Phosphorus — mg/dL to mmol/L

In short To convert serum phosphorus from mg/dL to mmol/L, multiply by 0.322851 — so one mg/dL is 0.322851 mmol/L, and one mmol/L is 3.0974 mg/dL. 2.5 mg/dL is 0.81 mmol/L; 3.5 mg/dL is 1.13 mmol/L; 4.5 mg/dL is 1.45 mmol/L. The factor comes from the molar mass, because the conversion uses the atomic mass of phosphorus, 30.974 g/mol, not the mass of the phosphate ion; that is why it differs for every analyte and why a single "lab unit converter" factor does not exist. Labs report this as "phosphorus" or as "phosphate" and mean the same measurement — the concentration of phosphorus atoms. The conversion therefore uses phosphorus's atomic mass of 30.974, not the mass of a phosphate ion, and a factor worked out from PO₄ would be about three times wrong. This converts units. It does not interpret results, does not compare your value against a reference interval, and is not a substitute for the clinician who ordered the test — reference intervals differ between laboratories, methods and populations.