Milliequivalents from a salt weight

What is this for?

Electrolyte orders are written in milliequivalents but products are formulated by weight. This calculator bridges the two using the molecular weight of the salt and the valence of the ion, in either direction.

How to use it

1. Choose whether you are starting from a weight in milligrams or from an order in milliequivalents.
2. Select the salt. The molecular weight and valence populate automatically; choose "Other" to enter them yourself.
3. Read the answer, and cross-check the millimole figure if the product is labelled that way.

Worked example

A potassium chloride tablet contains 750 mg. How many milliequivalents of potassium does it supply?

Answer: (750 × 1) ÷ 74.5 = 10 mEq. This is why 750 mg KCl tablets are labelled as 10 mEq.

Clinical pearls & pitfalls

• The molecular weight in the denominator is that of the whole salt, not of the ion alone. Using the atomic weight of potassium (39.1) instead of potassium chloride (74.5) nearly doubles the answer.
• Calcium products are a recurring trap: calcium chloride 1 g supplies about 13.6 mEq of calcium while calcium gluconate 1 g supplies only about 4.6 mEq. They are not interchangeable gram for gram.
• Milliequivalents describe charge. When a product is labelled in millimoles — as phosphate salts are — do not convert using a fixed valence.

Assumptions & limitations

• Assumes complete dissociation and a single fixed valence for the ion of interest.
• Does not apply to phosphate, whose charge varies with pH; phosphate is dosed in mmol of phosphorus.
• Uses anhydrous molecular weights unless otherwise indicated. Check whether your product is a hydrate.

References

• American Society of Health-System Pharmacists. ASHP Guidelines on Preventing Medication Errors with Electrolyte Solutions.
• United States Pharmacopeia–National Formulary monographs for electrolyte injections.
• Institute for Safe Medication Practices. ISMP List of High-Alert Medications in Acute Care Settings.

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