What is a milliequivalent (mEq)?

A milliequivalent is 1/1000 of an equivalent — the amount of an ion that can react with or replace one mole of hydrogen ions. For monovalent ions (Na+, K+, Cl-), 1 mmol = 1 mEq. For divalent ions (Ca2+, Mg2+), 1 mmol = 2 mEq.

How do I convert mg to mEq?

mEq = (mg ÷ molecular weight) × valence. For example, 23 mg of sodium (MW 23, valence 1) = (23 ÷ 23) × 1 = 1 mEq. For calcium (MW 40, valence 2): 20 mg = (20 ÷ 40) × 2 = 1 mEq.

What is the difference between mmol and mEq?

mmol (millimole) measures the amount of substance by particle count; mEq also accounts for ionic charge. For monovalent ions (valence = 1), mEq = mmol. For divalent ions (valence = 2), mEq = 2 × mmol.

What is the displayed ideal ion mOsm?

It is a particle-count estimate for the selected ion alone: 1 mmol of that ion is shown as approximately 1 mOsm of ideal ion particles. It is not total solution osmolarity because it excludes counterions, other solutes, non-ideal behavior, and the full compound dissociation stoichiometry.

Why does phosphate have two options?

Phosphate exists in different ionization states at physiological pH. Monobasic phosphate (H2PO4-) has valence 1; dibasic phosphate (HPO4 2-) has valence 2. The mix at pH 7.4 is roughly 80% dibasic. This calculator provides both forms so you can match your clinical context.

Is this a morphine milligram equivalent (MME) calculator?

No. Chemical milliequivalents (mEq) measure ionic charge. Morphine milligram equivalents (MME) are a separate opioid-dose concept and must not be calculated with this tool.

Can I enter the mass of potassium chloride or calcium gluconate?

No. The mg conversion uses the formula weight of the selected ion, not a salt, hydrate, tablet, or injectable product. First obtain the amount of the selected elemental ion from an authoritative product label.

Milliequivalent Calculator

Convert elemental-ion amounts between mEq, mmol, and mg, with an explicitly idealized ion-particle mOsm estimate. Do not enter the mass of a salt or medication product.

Electrolyte chemistry only. This is not an opioid Morphine Milligram Equivalent (MME) calculator and not a dosing tool. Enter mg only when it is the mass of the selected ion itself—not the mass of a salt, hydrate, tablet, or solution.

Input

Electrolyte / Ion

Input unit

Amount

Amount of the selected ion—not a compound, salt, or concentration.

Results

mEq

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mmol

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Ideal ion mOsm*

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Formula, scope, and sources

mEq = mmol × |charge|; mg = mmol × formula weight. Formula weights below describe the selected ion. They do not describe salts or dosage forms.

*mOsm limitation: the displayed estimate treats one mmol of the selected free ion as one ideal mOsm of ion particles. It excludes counterions, other solutes, incomplete dissociation, and osmotic coefficients, so it is not total solution osmolarity or osmolality.

For education and independent arithmetic checking only. Do not use this page alone to prepare, prescribe, or administer medication.

References: Merck Manual conversion tables and NIST atomic weights.

Quick Reference — Common Electrolytes (per 1 mEq)

Ion	Formula wt (g/mol)	Valence	mg per 1 mEq	mmol per 1 mEq

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Summary

Convert elemental-ion amounts between mEq, mmol, and mg, with an explicitly idealized ion-particle mOsm estimate. Do not enter the mass of a salt or medication product.

How it works

Select the electrolyte or ion from the dropdown.
Choose the input unit: mEq, mmol, mg, or mOsm.
Enter the numeric amount.
The calculator shows all four equivalent values in real time.
Use the copy button next to any result to copy that value to the clipboard.

Use cases

Study the charge relationship between mmol and mEq.
Convert an amount explicitly reported as elemental ion mass.
Cross-check stoichiometric arithmetic against a trusted source.
Compare monovalent and divalent ion conversions.
Pharmacy students learning electrolyte unit relationships.
Cross-check mEq-to-mmol conversions for monovalent vs. divalent ions.

Frequently Asked Questions

Last updated: 2026-09-20 · Reviewed by Nham Vu