Corrected calcium calculator (mg/dL)
Adjust serum calcium for a low albumin.
What this calculator is for
About half of the calcium in blood is bound to albumin. When albumin is low, as in liver disease, nephrotic syndrome, malnutrition, sepsis or after surgery, total calcium reads low even though the active, ionised calcium may be normal. Correcting for albumin stops you treating a number rather than the patient.
The formula
- In mg/dL: corrected calcium = measured calcium + 0.8 × (4.0 − albumin in g/dL).
- In mmol/L: corrected calcium = measured calcium + 0.02 × (40 − albumin in g/L).
To convert calcium, multiply mmol/L by 4 to get mg/dL (more exactly 4.008). The normal range is about 8.5–10.5 mg/dL (2.2–2.6 mmol/L); use your lab's range.
Interpreting the result
- Below about 8.5 mg/dL (2.2 mmol/L): true hypocalcaemia. Check magnesium, phosphate, vitamin D and PTH. Look for tingling, cramps, Chvostek's or Trousseau's sign and a long QT. Symptomatic or severe cases need IV calcium gluconate with ECG monitoring.
- Above about 10.5 mg/dL (2.6 mmol/L): hypercalcaemia. Above 12 mg/dL (3.0 mmol/L) is moderate and above 14 mg/dL (3.5 mmol/L) severe. The usual causes are primary hyperparathyroidism and cancer; start with IV fluids and senior advice.
Worked example
A patient with cirrhosis has calcium 7.6 mg/dL and albumin 2.0 g/dL. Corrected calcium = 7.6 + 0.8 × 2.0 = 9.2 mg/dL, which is normal. No calcium is needed.
Limitations and pitfalls
- The formula is an approximation and is unreliable in critical illness, severe hypoalbuminaemia and advanced kidney disease. Ionised calcium on a blood gas analyser is better when you have it.
- Alkalosis (for example from hyperventilation) lowers ionised calcium without changing total calcium.
- Low magnesium makes hypocalcaemia hard to correct; replace magnesium first.
- Haemolysed or EDTA-contaminated samples give falsely low calcium. Repeat a surprising result before acting.
- Do not mix units: check whether albumin is in g/dL or g/L.
See electrolytes on the ward, calcium gluconate, QTc calculator and the textbook chapter on calcium and parathyroid hormone.
Frequently asked questions
What is the formula for corrected calcium in mg/dL?
Corrected calcium = measured calcium + 0.8 × (4.0 − albumin in g/dL).
How do I convert calcium from mmol/L to mg/dL?
Multiply by 4 (more exactly 4.008). For example 2.2 mmol/L is about 8.8 mg/dL.
Is ionised calcium better than corrected calcium?
Yes. Ionised calcium measures the active fraction directly and is more reliable in critically ill patients, in kidney disease and when albumin is very low.
Why does low magnesium matter in hypocalcaemia?
Low magnesium reduces PTH release and action, so calcium stays low until magnesium is replaced.
A calculator supports, but does not replace, clinical judgement. Check results before acting on them.