Urine Phosphate Unit Converter
Urine Phosphate Unit Converter
Urinary phosphate excretion equals phosphate absorbed, so a reference interval is mostly a statement about diet. Convert mg/dL, mg/L and mmol/L, concentration or daily.
Urine Phosphate converter
Mass ⇄ molarUrine phosphate 45 mg/dL on a spot sample
Formula and conversion factors
mmol/24 h = mg/24 h × 0.0322854
mg/24 h = concentration in mg/L × collection volume in litres
- 30.9738
- the atomic weight of phosphorus. Laboratories report the element, not the phosphate ion, so the ion’s mass is never used
- 0.322854
- concentration factor, mg/dL to mmol/L
- 0.0322854
- excretion factor, mg/24 h to mmol/24 h — a tenth of the concentration factor, which also converts decilitres to litres
Worked example
Urine phosphate 45 mg/dL on a spot sample
45 × 0.322854 = 14.53 mmol/L
= 450 mg/L of elemental phosphorus
Over a 1.5 L collection that is 675 mg/24 h, which is 21.79 mmol/24 h — inside Mayo's 226 to 1,797 mg/24 h
Which says almost nothing on its own: pair it with the serum phosphate and calculate TmP/GFR or the fractional excretion
Phosphate: the two multipliers
| Conversion | Multiplier | Worked |
|---|---|---|
| mg/dL to mmol/L (a concentration) | 0.322854 | 45 mg/dL = 14.53 mmol/L |
| mg/24 h to mmol/24 h (an excretion) | 0.0322854 | 700 mg/24 h = 22.60 mmol/24 h |
| mg/dL to mg/L | 10 | 45 mg/dL = 450 mg/L |
What a urine phosphate is read against
| Source and basis | Figure | Unit |
|---|---|---|
| Mayo POU, 24-hour, adults 18 and over | 226 – 1,797 | mg/24 h |
| The same Mayo interval in millimoles | 7.30 – 58.02 | mmol/24 h |
| ARUP, phosphorus urine | No 24-hour interval; a phosphorus/creatinine ratio instead | mg/g |
| Renal phosphate wasting, by TmP/GFR | Below 2.5 | mg/dL, serum-paired |
| Renal phosphate wasting, by fractional excretion | Above 20 | %, serum-paired |
Excretion equals intake, and why that spoils the reference interval
At steady state, urinary phosphate excretion equals phosphate absorbed from the gut. That single fact explains almost everything about this test. It means a 24-hour urine phosphate is largely a measure of diet — Mayo’s adult interval runs from 226 to 1,797 mg/24 h, an eightfold span — and it means the number is close to meaningless without the serum phosphate beside it. A low urine phosphate with a low serum phosphate is an appropriately conserving kidney; the same low urine phosphate with a normal serum phosphate is someone who ate very little phosphate. The serum phosphate unit converter covers the blood side.
The first unit trap is shared with the serum page: laboratories report elemental phosphorus, not the phosphate ion, so the conversion uses the atomic weight 30.9738 and the concentration factor is 0.322854. The second trap is specific to urine. Converting a daily excretion, mg/24 h to mmol/24 h, uses 0.0322854 instead — a tenth of the size, because the concentration factor also carries decilitres to litres. Using the wrong one makes a perfectly ordinary 700 mg/24 h read as 226 mmol/24 h instead of 22.6.
What actually diagnoses renal phosphate wasting is not the urine phosphate at all but a paired calculation. The TmP/GFR gives the renal phosphate threshold through the Walton-Bijvoet nomogram and is the most sensitive of the available tests; the fractional excretion of phosphate is the quicker version, where above 20% with a low serum phosphate indicates an inappropriate leak and below 5% is the expected conservation response. Both need urine and serum phosphate and creatinine from around the same time.
It is worth noting that ARUP declines to publish a 24-hour reference interval for urine phosphorus at all, reporting an age- and sex-stratified phosphorus-to-creatinine ratio instead. Where two reference laboratories differ that far on whether a reference interval exists, the sensible reading is that the unqualified 24-hour number is not the measurement anyone should be acting on. And as with every timed collection, the daily figure needs the recorded volume: a concentration cannot supply it.
Frequently asked questions
Is urine phosphate reported as phosphate or as phosphorus?
As elemental phosphorus, which is why the conversion uses the atomic weight 30.9738 and the concentration factor is 0.322854. The mass of the phosphate ion is not used anywhere in clinical reporting of this test.
How do I convert a 24-hour urine phosphate from mg to mmol?
Multiply by 0.0322854. A 24-hour phosphate of 700 mg is 22.60 mmol. The mg/dL to mmol/L factor is ten times larger and must not be used on a daily figure.
What is a normal 24-hour urine phosphate?
Mayo gives 226 to 1,797 mg/24 h for adults, and the width is the point: at steady state the kidney excretes what the gut absorbed, so the interval describes diet. ARUP publishes no 24-hour interval at all, reporting a phosphorus-to-creatinine ratio instead.
How is renal phosphate wasting diagnosed then?
With a paired serum and urine sample, not with the urine phosphate alone. TmP/GFR is the most sensitive test; a fractional excretion of phosphate above 20% alongside a low serum phosphate indicates an inappropriate renal leak, while below 5% is the expected conservation response.
Can a spot phosphate concentration give the daily excretion?
No. Daily excretion is concentration multiplied by the collection volume, and the volume has to be measured. The same 45 mg/dL is 675 mg over 1.5 litres and 1,125 mg over 2.5 litres.
Related calculators
References
- Mayo Clinic Laboratories. Phosphorus, 24 hour, urine (POU) — reference values. Test catalogue, test ID 610832; 2026.
- ARUP Laboratories. Phosphorus, urine — phosphorus/creatinine reference intervals. Laboratory Test Directory 0020478, component test 0020214; 2026.
- Payne RB. Renal tubular reabsorption of phosphate (TmP/GFR): indications and interpretation. Ann Clin Biochem. 1998;35(2):201–206.
- Burtis CA, Ashwood ER, Bruns DE, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/