Urine Oxalate Unit Converter
Urine Oxalate Unit Converter
Urinary oxalate is the single most informative number in a stone work-up, and it is a 24-hour excretion. Convert mg/24 h, µg/24 h, µmol/24 h and nmol/24 h.
Urine Oxalate converter
Mass ⇄ molar, per 24 hA 24-hour urinary oxalate reported as 40.5 mg, collection complete
The conversion, and the molecular weight behind it
nmol/24 h = µg/24 h × 11.3613
mmol/24 h = mg/24 h × 0.0113613
mg/24 h = µmol/24 h ÷ 11.3613
- 88.018
- the molar mass of the oxalate DIANION, C2O4 2−, which is the species in urine and the mass Mayo and Labcorp convert with
- 90.034
- the molar mass of oxalic ACID — 2.3% larger, and the reason two published conversion factors for the same test exist
- per 24 hours, not per litre
- the collection volume is already inside the number; a concentration in the bottle says mostly how much the patient drank
Worked example
A 24-hour urinary oxalate reported as 40.5 mg, collection complete
40.5 × 11.3613 = 460 µmol/24 h
= 0.46 mmol/24 h, which is exactly the upper limit Mayo prints beside its own 40.5 mg/24 h — the two agree only if the dianion mass is used
At the top of Mayo's interval, inside ARUP's male interval (16–49 mg), and above Labcorp's female interval (4–31 mg)
A concentration, not an excretion: this number says nothing about the oxalate per litre, and nothing about supersaturation without the volume
Oxalate: the two molar masses, and what each does to the factor
| Basis | Molar mass | mg/24 h to µmol/24 h | Who uses it |
|---|---|---|---|
| Oxalate dianion, C2O4 2− | 88.018 | 11.3613 | Mayo (its own mmol and mg limits agree only with this); Labcorp’s SI table, which gives 11.4 |
| Oxalic acid, C2H2O4 | 90.034 | 11.1077 | GlobalRPh’s conversion table; the familiar “0.5 mmol/day is about 45 mg/day” |
| Difference | 2.3% | — | Four times the 0.5% a self-consistency check tolerates, so the choice is not cosmetic |
24-hour urine oxalate: what the sources actually say
| Source | Interval | Unit |
|---|---|---|
| Mayo OXU, 16 years and over | 9.7 – 40.5 | mg/24 h |
| Mayo OXU, the same interval in SI | 0.11 – 0.46 | mmol/24 h |
| ARUP, male 13 and over | 16 – 49 | mg/day |
| ARUP, female 13 and over | 13 – 40 | mg/day |
| ARUP, 0 – 12 years | 7 – 31 | mg/day |
| Labcorp 003970, adult male | 7 – 44 | mg/24 h |
| Labcorp 003970, adult female | 4 – 31 | mg/24 h |
| Hyperoxaluria, commonly quoted threshold | ≥ 40 – 45 | mg/day |
Why oxalate is the number worth getting right
Calcium oxalate is the commonest stone, and of the two ions in it oxalate is the one that moves supersaturation most for a given change. A 20% rise in urinary oxalate does more to the calcium oxalate supersaturation than a 20% rise in urinary calcium, which is why a stone panel that reports calcium carefully and oxalate casually is reporting the wrong one carefully. The companion number on the same collection is urine calcium, and the inhibitor that offsets both is urine citrate.
This is a 24-hour test and the units reflect that: milligrams or micromoles per collection, with no volume in the denominator. That makes the collection itself part of the result. An under-collected bottle produces a low oxalate in a patient excreting normally, and the only way to see it is the creatinine on the same specimen — see the collection adequacy calculator. Converting a spot oxalate concentration into a daily excretion requires the measured volume and nothing else will do.
The arithmetic has a genuine trap that most conversion tables hide. Oxalate can be treated as the dianion, molar mass 88.018, or as oxalic acid, 90.034, and the two give factors 2.3% apart. Mayo publishes its reference interval in both units at once, and the pair it prints reproduces only with the dianion; Labcorp’s own SI table gives 11.4 µmol per milligram, again the dianion. The widely repeated clinical shorthand that 0.5 mmol/day is about 45 mg/day is the oxalic-acid figure. Both are in print, so quoting the basis matters as much as quoting the number.
What moves the result: dietary oxalate and, more than most clinicians expect, dietary calcium — a low-calcium diet raises urinary oxalate, because unbound oxalate in the gut is absorbed. Fat malabsorption after bariatric surgery or in inflammatory bowel disease does the same thing more dramatically, and vitamin C in gram doses is metabolised to oxalate. Marked, sustained elevation in a child, or a stone-forming adult excreting far above 100 mg/24 h, is the pattern that should prompt genotyping for primary hyperoxaluria rather than another dietary review. A full panel is read by the 24-hour urine stone-risk interpreter.
Frequently asked questions
How do I convert urine oxalate from mg/24 h to mmol/24 h?
Multiply by 0.0113613, which is one divided by the molar mass of the oxalate dianion, 88.018. A 24-hour oxalate of 40.5 mg is 0.46 mmol, which is exactly what Mayo prints beside that figure. For micromoles multiply by 11.3613 instead.
Why do different sources give different oxalate conversion factors?
Because they disagree about what is being weighed. Treating the measurand as the oxalate dianion gives 11.36 µmol per mg; treating it as oxalic acid gives 11.11. Mayo’s published mg and mmol limits and Labcorp’s SI table both match the dianion; GlobalRPh’s table and the familiar “45 mg is 0.5 mmol” shorthand match the acid. The 2.3% gap matters at a threshold and nowhere else.
What counts as hyperoxaluria?
Above roughly 40 to 45 mg/day is the usual threshold, but the laboratory interval you are reading against may put the upper limit anywhere from 31 to 49 mg/day. Mild elevation is usually dietary or enteric. Primary hyperoxaluria typically produces much higher and persistent excretion, often with stones in childhood.
Can a spot urine oxalate replace the 24-hour collection?
Not for stone risk. A spot sample can be reported as an oxalate-to-creatinine ratio, which at least corrects for dilution, but the supersaturation calculations a stone panel exists to drive need the daily excretion and the volume. A bare spot concentration mostly measures how much the patient drank.
Does cutting dietary calcium reduce urinary oxalate?
It does the opposite. Calcium binds oxalate in the gut and keeps it out of the circulation, so a low-calcium diet raises urinary oxalate and raises stone risk. Normal dietary calcium taken with meals is the standard advice for a calcium-oxalate stone former.
Related calculators
References
- Mayo Clinic Laboratories. Oxalate, 24 hour, urine (OXU) — reference values. Test catalogue, test ID 606737; 2026.
- ARUP Laboratories. Oxalate, urine — per 24 h, age- and sex-stratified reference intervals. Laboratory Test Directory 0020482 (component 0020255); 2026.
- Labcorp. Oxalate, quantitative, 24-hour urine — test 003970, reference interval. Test menu; 2026.
- Labcorp. SI unit conversion table — urine oxalate mg/24 h x 11.4 to umol/d; VMA mg/24 h x 5.05 to umol/d; urine metanephrines ug/24 h x 5.07 to nmol/d. Test menu resources; 2026.
- PubChem. Oxalate(2-), CID 71081, and oxalic acid, CID 971. National Library of Medicine; accessed October 2026.
- Human Metabolome Database. Oxalic acid, HMDB0002329 — average mass 90.0349.
- Overview of hyperoxaluria — diagnostic thresholds for 24-hour urinary oxalate. Urology Times; accessed October 2026.
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/