Phosphate Unit Converter
Phosphate Unit Converter
Convert serum phosphate between mg/dL, mmol/L and mg/L — and see why the factor uses the atomic weight of phosphorus rather than that of phosphate.
Phosphate converter
Mass ⇄ molarSerum phosphate 3.5 mg/dL
Formula and conversion factors
mg/dL = mmol/L ÷ 0.322854
mg/L = mg/dL × 10
- 0.322854
- derived from the atomic weight of elemental phosphorus, 30.9738 Da
- not PO₄
- laboratories report inorganic phosphate as elemental phosphorus, so the formula mass of the phosphate ion, near 95 Da, must not be used — it would understate the molar result threefold
- mg/L
- ten times the mg/dL figure, because a decilitre is 100 mL
- age
- the reference interval is markedly higher in children and adolescents than in adults, and falls to the adult range only after skeletal maturity
Worked example
Serum phosphate 3.5 mg/dL
3.5 × 0.322854 = 1.13 mmol/L
3.5 × 10 = 35.0 mg/L
Within the adult interval of 0.80–1.45 mmol/L, which is 2.48–4.49 mg/dL
Serum phosphate across the three units
| mg/dL | mmol/L | mg/L |
|---|---|---|
| 1.00 — severe hypophosphataemia | 0.32 | 10.0 |
| 2.48 — typical lower limit | 0.80 | 24.8 |
| 3.50 | 1.13 | 35.0 |
| 4.49 — typical upper limit | 1.45 | 44.9 |
| 6.00 | 1.94 | 60.0 |
| 8.00 | 2.58 | 80.0 |
Why the elemental phosphorus convention matters
| Species | Mass used | mg/dL → mmol/L factor | |
|---|---|---|---|
| What laboratories actually report | Elemental phosphorus (P) | 30.97 Da | 0.322854 — correct |
| A plausible but wrong assumption | Phosphate ion (PO₄³⁻) | 94.97 Da | 0.105 — understates the molar result about threefold |
| Another wrong assumption | Hydrogen phosphate (HPO₄²⁻) | 95.98 Da | 0.104 — same error |
Phosphorus, not phosphate — and why the distinction is not pedantry
Serum phosphate is reported in mg/dL in the United States and in mmol/L elsewhere, with a factor of 0.322854 between them. A phosphate of 3.5 mg/dL is 1.13 mmol/L. That factor is derived from the atomic weight of elemental phosphorus, 30.9738 daltons, and not from the mass of the phosphate ion, and the reason is a genuine and frequent source of confusion rather than a technicality.
Although the test is universally called phosphate, what the laboratory measures and reports is the phosphorus content of the inorganic phosphate in the sample. Inorganic phosphate in serum is not one species: it is a pH-dependent mixture of hydrogen phosphate and dihydrogen phosphate, so there is no single molecular mass that could be used. Expressing the result as elemental phosphorus removes that ambiguity. Anyone who derives a conversion factor from the formula mass of PO₄, near 95 daltons, will produce a molar value roughly a third of the true one, and the error is silent because the number still looks plausible.
Interpretation depends heavily on age and on renal function. Children and adolescents run substantially higher phosphate concentrations than adults throughout skeletal growth, and applying an adult interval to a child manufactures abnormal results. In adults the commonest cause of a high phosphate is reduced renal excretion in chronic kidney disease, where phosphate is managed as part of mineral and bone disorder alongside calcium, PTH and vitamin D rather than as an isolated number.
A low phosphate is often the more urgent finding and is regularly missed. Refeeding after starvation, the treatment of diabetic ketoacidosis, alcohol misuse, phosphate binders and renal phosphate wasting all drive it down, and severe hypophosphataemia below roughly 0.32 mmol/L causes respiratory muscle weakness, rhabdomyolysis and haemolysis. Falsely high results are also worth excluding: haemolysis and delayed separation of the sample release intracellular phosphate and can prompt treatment of an artefact.
Frequently asked questions
How do I convert phosphate from mg/dL to mmol/L?
Multiply by 0.322854. A phosphate of 3.5 mg/dL is 1.13 mmol/L. To go the other way, divide the mmol/L figure by 0.322854.
Why is the factor based on phosphorus and not on phosphate?
Laboratories report the phosphorus content of inorganic phosphate, so the atomic weight of P, 30.97 Da, is the right mass to use. Serum phosphate is a pH-dependent mixture of hydrogen and dihydrogen phosphate with no single molecular mass, which is why the phosphorus convention exists. Using the formula mass of PO₄, near 95 Da, understates the molar result about threefold.
Is a phosphate of 5 mg/dL abnormal?
In an adult it is above the usual interval and suggests reduced renal excretion or a spurious result from haemolysis. In a growing child or adolescent it can be entirely normal, because the paediatric interval runs considerably higher. The age-appropriate range has to be applied before the number means anything.
When is a low phosphate urgent?
Below roughly 0.32 mmol/L, about 1.0 mg/dL, hypophosphataemia causes respiratory muscle weakness, rhabdomyolysis and haemolysis and needs replacement rather than observation. Refeeding after starvation and the treatment of diabetic ketoacidosis are the classic settings, and phosphate should be monitored through both.
Can a phosphate result be falsely high?
Yes. Haemolysis and delayed separation of the sample release phosphate from red cells and raise the measured value. A high phosphate that does not fit the clinical picture or the renal function should be repeated on a properly handled sample.
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References
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD-MBD Update Work Group. KDIGO 2017 clinical practice guideline update for the diagnosis, evaluation, prevention, and treatment of chronic kidney disease–mineral and bone disorder (CKD-MBD). Kidney Int Suppl. 2017;7(1):1–59.
- Felsenfeld AJ, Levine BS. Approach to treatment of hypophosphatemia. Am J Kidney Dis. 2012;60(4):655–661.
- Wagner CA. The basics of phosphate metabolism. Nephrol Dial Transplant. 2024;39(2):190–201.