its charge varies with pH.
Errors this conversion invites
- Forgetting the valence. Dividing milligrams by molecular weight gives millimoles, not milliequivalents. For calcium and magnesium the two differ by a factor of two.
- Using the anhydrous molecular weight for a hydrated salt. MgSO₄ is 120.37 and MgSO₄·7H₂O is 246.47. The error is more than a factor of two in the wrong direction.
- Confusing the mass of the salt with the mass of the element. One gram of NaCl contains 393 mg of sodium; both correspond to 17.1 mEq.
- Computing milliequivalents for phosphate. Its charge is pH-dependent, which is why phosphate is dosed in millimoles. Read the potassium or sodium content in mEq separately.
- Reading the anion's valence when you want the cation's. One millimole of CaCl₂ is 1 mmol of calcium but 2 mmol of chloride, even though both supply 2 mEq.
- Slipping a factor of ten between % w/v and mg/mL. A 1% w/v solution is 10 mg/mL, not 1.
- Comparing a serum concentration to a dose. A serum potassium in mEq/L and an ordered dose in mEq are different quantities that happen to share a unit.
Where milliequivalents appear in practice
Milliequivalents run through four separate workflows. In IV admixture, additives are ordered in mEq and drawn from concentrates labelled in mEq/mL, which is the conversion this page automates. In parenteral nutrition, every electrolyte is prescribed per litre or per day in mEq, except phosphate in mmol, and compatibility limits — particularly the calcium-phosphate solubility product — are checked in those units. In laboratory interpretation, sodium, potassium, chloride and bicarbonate are reported in mEq/L in the United States and in mmol/L elsewhere, which for these monovalent ions are numerically identical. In oral supplementation, potassium chloride tablets are labelled in mEq while the tablet mass is in milligrams.
The laboratory use connects directly to acid-base work: the anion gap is a milliequivalent balance, subtracting measured anions from measured cations, and it only works because charge, not mass, is what balances. Calcium interpretation needs its own correction for albumin binding, which is the corrected calcium calculation.
On the preparation side, once a concentration is settled, mixing two strengths to reach a third is alligation, and stepping a concentrate down through several stages is a serial dilution. The volume of fluid the electrolytes go into is set separately — for children by the maintenance fluid calculation — and the finished product is then billed on quantity dispensed through a days supply calculation.
Key terms
- Equivalent
- One mole of charge. For a monovalent ion it equals one mole of the ion; for a divalent ion, half a mole.
- Valence (z)
- The magnitude of the charge on an ion: 1 for Na⁺ and Cl⁻, 2 for Ca²⁺ and SO₄²⁻, 3 for citrate³⁻.
- Equivalent weight
- Molecular weight divided by valence — the mass that supplies one equivalent. In milligrams per milliequivalent it is numerically identical.
- Water of hydration
- Water molecules built into a crystalline salt. It contributes to the weighed mass and must be included in the molecular weight used.
- % w/v
- Grams of solute per 100 mL of solution. One percent w/v is 10 mg/mL.
- Osmole
- A mole of osmotically active particles. A millimole of NaCl gives about 2 mOsm because it dissociates into two ions, which is a different count again from either mmol or mEq.