Milliequivalents and Millimoles Calculation

The quantities of electrolytes administered to patients are usually expressed in terms of mEq. Weight units such as mg or g are not often used for electrolytes because the electrical properties of ions are best expressed as mEq.

An equivalent (Eq) is the weight of a substance that supplies 1 unit of charge. 

An equivalent weight is the weight, in g, of an atom or radical, divided by the valence of the atom or radical.

A mEq is 1/1000 of an Eq.

The equivalent weight of a compound may be determined by dividing its formula or MW in g by the valence of its largest valence ion.

A mole equals one gram-atomic weight or gram-molecular weight of a substance. 

A millimole equals 1/1000 of a mole.

Milliequivalents Practice Problems

1. Calculate the mEq weight of calcium. [Note—Calcium has a MW of 40.08, and the valence of calcium is 2+.]

Eq weight = 40.08 g/2 = 20.04 g

mEq weight = 20.04 g/1000 = 0.02004 g = 20.04 mg

2. Calculate the quantity, in mEq, of potassium in a 250-mg Penicillin V Potassium Tablet. [Note—Penicillin V potassium has a MW of 388.48 g, there is one potassium atom in the molecule, and the valence of potassium is 1+.]

Eq weight = 388.48 g/1 = 388.48 g

mEq weight = 388.48 g/1000 = 0.38848 g = 388.48 mg

(250 mg/tablet)/(388.48 mg/mEq) = 0.644 mEq of potassium/tablet

3. Calculate the mEq of magnesium and sulfate in a 2-mL dose of 50% Magnesium Sulfate Injection. [Note—Magnesium sulfate (MgSO4 • 7H2O) has a MW of 246.47, and the highest valence ion is magnesium 2+ and sulfate 2−.]

(50 g/100 mL) × (2 mL/dose) = 1 g/dose

Eq weight = 246.47 g/2 = 123.24 g/Eq

(1g/dose)/(123.24 g/Eq) = 0.008114 Eq = 8.114 mEq of both magnesium and sulfate per dose

4. A vial of sodium chloride injection contains 3 mEq/mL of sodium chloride. Calculate the strength, in % w/v, of the injection. [Note—Sodium chloride has a MW of 58.44.]


(0.1753 g/mL) × 100 mL = 17.53 g in 100 mL = 17.53% w/v

5. Calculate the weight of potassium in mmol. [Note—Potassium has a MW of 39.1.]

The weight of 1 mol is 39.1 g and the weight in mmol is:

39.1 g/1000 = 0.0391 g or 39.1 mg

6. Calculate the mmol of penicillin V potassium in a 250-mg Penicillin V Potassium Tablet. [Note—Penicillin V potassium has a MW of 388.48.]

The weight of 1 mol is 388.48 g, and the weight of 1 mmol is:

388.48 g/1000 = 0.38848 g or 388.48 mg

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