Renal & electrolytes
Corrected calcium calculator
Total calcium lies when the albumin is low — one correction shows what the ionised fraction is actually doing.
How to read the result
Corrected calcium (mmol/L) falls into these published bands.
A malnourished inpatient with a total calcium of 2.10 mmol/L and a serum albumin of 30 g/L.
Corrected calcium 2.30 mmol/L — normal
The borderline-low total is an albumin artefact: corrected, the value sits comfortably within the 2.10–2.55 mmol/L reference range, and no calcium replacement is warranted on this number alone.
Who this is for
For clinicians interpreting an abnormal calcium alongside an abnormal albumin — malnutrition, chronic disease, critical illness, oncology wards. Roughly 40% of circulating calcium is protein-bound, so a low albumin drags the total down without touching the physiologically active ionised fraction; the Payne correction restores the comparison to the 2.10–2.55 mmol/L reference range.
How it works
What it calculates
Serum calcium corrected for albumin: roughly 40% of circulating calcium is protein-bound, so hypoalbuminaemia lowers total calcium without changing the physiologically active ionised fraction.
When to use it
Any abnormal total calcium in a patient with an abnormal albumin — common in malnutrition, chronic disease, and critical illness.
Limitations
The correction is an approximation; at extremes of albumin or pH, measure ionised calcium on a blood gas instead. Reference range for corrected calcium: 2.10–2.55 mmol/L.
Built on published evidence
- Corrected Ca = total Ca + 0.02 × (40 − albumin). Reference: Payne et al., BMJ 1973
Codification last reviewed 2026-08-28, and checked against published reference values in automated tests on every release — see the full evidence ledger.
Questions clinicians ask
Which formula does the correction use?
The Payne formula (BMJ 1973): corrected calcium = total calcium + 0.02 × (40 − albumin), with calcium in mmol/L and albumin in g/L — the standard correction on South African laboratory reports.
When should I measure ionised calcium instead?
At extremes of albumin or pH the correction becomes unreliable — measure ionised calcium on a blood gas instead. The correction is an approximation, not a substitute for the direct measurement when the decision is close.
How is hypercalcaemia graded?
Mild is 2.56–3.00 mmol/L (assess the cause; usually not an emergency in itself), moderate is 3.00–3.50 (assess symptoms, hydrate, investigate urgently), and above 3.50 mmol/L is severe hypercalcaemia requiring emergency management.
Why does a low albumin lower the total calcium?
About 40% of circulating calcium travels bound to albumin, so hypoalbuminaemia lowers the total calcium the lab measures while the free, physiologically active ionised fraction stays unchanged — the correction estimates what the total would be at a normal albumin.
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