Calcium Unit Converter
Calcium Unit Converter
Convert total calcium between mg/dL, mmol/L, mEq/L and mg/L — and get the divalent step right, because mEq/L is twice the mmol/L figure, not the same as it.
Calcium converter
Mass ⇄ molarTotal calcium 9.5 mg/dL
Formula and conversion factors
mg/dL = mmol/L ÷ 0.249513
mEq/L = mmol/L × 2
mg/L = mg/dL × 10
- 0.249513
- derived from the atomic weight of calcium, 40.078 Da
- × 2
- calcium carries two charges, so one millimole supplies two milliequivalents; mEq/L is double the mmol/L figure and is never equal to it
- mg/L
- ten times the mg/dL figure, because a decilitre is 100 mL
- albumin
- roughly 40% of total calcium is protein-bound, so the total tracks the albumin while the physiologically active ionised fraction does not
Worked example
Total calcium 9.5 mg/dL
9.5 × 0.249513 = 2.37 mmol/L
2.37 × 2 = 4.74 mEq/L — calcium is divalent, so the milliequivalent figure is double the millimole figure
Within the adult interval of 2.20–2.60 mmol/L, which is 8.82–10.42 mg/dL
Total calcium across the four units
| mg/dL | mmol/L | mEq/L | mg/L |
|---|---|---|---|
| 6.00 | 1.50 | 2.99 | 60.0 |
| 8.82 — typical lower limit | 2.20 | 4.40 | 88.2 |
| 9.50 | 2.37 | 4.74 | 95.0 |
| 10.42 — typical upper limit | 2.60 | 5.20 | 104.2 |
| 12.00 | 2.99 | 5.99 | 120.0 |
| 14.00 | 3.49 | 6.99 | 140.0 |
Three different calcium measurements
| Measurement | What it is | Moves with albumin? | When to use it |
|---|---|---|---|
| Total calcium | Protein-bound plus complexed plus ionised | Yes — about 40% is bound, mostly to albumin | Routine screening and monitoring in a patient with normal proteins |
| Albumin-corrected calcium | Total calcium adjusted arithmetically for albumin | Corrected for, but only approximately | A quick adjustment in stable outpatients; unreliable in critical illness, dialysis and myeloma |
| Ionised calcium | The free, physiologically active fraction, measured directly | No | Critical illness, massive transfusion, dialysis, acid-base disturbance, discordant results |
The divalent step, and why a total calcium never stands alone
Total calcium is reported in mg/dL in the United States and in mmol/L almost everywhere else, with a factor of 0.249513 between them taken from the atomic weight of calcium, 40.078 daltons. A calcium of 9.5 mg/dL is 2.37 mmol/L. Two further units appear on reports: mg/L, which is simply ten times the mg/dL figure, and mEq/L, which is where the mistakes happen.
Calcium is divalent. It carries two positive charges, so one millimole of calcium supplies two milliequivalents, and mEq/L is always twice the mmol/L figure. A calcium of 2.37 mmol/L is 4.74 mEq/L. Treating the two as interchangeable, which is easy to do because they look alike and because monovalent ions such as sodium and potassium genuinely are interchangeable between the two units, halves or doubles the result. Magnesium behaves the same way and is misread the same way.
The larger issue is not arithmetic at all. About 40 per cent of circulating calcium is bound to protein, mostly albumin, and only the free ionised fraction is physiologically active. A total calcium therefore falls with the albumin in nephrotic syndrome, chronic liver disease, malnutrition and critical illness, in patients whose ionised calcium is entirely normal. The traditional albumin correction adjusts for this, but it is an approximation derived from a healthy outpatient population and it performs poorly in the critically ill, in dialysis patients and in myeloma.
Where the ionised calcium genuinely matters — intensive care, massive transfusion with citrate, dialysis, a marked acid-base disturbance, or a total and a corrected calcium that disagree with the clinical picture — it should be measured directly on an appropriately handled sample. It has its own reference interval, around 1.15 to 1.30 mmol/L, and it is not a unit conversion of the total. A persistently raised total calcium is then worked up with PTH, which separates primary hyperparathyroidism from the PTH-independent causes.
Frequently asked questions
How do I convert calcium from mg/dL to mmol/L?
Multiply by 0.249513, a factor derived from the atomic weight of calcium, 40.078 Da. A calcium of 9.5 mg/dL is 2.37 mmol/L. To go the other way, divide the mmol/L figure by 0.249513.
Is mEq/L the same as mmol/L for calcium?
No, and this is the commonest error with calcium. Calcium is divalent, so one millimole carries two milliequivalents and mEq/L is exactly twice the mmol/L figure. A calcium of 2.37 mmol/L is 4.74 mEq/L, and magnesium behaves the same way.
Why does my calcium need to be read against the albumin?
About 40% of total calcium is bound to protein, mostly albumin, and only the free ionised fraction is active. A low albumin therefore lowers the total calcium in a patient whose ionised calcium is normal. The corrected calcium calculator makes that adjustment, though it is only an approximation.
Can I convert a total calcium into an ionised calcium?
No. Ionised calcium is a separate direct measurement with its own reference interval of roughly 1.15 to 1.30 mmol/L, and it depends on pH, albumin and complexing anions rather than on the total alone. Where it matters, in critical illness, dialysis or citrate exposure, it has to be measured.
What should be done about a raised calcium?
Confirm it on a repeat sample taken without a prolonged tourniquet, and check the albumin. Then measure PTH: a raised or inappropriately normal PTH indicates primary hyperparathyroidism, while a suppressed PTH points to malignancy or another PTH-independent cause.
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References
- Bilezikian JP, Khan AA, Silverberg SJ, et al. Evaluation and management of primary hyperparathyroidism: summary statement and guidelines from the Fifth International Workshop. J Bone Miner Res. 2022;37(11):2293–2314.
- Payne RB, Little AJ, Williams RB, Milner JR. Interpretation of serum calcium in patients with abnormal serum proteins. Br Med J. 1973;4(5893):643–646.
- Baird GS. Ionized calcium. Clin Chim Acta. 2011;412(9–10):696–701.
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.