Corrected Calcium Calculator

The formula

The most widely used albumin-correction equation (with a normal reference albumin of 4.0 g/dL) is:

Corrected Ca = measured Ca + 0.8 × (4.0 − albumin)

Where calcium is in mg/dL and albumin is in g/dL. The factor 0.8 reflects that total serum calcium rises by roughly 0.8 mg/dL for every 1.0 g/dL increase in albumin. When albumin is below 4.0 g/dL, the correction term is positive and the corrected value is higher than the measured value.

Worked example

A patient has a measured total calcium of 8.0 mg/dL and an albumin of 2.0 g/dL:

• Correction term: 0.8 × (4.0 − 2.0) = 0.8 × 2.0 = 1.6 mg/dL
• Corrected Ca = 8.0 + 1.6 = 9.6 mg/dL

So a result that looked low (8.0) is actually within the normal range once the low albumin is accounted for, the hypocalcemia was apparent, not true.

Quick reference

A common adult reference range for total calcium is roughly 8.5 → 10.5 mg/dL, though labs vary.

Pitfalls and limitations

• Use consistent units. This equation assumes calcium in mg/dL and albumin in g/dL. If your lab reports calcium in mmol/L, convert first (1 mmol/L ≈ 4.0 mg/dL).
• It is an estimate, not a measurement. The formula is a population approximation. In critically ill patients it correlates poorly with directly measured ionized calcium, which is the gold standard when accuracy matters.
• Acid–base status matters. Alkalosis increases calcium binding to albumin and lowers the free fraction; the simple correction does not capture pH effects.
• Reference albumin varies. Some sources use 4.0 g/dL, others 4.4 g/dL, which shifts the result. This calculator uses 4.0 g/dL.