Potassium Unit Converter
Potassium Unit Converter
Convert potassium between mmol/L, mEq/L and mg/dL. Potassium is monovalent, so mmol/L and mEq/L are the same number — the real risk on this test is the sample, not the arithmetic.
Potassium converter
Monovalent — mmol/L = mEq/LPotassium 4.1 mmol/L
Formula and conversion factors
mg/dL = mmol/L × 3.910
mEq/L = mmol/L (potassium is monovalent)
- 0.255766
- 10 ÷ 39.10, the atomic mass of potassium — mg/dL × 10 ÷ MW gives mmol/L
- mEq/L
- potassium carries a single charge, so millimoles and milliequivalents are the same number
- serum against plasma
- serum potassium runs about 0.2–0.4 mmol/L higher than plasma because platelets release potassium as the sample clots
- why mg/dL is offered
- serum potassium is not reported in mg/dL; the unit appears in dietary figures and older literature
Worked example
Potassium 4.1 mmol/L
4.1 mmol/L — the unit almost every laboratory reports
4.1 mmol/L = 4.1 mEq/L — potassium is monovalent, so the two are the same number
4.1 ÷ 0.255766 = 16.0 mg/dL, a mass unit seen on supplement labels rather than on a serum report
Within the adult reference interval of 3.5–5.0 mmol/L
Thresholds across the units
| mmol/L | mEq/L | mg/dL | |
|---|---|---|---|
| Adult reference interval | 3.5 – 5.0 | 3.5 – 5.0 | 13.7 – 19.5 |
| Severe hypokalaemia | < 2.5 | < 2.5 | < 9.8 |
| Hyperkalaemia — treat | > 6.0 | > 6.0 | > 23.5 |
| Severe hyperkalaemia — emergency | > 6.5 | > 6.5 | > 25.4 |
Spurious hyperkalaemia — what to look for
| Cause | Mechanism | Clue |
|---|---|---|
| Haemolysis in the sample | Potassium leaks from red cells | Haemolysis index flagged; repeat with a clean venepuncture |
| Fist clenching or a tight tourniquet | Local muscle releases potassium | Isolated high result in a well patient |
| Delayed separation or a cold sample | Membrane pumps fail and potassium leaks out | Sample sent from a remote site or held overnight |
| Marked leucocytosis or thrombocytosis | Cells release potassium as the sample clots | Serum high, plasma normal |
| EDTA carry-over from a full blood count tube | Order-of-draw error | Very high potassium with a very low calcium |
The number is easy; the sample is not
Potassium is reported in mmol/L in the United Kingdom and in mEq/L in the United States, and the two are the same number. Potassium carries a single positive charge, so one millimole is one milliequivalent and no conversion applies. The mg/dL column converts by 10 ÷ 39.10, the atomic mass, or 0.2558 mmol/L per mg/dL; serum potassium is never reported that way in clinical practice, and the unit is offered for dietary figures and older literature.
Almost every difficulty with a potassium result is pre-analytical. Haemolysis releases potassium from red cells, a tight tourniquet or a clenched fist releases it from muscle, a sample left unspun or kept cold leaks it across failing membranes, and carry-over from an EDTA tube produces a spectacular potassium alongside an unbelievable calcium. Serum also runs 0.2 to 0.4 mmol/L above plasma, because platelets release potassium as the clot forms — a difference that becomes large in marked thrombocytosis.
The decision that follows is therefore not arithmetic. In a well patient with a normal electrocardiogram, a raised potassium should be repeated before anything else happens. In an unwell patient, or one with a rate of rise, an arrhythmia or characteristic electrocardiographic change, treatment starts while the repeat is running: calcium to stabilise the myocardium, insulin with dextrose and a beta-2 agonist to shift potassium into cells, and then something that actually removes it.
Hypokalaemia has its own trap. Potassium replacement fails while magnesium is low, because magnesium depletion drives renal potassium wasting through the ROMK channel and the loss simply continues. Checking and replacing magnesium is part of treating hypokalaemia, not an afterthought. Beyond that, the useful next tests are acid-base status and a urinary potassium or transtubular gradient, which separate renal loss from gastrointestinal loss and from transcellular shift.
Frequently asked questions
Is mmol/L the same as mEq/L for potassium?
Yes. Potassium is monovalent, so a millimole carries one milliequivalent of charge and the two figures are identical. A potassium of 4.1 mmol/L is 4.1 mEq/L, and no conversion should be applied.
How do I convert potassium from mg/dL to mmol/L?
Multiply by 0.2558, which is 10 divided by the atomic mass of potassium, 39.10. A value of 16 mg/dL is 4.1 mmol/L. To go the other way, multiply the mmol/L figure by 3.910.
Why is serum potassium higher than plasma potassium?
Platelets release potassium as the sample clots, so serum runs about 0.2 to 0.4 mmol/L above plasma from the same patient. The difference widens in marked thrombocytosis, and the reference interval should match the sample type.
What causes a falsely high potassium?
Haemolysis, a clenched fist or tight tourniquet, delayed separation or a cold sample, marked leucocytosis or thrombocytosis, and EDTA carry-over from a full blood count tube. A very high potassium with a very low calcium is EDTA contamination until proved otherwise.
Why is my patient's hypokalaemia not correcting?
Check magnesium. Magnesium depletion drives renal potassium wasting, and replacement fails while it persists. Correcting magnesium is part of treating hypokalaemia rather than a separate issue to address later.
Related calculators
References
- Alfonzo A, Harrison A, Baines R, et al. Clinical Practice Guidelines: Treatment of Acute Hyperkalaemia in Adults. UK Kidney Association; 2020.
- Palmer BF, Clegg DJ. Diagnosis and treatment of hyperkalemia. Cleve Clin J Med. 2017;84(12):934–942.
- Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.