Urine Magnesium Unit Converter
Urine Magnesium Unit Converter
In hypomagnesaemia a urine magnesium shows whether the kidney is the leak, but only alongside the serum value. Convert mg/dL, mg/L, mmol/L and mEq/L.
Urine Magnesium converter
Divalent — mEq/L = 2 × mmol/LUrine magnesium 7 mg/dL on a spot sample, serum magnesium 0.52 mmol/L
Formula and conversion factors
mmol/24 h = mg/24 h × 0.0411438
mEq = 2 × mmol (magnesium is divalent)
- 0.411438
- concentration factor, from the atomic weight of magnesium, 24.305 — IUPAC gives an interval of 24.304 to 24.307 and this is the conventional value inside it
- 0.0411438
- excretion factor, mg/24 h to mmol/24 h
- divalent
- magnesium carries two charges, so mEq is twice mmol — the commonest magnesium reporting error
Worked example
Urine magnesium 7 mg/dL on a spot sample, serum magnesium 0.52 mmol/L
7 × 0.411438 = 2.88 mmol/L
= 5.76 mEq/L, because magnesium is divalent
Over a 1.5 L collection, 105 mg/24 h = 4.32 mmol/24 h
Continuing to excrete that much with a serum magnesium of 0.52 mmol/L is inappropriate — calculate the fractional excretion to confirm renal wasting
Magnesium: the two multipliers, and the valency
| Conversion | Multiplier | Worked |
|---|---|---|
| mg/dL to mmol/L (a concentration) | 0.411438 | 7 mg/dL = 2.88 mmol/L |
| mg/24 h to mmol/24 h (an excretion) | 0.0411438 | 100 mg/24 h = 4.11 mmol/24 h |
| mmol to mEq | 2 | Magnesium is divalent |
| mg/dL to mg/L | 10 | 7 mg/dL = 70 mg/L |
24-hour urine magnesium, and the renal response in hypomagnesaemia
| Source and basis | Figure | Unit |
|---|---|---|
| Mayo MAGU, 24-hour, adults 18 to 83 | 51 – 269 | mg/24 h |
| ARUP, 24-hour collection | 12 – 199 | mg/day |
| Mayo’s interval in millimoles | 2.10 – 11.07 | mmol/24 h |
| Elisaf, extrarenal loss (fractional excretion) | Below 2 | % |
| Elisaf, renal magnesium wasting (fractional excretion) | Above 4 | % |
Is the kidney the leak?
Hypomagnesaemia has two kinds of cause, and a urine magnesium is how you tell them apart. If the gut is the source — diarrhoea, malabsorption, poor intake — the kidney responds by reabsorbing almost all the filtered magnesium, and the urine magnesium falls close to nothing. If the kidney is the source — loop and thiazide diuretics, calcineurin inhibitors, cisplatin, aminoglycosides, proton pump inhibitors, or an inherited tubulopathy — magnesium keeps appearing in the urine despite the low serum level, which is the definition of inappropriate. The serum side is on the serum magnesium unit converter.
That makes the urine magnesium a test that only works in pairs. A 24-hour magnesium of 100 mg means one thing with a serum magnesium of 0.85 mmol/L and the opposite with a serum magnesium of 0.45 mmol/L, and no reference interval can capture that. The formal version of the comparison is the fractional excretion of magnesium, which also corrects for the roughly 30% of serum magnesium that is protein-bound and never filtered: below 2% is the expected renal response in a hypomagnesaemic patient, above 4% is renal wasting.
Two unit hazards sit on this page, and they are of different sizes. Magnesium is divalent, so a milliequivalent figure is twice the millimole figure — the classic magnesium reporting error, and a twofold one. Larger is the concentration-against-excretion confusion: mg/dL becomes mmol/L by multiplying by 0.411438, while mg/24 h becomes mmol/24 h by multiplying by 0.0411438, a tenth of the size. Both derive from the atomic weight 24.305, for which IUPAC publishes an interval of 24.304 to 24.307 rather than a single value.
The published 24-hour intervals are worth seeing side by side, because they disagree badly: Mayo gives 51 to 269 mg/24 h and ARUP 12 to 199 mg/day. Magnesium intake varies widely and there is no set point for excretion, so neither laboratory is wrong — the spread is the real property of the analyte. Use the interval on your own report, and where the question is renal wasting, ask it with a paired sample rather than with the urine figure alone.
Frequently asked questions
Is urine magnesium in mEq the same as mmol?
No. Magnesium is divalent, so one millimole is two milliequivalents. A urine magnesium of 2.88 mmol/L is 5.76 mEq/L, and treating one as the other is a twofold error.
How do I convert a 24-hour urine magnesium from mg to mmol?
Multiply by 0.0411438, one over the atomic weight 24.305. A 24-hour magnesium of 100 mg is 4.11 mmol. The mg/dL to mmol/L factor is ten times larger and does not apply to a daily figure.
What urine magnesium indicates renal magnesium wasting?
The urine figure alone cannot say — it has to be read against the serum magnesium. A fractional excretion of magnesium above 4% in a hypomagnesaemic patient indicates renal wasting, and below 2% indicates an appropriate renal response, so the loss is from the gut.
Why do Mayo and ARUP give such different 24-hour intervals?
Because urinary magnesium excretion tracks intake and there is no physiological set point for it. Mayo gives 51 to 269 mg/24 h and ARUP 12 to 199 mg/day; both are descriptions of their own reference populations’ diets. Use the interval your laboratory printed.
What drugs cause renal magnesium wasting?
Loop and thiazide diuretics, calcineurin inhibitors such as ciclosporin and tacrolimus, cisplatin, aminoglycosides and proton pump inhibitors are the common ones, alongside inherited tubulopathies such as Gitelman syndrome.
Related calculators
References
- Mayo Clinic Laboratories. Magnesium, 24 hour, urine (MAGU) — reference values. Test catalogue, test ID 610768; 2026.
- ARUP Laboratories. Magnesium, urine — reference interval per 24 hours. Laboratory Test Directory 0020477, component test 0020212; 2026.
- Elisaf M, Panteli K, Theodorou J, Siamopoulos KC. Fractional excretion of magnesium in normal subjects and patients with hypomagnesemia. Magnes Res. 1997;10(4):315–320.
- Commission on Isotopic Abundances and Atomic Weights, IUPAC. Standard Atomic Weights 2024. Accessed October 2026.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/