This corrected calcium calculator adjusts total serum calcium for albumin — because roughly 40% of calcium rides on albumin, a low albumin makes total calcium look falsely low.

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Corrected Ca = Measured Ca + 0.8 × (4.0 − Albumin)

Payne’s formula, used at bedsides since 1973. It belongs to the same electrolyte-interpretation family as our Anion Gap Calculator — which carries its own albumin correction for the same reason. Part of our Health Calculators collection.

Worked Example (Verified)

  • Calcium 8.0 mg/dL, albumin 2.0 g/dL (defaults): 8.0 + 0.8 × 2.0 = 9.6 mg/dL — a “low-normal” value that is actually mid-normal; treating the raw 8.0 as hypocalcemia would be an albumin illusion
  • The mirror case: calcium 10.2 with albumin 5.0 corrects to 9.4 — dehydration’s high albumin inflating the total
  • Each 1 g/dL of albumin moves the correction by 0.8 mg/dL — larger than many lab reference-range widths, which is why the correction is routine, not exotic
  • SI users: mg/dL ÷ 4 ≈ mmol/L for calcium (9.6 → 2.40 mmol/L)

When the Formula Struggles

  • Critical illness and CKD: binding shifts with pH and anions; ionized calcium decides.
  • Multiple myeloma: paraproteins bind calcium (pseudohypercalcemia) — total high, ionized normal.
  • Severe acid-base disturbance changes ionized fraction without changing total at all.
  • Use the albumin drawn at the same time as the calcium; stale albumin corrects fiction.

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Frequently Asked Questions

How do you calculate corrected calcium?

Corrected Ca (mg/dL) = measured total Ca + 0.8 × (4.0 − albumin in g/dL). For calcium 8.0 with albumin 2.0: 8.0 + 1.6 = 9.6 mg/dL. The formula (Payne, 1973) estimates what the total calcium would read at a normal albumin of 4.0.

Why does albumin change calcium results?

About 40% of blood calcium is bound to albumin and biologically inactive; ~50% is free (ionized, active) and ~10% is complexed. Low albumin lowers the bound pool and therefore the total — even when the active ionized fraction is perfectly normal.

Corrected calcium vs ionized calcium — which is better?

Ionized calcium is the direct, definitive measurement and wins whenever available and the stakes are real (ICU, CKD, myeloma, symptoms). The corrected value is a screening estimate for routine panels where ionized testing isn’t drawn — good arithmetic, honest limits.

What is a normal corrected calcium?

Reference ranges cluster around 8.5–10.5 mg/dL (2.12–2.62 mmol/L) for total/corrected calcium, with lab variation. Values outside after correction deserve the standard workup (PTH, vitamin D, phosphate, magnesium) rather than repeat correction gymnastics.

Does the formula work in SI units?

Yes, with converted constants: corrected Ca (mmol/L) = measured + 0.02 × (40 − albumin in g/L). This page takes mg/dL and g/dL and prints the mmol/L equivalent alongside.

Is this corrected calcium calculator free?

Yes — free, instant, no sign-up. A clinical estimation aid, not a substitute for laboratory judgment.