mmol/L to mEq/L Conversion Calculator

mmol/L to mEq/L Converter

mmol/L to mEq/L: Why Your Calcium Number Changes and Your Sodium Doesn’t

Here’s the thing that actually confuses people about this conversion, and it’s not what most calculators lead with. Look at your sodium in mmol/L and mEq/L, and it’s the same number both times. Look at your calcium the same way, and suddenly the mEq/L figure is double the mmol/L one. Same lab, same blood draw, two electrolytes behaving completely differently. That’s not a mistake on the report. That’s the whole conversion, and once you see why, the rest of it stops being confusing.

Is mmol/L the Same as mEq/L?

Sometimes, and it depends entirely on which electrolyte you’re looking at. mmol/L counts particles, plain and simple, how many millimoles of a substance are floating around in a liter of blood. mEq/L counts something slightly different: the electrical charge those particles are carrying. For an ion that carries a single charge, sodium or potassium, one particle equals one unit of charge, so the two units land on the same number. For an ion that carries a double charge, calcium or magnesium, each particle is worth two units of charge, so the mEq/L number ends up exactly twice the mmol/L number.

Why Sodium and Potassium Don’t Change, But Calcium Does

This comes down to something called valence, which is just a way of describing how many charges a particular ion carries. Sodium (Na+) and potassium (K+) are both monovalent, one charge each, so converting between mmol/L and mEq/L for them is really just relabeling the same number. Calcium (Ca2+) and magnesium (Mg2+) are divalent, two charges each, so their mEq/L value is always double their mmol/L value. It’s not that the lab measured something different. It’s that mEq/L is asking a different question than mmol/L, and for a two-charge ion, that question has a bigger answer.

The Actual Formula

mEq/L = mmol/L × valence

And going backward:

mmol/L = mEq/L ÷ valence

• Sodium, potassium, chloride, bicarbonate: valence of 1, so the numbers match exactly.
• Calcium, magnesium: valence of 2, so mEq/L is always double mmol/L.
• Phosphate: valence of 3 in most clinical contexts, so mEq/L is triple mmol/L.

Your Calcium Says 2.4 mmol/L. Here’s What That Is in mEq/L

This is the exact scenario that sends people looking for this conversion. A calcium result of 2.4 mmol/L (a normal, healthy value) becomes 4.8 mEq/L once you apply calcium’s valence of 2:

2.4 mmol/L × 2 = 4.8 mEq/L

If you’re used to seeing calcium reported in mEq/L and someone hands you a 2.4, it can look alarmingly low at first glance, until you remember it’s not on the same scale. The number didn’t shrink. The unit changed what it’s counting.

Why Sodium Doesn’t Do This

A sodium result of 140 mmol/L stays exactly 140 in mEq/L, because sodium only carries a single charge:

140 mmol/L × 1 = 140 mEq/L

That’s why sodium and potassium panels can genuinely be read interchangeably between the two units, while calcium and magnesium panels absolutely cannot.

Common Electrolytes, Side by Side

Electrolyte	Valence	Example mmol/L	Converts to mEq/L
Sodium (Na+)	1	140 mmol/L	140 mEq/L
Potassium (K+)	1	4.0 mmol/L	4.0 mEq/L
Chloride (Cl-)	1	100 mmol/L	100 mEq/L
Bicarbonate (HCO3-)	1	24 mmol/L	24 mEq/L
Calcium (Ca2+)	2	2.4 mmol/L	4.8 mEq/L
Magnesium (Mg2+)	2	0.85 mmol/L	1.7 mEq/L

 

Typical Reference Ranges in Both Units

Always defer to the range printed on your own report, since these shift slightly by lab. These are general adult ranges for orientation.

Electrolyte	Normal range (mmol/L)	Normal range (mEq/L)
Sodium	135 – 145	135 – 145
Potassium	3.5 – 5.0	3.5 – 5.0
Chloride	98 – 106	98 – 106
Calcium (total)	2.2 – 2.6	4.4 – 5.2
Magnesium	0.7 – 1.0	1.4 – 2.0

 

Why Does the US Even Use mEq/L?

It’s mostly history. mEq/L took hold in American medicine decades ago because it maps directly onto how electrolytes behave in IV fluids and acid-base balance, where charge, not just particle count, drives what happens in the body. Most of the rest of the world standardized on mmol/L instead, since it’s the SI unit and works cleanly across all of chemistry, not just electrolytes. Neither system is wrong. You just need to know which one you’re looking at before you compare a number against a reference range.

The Short Version

If you’re looking at sodium, potassium, chloride, or bicarbonate, mmol/L and mEq/L are the same number, no math required. If you’re looking at calcium or magnesium, double the mmol/L figure to get mEq/L, or halve the mEq/L figure to get mmol/L. The mismatch isn’t an error on your lab report, it’s just two different ions being measured two different ways.

Want to skip the mental math? Our mmol/L to mEq/L Conversion Calculator handles the valence automatically for sodium, potassium, calcium, magnesium, and other common electrolytes, in both directions.