Formula
mg/dL: corrected calcium = measured calcium + 0.8 × (4.0 − albumin in g/dL). mmol/L: corrected calcium = measured calcium + 0.02 × (40 − albumin in g/L).
This tool applies a commonly used simplified albumin correction to total serum calcium. Some circulating calcium is bound to albumin, so total calcium can change when albumin changes even if the biologically active fraction behaves differently. The corrected value is a formula estimate of total calcium at a reference albumin. It is not an ionized calcium measurement and should never be labeled as one. The calculation does not incorporate blood pH or other calcium-binding substances.
Worked example
For measured calcium 8 mg/dL and albumin 3 g/dL, the result is 8 + 0.8 × (4 − 3) = 8.8 mg/dL. In the SI convention, calcium 2 mmol/L and albumin 30 g/L gives 2 + 0.02 × (40 − 30) = 2.2 mmol/L.
How to use this calculator
- Select the calcium unit and enter the measured total calcium.
- Choose the albumin unit independently, then enter albumin from a compatible sample.
- Select Calculate to apply the convention for the chosen calcium unit.
- Read the limitations and compare only with an appropriate laboratory interval.
When not to use this
Do not rely on this correction in advanced kidney disease, critical illness, or other situations where protein binding and acid–base balance differ from the assumptions behind the equation. It can misclassify calcium status, particularly with low albumin. Direct ionized calcium measurement is more appropriate when an accurate assessment of physiologically active calcium is needed. Laboratory handling also matters for ionized calcium. The calculator cannot decide whether either test is indicated or interpret a person’s clinical condition.
Reference ranges
| Measure | Context |
|---|---|
| Typical total calcium example | 8.5–10.5 mg/dL |
| Approximately equivalent SI interval | 2.12–2.62 mmol/L |
| Correction reference albumin | 4.0 g/dL or 40 g/L |
Ranges are reference only, vary by laboratory and population, and are not diagnostic thresholds. Calcium conversion uses 1 mmol/L = 4.008 mg/dL; albumin uses 1 g/dL = 10 g/L. The requested coefficients 0.8 and 0.02 are rounded conventions, so converting inputs and recalculating can yield a very small difference. Payne’s 1973 paper is a historical source for protein adjustment, but its original equation used a coefficient of 1.0 rather than this simplified 0.8 coefficient. Modern validation studies have questioned routine use of albumin-adjusted calcium.
References
Frequently asked questions
Is corrected calcium the same as ionized calcium?
No. Corrected calcium is an estimate derived from total calcium and albumin. Ionized calcium is a separate measurement of the unbound fraction.
Can I use albumin reported in g/L?
Yes. Select g/L. The calculator converts albumin to the unit needed by the selected calcium equation.
Why can the unit versions differ slightly?
The SI coefficient 0.02 is rounded. It is close to, but not exactly, the conversion of the 0.8 coefficient using 4.008.
Is this the original Payne equation?
No. It is the commonly used simplified correction. The original Payne paper is included as a historical reference with that distinction stated explicitly.
Can this replace a laboratory assessment?
No. Formula correction can be misleading. The result must be assessed with the original measurements and appropriate professional judgment.
Related calculators
Clinical reviewer: [REVIEWER NAME AND CREDENTIALS]
Last reviewed: [DATE — complete after qualified clinical review]. Clinical review is pending.