Magnesium Unit Converter
Magnesium Unit Converter
Magnesium is divalent: mEq/L is exactly twice mmol/L, and mg/dL is a different scale again. Convert between all three without making the twofold error.
Magnesium converter
Divalent — mEq/L = 2 × mmol/LMagnesium 2.0 mg/dL
Formula and conversion factors
mEq/L = mmol/L × 2 (magnesium is divalent)
mEq/L = mg/dL × 0.8229
mg/dL = mmol/L × 2.4305
- 0.411438
- 10 ÷ 24.305, the atomic mass of magnesium — mg/dL × 10 ÷ MW gives mmol/L
- mEq/L = 2 × mmol/L
- milliequivalents count charge and millimoles count particles. Magnesium carries two charges, so the equivalent figure is exactly double the molar one
- the twofold error
- reading an mEq/L threshold as though it were mmol/L halves it; reporting a mmol/L result as mEq/L doubles it. This is the classic magnesium reporting mistake
- the 2.43-fold error
- mmol/L and mg/dL differ by 2.4305, so mistaking a US mg/dL result for a UK mmol/L one — or the reverse — is a separate and larger slip
Worked example
Magnesium 2.0 mg/dL
2.0 × 0.411438 = 0.82 mmol/L
2.0 × 0.822876 = 1.65 mEq/L — magnesium is divalent, so mEq/L is exactly twice mmol/L
Within the adult reference interval of 0.70–1.05 mmol/L, equivalently 1.40–2.10 mEq/L
The same magnesium in three units
| mg/dL | mmol/L | mEq/L | |
|---|---|---|---|
| Adult reference interval | 1.70 – 2.55 | 0.70 – 1.05 | 1.40 – 2.10 |
| A typical mid-range result | 2.00 | 0.82 | 1.65 |
| Hypomagnesaemia | < 1.70 | < 0.70 | < 1.40 |
| Severe, usually symptomatic | < 1.22 | < 0.50 | < 1.00 |
Magnesium sulfate in pre-eclampsia — where the twofold error does harm
| mmol/L | mEq/L | mg/dL | |
|---|---|---|---|
| Therapeutic range | 2.0 – 3.5 | 4.0 – 7.0 | 4.9 – 8.5 |
| Loss of deep tendon reflexes | about 4 – 5 | about 8 – 10 | about 10 – 12 |
| Respiratory depression | about 5 – 6.5 | about 10 – 13 | about 12 – 16 |
| Cardiac arrest | > 12.5 | > 25 | > 30 |
Divalent, and reported three different ways
Magnesium is the one common electrolyte where mEq/L and mmol/L are not the same number. It carries two positive charges, so each millimole supplies two milliequivalents and mEq/L is exactly twice mmol/L. A magnesium of 0.82 mmol/L is 1.65 mEq/L. The mass unit adds a third scale: 10 ÷ 24.305 gives 0.4114 mmol/L per mg/dL, so the same result is 2.00 mg/dL. Three units, two different multipliers, and no identity anywhere.
That matters because reporting genuinely differs by country. United States laboratories report magnesium in mg/dL and American obstetric and critical care literature often uses mEq/L, while United Kingdom and most European laboratories report mmol/L. A result read across those conventions without converting is wrong by a factor of two, of 2.43, or — mistaking mEq/L for mg/dL — of 1.21. The twofold slip is the classic one, and it runs in the dangerous direction when a therapeutic threshold quoted in milliequivalents is applied to a molar result.
The second difficulty is biological. Less than 1 per cent of body magnesium is in the serum, the rest being in bone and inside cells, and the serum concentration is defended long after stores are depleted. A normal magnesium therefore does not exclude deficiency in a patient on a proton pump inhibitor, a loop or thiazide diuretic, cisplatin or a calcineurin inhibitor, or with alcohol dependence or chronic diarrhoea. Where the history fits, a therapeutic trial of replacement is often more informative than the number.
Hypomagnesaemia is also why other electrolytes will not correct. It drives renal potassium wasting, so hypokalaemia stays refractory until magnesium is replaced, and it suppresses parathyroid hormone release and end-organ response, so hypocalcaemia does the same. Checking magnesium is part of working up either. At the other end, hypermagnesaemia is nearly always iatrogenic, and its early signs — lost deep tendon reflexes, then respiratory depression — are clinical rather than biochemical.
Frequently asked questions
Is mEq/L the same as mmol/L for magnesium?
No, and this is the important difference. Magnesium is divalent, so mEq/L is exactly twice mmol/L. A magnesium of 0.82 mmol/L is 1.65 mEq/L. Treating them as identical, as you can for sodium or potassium, gives a twofold error.
How do I convert magnesium from mg/dL to mmol/L?
Multiply by 0.4114, which is 10 divided by the atomic mass of magnesium, 24.305. A value of 2.0 mg/dL is 0.82 mmol/L. To go the other way, multiply the mmol/L figure by 2.4305.
How do I convert magnesium from mg/dL to mEq/L?
Multiply by 0.8229, which is twice the mmol/L factor because magnesium is divalent. A magnesium of 2.0 mg/dL is 1.65 mEq/L. Equivalently, convert to mmol/L first and then double it.
Does a normal serum magnesium mean magnesium stores are normal?
No. Under 1 per cent of body magnesium is in the serum, and the concentration is defended long after stores fall. In a patient on a proton pump inhibitor, a diuretic, cisplatin or a calcineurin inhibitor, deficiency is common with a normal serum value.
Why will my patient's potassium not correct?
Check magnesium. Hypomagnesaemia drives renal potassium wasting, so replacement fails while it persists. It also suppresses parathyroid hormone release and action, which makes hypocalcaemia equally refractory until magnesium is replaced.
Related calculators
References
- Ayuk J, Gittoes NJL. Contemporary view of the clinical relevance of magnesium homeostasis. Ann Clin Biochem. 2014;51(Pt 2):179–188.
- Lu JF, Nightingale CH. Magnesium sulfate in eclampsia and pre-eclampsia: pharmacokinetic principles. Clin Pharmacokinet. 2000;38(4):305–314.
- 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.