# mEq/L and mmol/L, and why valence matters

| Audience | Knowledge level | Intent | Topic | Mode | Turns |
|---|---|---|---|---|---:|
| Science-literate adult | Intermediate | Medication and numerical reasoning | Electrolyte units | Non-thinking | 2 |

**Highlights:** Unit conversion · Confirms the user's hypothesis · Extends the rule to other ions

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### Turn 1

**User**

> I'm reading a paper that reports a serum potassium of 4.2 mEq/L. I'm more used to mmol/L. Is there a conversion, and are they the same thing?

**Med Advisor**

Yes, they're the same thing — mEq/L and mmol/L are equivalent units for potassium concentration.

**Conversion:**  
1 mEq/L = 1 mmol/L. So a serum potassium of **4.2 mEq/L is exactly 4.2 mmol/L**.

A few practical points:

- **Reference range** is often given in mmol/L (e.g., roughly 3.5–5.0 mmol/L), so 4.2 falls within the typical normal range.
- Potassium is reported in both units across labs, and the conversion is a simple 1:1 ratio.
- If you see a potassium value in both units, they're interchangeable for clinical interpretation.

One caveat: unit labels can occasionally be misread or inconsistently reported, but when both are mEq/L or both mmol/L, the number is identical. If you have the full lab report, the units are usually specified clearly. For any personal or patient-specific interpretation, a clinician or lab report should be consulted.

### Turn 2

**User**

> That makes sense — is the 1:1 ratio because potassium is monovalent, so mEq equals mmol times valence? Would that mean a divalent ion like calcium or magnesium wouldn't be a 1:1 conversion?

**Med Advisor**

Yes — that's exactly right.

**Why the 1:1 ratio:**  
Potassium has a valence of +1. The formula for milliequivalents is:

**mEq = mmol × valence**

So for potassium:

**mEq = mmol × 1**  
→ mEq/L = mmol/L

**For divalent ions (valence 2):**  
Calcium and magnesium are both +2, so:

**mEq = mmol × 2**

Which means:

**1 mmol/L of calcium = 2 mEq/L**  
**1 mmol/L of magnesium = 2 mEq/L**

So calcium and magnesium are **not** a 1:1 conversion — their mEq values are double their mmol values. That's why calcium and magnesium are often reported in mg/dL or mmol/L rather than mEq/L, and why a lab report's unit label matters for interpretation.

**Quick reference:**
- Potassium: 1:1 (monovalent)
- Calcium: 1 mmol = 2 mEq
- Magnesium: 1 mmol = 2 mEq

This distinction is important when comparing values across labs or units, especially in clinical settings where different units are used. For any specific lab report or patient interpretation, a clinician or the lab's documentation should guide the meaning.

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*Illustrative examples, selected to show response style. The model's replies are unedited. Not medical advice.*