Corrected calcium calculator.

How albumin-adjusted calcium is calculated

Adjusted calcium (mmol/L) = total calcium (mmol/L) + 0.02 × (40 - albumin in g/L). In conventional units that is about 0.8 mg/dL added for every 1 g/dL that albumin sits below 4.0, which is the form most people remember: corrected calcium = measured calcium + 0.8 × (4.0 - albumin). The 0.02 version is the "previously published equation" quoted in the James 2008 paper under Sources. The calculator works in mmol/L and g/L internally, converting mg/dL with the factor 4.008 and g/dL by multiplying by 10, so the conventional-unit answer is the same arithmetic with a coefficient of 0.802 rather than exactly 0.8.

Why adjust at all: much of the calcium in blood travels bound to albumin, so a low albumin pulls the total calcium down even when the physiologically active ionised fraction is unchanged. The adjustment tries to estimate what total calcium would be at a normal albumin of 40 g/L (4.0 g/dL). The page also shows the Payne equation exactly as tabulated in the 2023 Phylactou comparison, total calcium + 0.0246 × (40.4 - albumin) in mmol/L, because the original Payne coefficients are not quite the same as the rounded rule of thumb. For this example they differ by 0.3 mg/dL.

Worked example

Total calcium 8.0 mg/dL is 8.0 / 4.008 = 1.996 mmol/L. Albumin 2.5 g/dL is 25 g/L. The adjustment is 0.02 × (40 - 25) = 0.30 mmol/L, so adjusted calcium is 2.296 mmol/L, which is 9.20 mg/dL. The Payne tabulated form gives 1.996 + 0.0246 × (40.4 - 25) = 2.375 mmol/L, or 9.52 mg/dL.

Limits matter more here than for most formulas. James and colleagues found that a locally derived equation differed from the published ones and changed how many low-albumin patients were classed as low, normal or high calcium; they advise each laboratory to derive its own regression. Phylactou and colleagues concluded that adjusting total calcium for albumin did not outperform the unadjusted value in predicting ionised calcium across an unrestricted dataset. This calculator therefore gives no pass or fail label and no reference range: the interval printed on your lab report is the one that applies.