Calcium — mmol/L to mg/dL

In short To convert serum calcium from mmol/L to mg/dL, multiply by 4.00781 — so one mmol/L is 4.00781 mg/dL, and one mg/dL is 0.249513 mmol/L. 8.5 mg/dL is 2.12 mmol/L; 10.0 mg/dL is 2.50 mmol/L; 10.5 mg/dL is 2.62 mmol/L. The factor comes from the molar mass, because calcium has an atomic mass of 40.078 g/mol; that is why it differs for every analyte and why a single "lab unit converter" factor does not exist. Calcium is the analyte where a third unit turns up and causes real confusion: mEq/L. For sodium and potassium, mEq/L and mmol/L are identical because each ion carries one charge — for calcium and magnesium they are not, because those ions carry two, so 2.5 mmol/L is 5.0 mEq/L. Divide by four rather than eight and the answer is exactly double. This converts units. It does not interpret results, does not compare your value against a reference interval, and is not a substitute for the clinician who ordered the test — reference intervals differ between laboratories, methods and populations.