What is the conversion factor for calcium from mg/dL to mmol/L?

Divide by 4.008. The molar mass of calcium is 40.08 g/mol, and there are 10 dL per L.

What is the conversion factor for albumin from g/dL to g/L?

Multiply by 10. One g/dL equals 10 g/L.

What happens if I use mg/dL calcium in the Payne equation without converting?

The equation treats it as mmol/L. A calcium of 8.5 mg/dL would be read as 8.5 mmol/L, producing a false hypercalcemia result near 9 mmol/L.

Is a correct unit conversion enough to trust the corrected calcium result?

No. It only confirms arithmetic consistency. The Payne equation remains invalid in ICU, CKD, and acidotic patients regardless of unit accuracy.

Unit-Factor Checks for Calcium and Albumin Formulas

A resident on call sees a corrected calcium of 2.22 mmol/L on their phone and is about to order IV calcium. The result is wrong because someone entered the albumin as 3.5 g/dL and forgot to multiply by 10. The unit-factor checks for calcium and albumin formulas catch this before treatment starts.

A reader can verify proportional unit conversions before applying a formula designed for a specific unit system. You have the Payne equation in front of you. It requires calcium in mmol/L and albumin in g/L. Your lab report gives calcium in mg/dL and albumin in g/dL. You need a reliable check that your conversion is correct before you trust the result.

A Worked Example

Work one concrete set of numbers. A patient has a total calcium of 8.5 mg/dL and an albumin of 3.5 g/dL. The Payne equation is corrected Ca = total Ca + 0.02 × (40 − albumin g/L).

Step one: convert calcium. The IFCC defines the molar mass of calcium as 40.08 g/mol. The conversion from mg/dL to mmol/L is divide by 4.008. 8.5 / 4.008 = 2.12 mmol/L (rounded).

Step two: convert albumin. Multiply g/dL by 10 to get g/L. 3.5 × 10 = 35 g/L.

Step three: apply the equation. corrected Ca = 2.12 + 0.02 × (40 − 35) = 2.12 + 0.02 × 5 = 2.12 + 0.10 = 2.22 mmol/L.

That value falls within a normal reference interval and would not trigger alarm. Now imagine the same numbers without conversion: Ca = 8.5 (as if it were mmol/L) and albumin = 3.5 (as if it were g/L). The equation returns 8.5 + 0.02 × (40 − 3.5) = 8.5 + 0.73 = 9.23 mmol/L. That false result is a life-threatening hypercalcemia reading. No patient with those true values has a calcium of 9.23 mmol/L. The unit conversion is not optional.

Calculation Steps

Use dimensional analysis to verify your conversion factors. For calcium you have a result in mg/dL. You need mmol/L. Write the conversion as a chain:

(mg/dL) × (1 mmol / 40.08 mg) × (10 dL / 1 L) = mmol/L.

The mg cancels. The dL cancels. What remains is mmol/L. That is the unit you need. For albumin: (g/dL) × (10 dL / 1 L) = g/L. The dL cancels, leaving g/L.

If you are working by hand and want a fast sanity check, use a proportion calculator at the point where you need to confirm the multiplier. Enter the lab value and the known conversion factor. If the result does not match your manual calculation, recheck every step before proceeding.

The conversion factor for calcium is exact: 1 mmol/L = 4.008 mg/dL. The factor for albumin is exact: 1 g/dL = 10 g/L. There is no rounding in the definitions. Rounding only happens when you apply them to a real patient value. Round to two decimal places for calcium and to the nearest whole number for albumin in g/L. The Payne equation uses the g/L integer 40 as the normal albumin, not 4.0 g/dL.

Why the Method Works

The failure case is the same every time: mixing unit systems. A user inputs total Ca in mg/dL and albumin in g/dL directly into the equation without conversion. The equation treats the calcium as if it were mmol/L and the albumin as if it were g/L. The result is meaningless and clinically dangerous.

Test your work with a known pair. If total Ca = 10.0 mg/dL and albumin = 4.0 g/dL, the correct conversions are Ca = 2.50 mmol/L and albumin = 40 g/L. The Payne equation then gives corrected Ca = 2.50 mmol/L (since 40 − 40 = 0). If your conversion gives anything else, stop and find the error.

The limit of this approach is that unit checks only verify arithmetic consistency. They do not validate the clinical appropriateness of the Payne equation. The formula is a crude population-level statistical patch derived from a single UK hospital cohort in 1973. It fails in patients with acid-base disturbances, critical illness, or dysproteinemias, precisely the patients whose ionized calcium status matters most. KDIGO 2017 explicitly advises against relying on albumin-adjusted calcium as the primary measure in CKD. A correct unit conversion does not make the clinical output correct.

What goes wrong most often: the user knows the formula but forgets the albumin is in g/L. They see 4.0 on the lab report and plug it in. The equation then subtracts 4 from 40, giving an artificially large correction. The corrected calcium jumps into a range that looks like hypercalcemia, triggering unnecessary interventions. The unit check is the single step that prevents this.