Which foods are highest in oxalate?

Spinach is the single highest common source (a half-cup cooked can exceed 700 mg). Other very-high foods include almonds, beets, rhubarb, Swiss chard, potatoes with skin, navy beans, soy, black tea, and dark chocolate in quantity. OxalWise organizes 300+ foods into low, moderate, high, and very-high bands per serving.

How much oxalate per day is recommended for kidney stone formers?

No urology guideline sets a numeric daily oxalate limit. The widely used figure — under 100 mg of dietary oxalate per day, with 50 mg for stricter cases — comes from the University of Chicago Kidney Stone Program. The right number is individual: a 24-hour urine test and your care team decide it.

Does boiling remove oxalate from food?

Yes, the soluble fraction — and only if you discard the water. Chai and Liebman (Journal of Agricultural and Food Chemistry, 2005) measured a 30-87% drop in soluble oxalate from boiling, 5-53% from steaming, and none from baking. The oxalate is not destroyed; it leaches out, and the study recovered essentially all of it in the cooking water. Boil, drain, and never reuse the water.

Should stone formers avoid calcium?

No — normal dietary calcium taken with meals binds oxalate in the gut so less is absorbed, and cutting calcium can raise stone risk. It is the most counterintuitive, best-replicated finding in stone prevention.

Sources & references

OxalWise is a reference, so it only works if you can check it. Every health claim in the app traces to one of the sources below, each linked to the source itself.

Where the guidelines are silent, this app says so rather than inventing a consensus: no urology guideline sets a numeric daily oxalate limit, and the widely used 100 mg/day figure is attributed to the named clinical program it actually comes from.

Clinical guidelines

The management advice in The Science guide — fluid, calcium, sodium, citrate — comes from these. Note what they do not say: neither sets a numeric daily oxalate limit.

Medical Management of Kidney Stones: AUA Guideline — Pearle MS, Goldfarb DS, Assimos DG, Curhan G, Denu-Ciocca CJ, Matlaga BR, Monga M, Penniston KL, Preminger GM, Turk TMT, White JR, Journal of Urology 192(2):316–324, 2014. The fluid target (produce at least 2.5 L of urine a day), the 1,000–1,200 mg/day dietary calcium range taken with meals, the 2,300 mg/day sodium ceiling, the "1 in 11 people in the US have had a stone" figure, and the "at least 50% recur within 10 years" figure. This PDF is free to read. The guideline is also published as Pearle et al., J Urol 2014 — doi.org/10.1016/j.juro.2014.05.006.
Medical Management of Kidney Stones: AUA Guideline (2026) — current edition — American Urological Association, American Urological Association, 2026. The AUA has revised this guideline since the 2014 publication above, and the current edition now covers children as well as adults. Cited so you can check the latest wording yourself rather than take our summary of the 2014 text as current practice.
EAU Guidelines on Urolithiasis — European Association of Urology, Urolithiasis Guidelines Panel, European Association of Urology (uroweb.org), 2026 edition. The second guideline behind the calcium and sodium advice. The EAU says only that "excessive intake of oxalate-rich products should be limited or avoided" — it sets no milligram figure for dietary oxalate. Its mmol/day numbers are urinary excretion, not what you eat, which is easy to misread.

Where the food numbers come from

The oxalate values in the Food Atlas, and the reason this app shows bands instead of decimals.

Oxalate Table 2023 (spreadsheet) — Nutrition Questionnaire Service Center — Harvard T.H. Chan School of Public Health, Department of Nutrition — measurements by the laboratory of Dr John Knight, University of Alabama School of Medicine, Harvard T.H. Chan School of Public Health, 2023. The primary source for the Food Atlas. It is published per household serving rather than per 100 g, which is why this app reports per serving. The table itself downloads from that page as "Oxalate Table 2023 (.xlsx)". Note: the older regepi.bwh.harvard.edu address for this table is dead — the server no longer exists — even though it still ranks highly in search.
Oxalate content of food: a tangled web — Attalla K, De S, Monga M, Urology 84(3):555–560, 2014. Why this app shows bands, not decimals. Published values for the same food disagree sharply — this paper found spinach reported anywhere from 364 to 1,145 mg per 100 g. If the sources cannot agree within a factor of three, a decimal point is false precision.

Individual studies

Cited wherever a specific number in the app depends on one particular piece of research.

Effect of different cooking methods on vegetable oxalate content — Chai W, Liebman M, Journal of Agricultural and Food Chemistry 53(8):3027–3030, 2005. The cooking numbers in The Science guide and the FAQ: boiling cut soluble oxalate by 30–87%, steaming by 5–53%, and baking not at all. The oxalate is not destroyed — the study recovered essentially all of it in the cooking water, which is why draining is the whole trick.
Schedule of taking calcium supplement and the risk of nephrolithiasis — Domrongkitchaiporn S, Sopassathit W, Stitchantrakul W, Prapaipanich S, Ingsathit A, Rajatanavin R, Kidney International 65(5):1835–1841, 2004. The calcium-supplement timing advice: the same supplement taken at bedtime raised calcium oxalate supersaturation, while the same amount split across meals did not. Worth knowing the limits — this was a crossover study in 32 healthy young men with no history of stones, not in stone formers.
Effect of ascorbic acid consumption on urinary stone risk factors — Traxer O, Huet B, Poindexter J, Pak CY, Pearle MS, Journal of Urology 170(2 Pt 1):397–401, 2003. The vitamin C answer in the FAQ: 2 g a day of ascorbic acid raised urinary oxalate by 33% in stone formers and 20% in non-stone-formers.
Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones — Ferraro PM, Curhan GC, Gambaro G, Taylor EN, American Journal of Kidney Diseases 67(3):400–407, 2016. The rest of the vitamin C answer: supplemental vitamin C of 1,000 mg a day or more tracked with modestly higher stone risk in men, with no association found in women — and vitamin C from food was not linked to higher risk in either. Free full text.
Comparison of dietary calcium with supplemental calcium and other nutrients as factors affecting the risk for kidney stones in women — Curhan GC, Willett WC, Speizer FE, Spiegelman D, Stampfer MJ, Annals of Internal Medicine 126(7):497–504, 1997. The Nurses’ Health Study finding behind the calcium paradox: in women, dietary calcium tracked with fewer stones while supplemental calcium tracked with slightly more.
A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones — Curhan GC, Willett WC, Rimm EB, Stampfer MJ, New England Journal of Medicine 328(12):833–838, 1993. The same finding in men, from the Health Professionals Follow-up Study — the original large cohort result that overturned the intuition that stone formers should cut calcium.
Influence of Age and Geography on Chemical Composition of 98,043 Urinary Stones from the USA — Katz JE, Soodana-Prakash N, Jain A, Parmar M, Smith N, Kryvenko O, Austin GW, Shah HN, European Urology Open Science 34:19–26, 2021. The stone-composition figure on the home page and in The Science guide: calcium oxalate was the most common type, at 79.2% of pure stones. Free full text. The widely repeated "about 80% of stones are calcium oxalate" is often a misquote of an older figure for all calcium stones — this is the specific number.
Prevalence of kidney stones in the United States — Scales CD Jr, Smith AC, Hanley JM, Saigal CS; Urologic Diseases in America Project, European Urology 62(1):160–165, 2012. The "1 in 11" figure — self-reported lifetime prevalence of 8.8% in the US (NHANES 2007–2010). Free full text.
The first kidney stone — Uribarri J, Oh MS, Carroll HJ, Annals of Internal Medicine 111(12):1006–1009, 1989. The recurrence figures: pooled across six retrospective studies, cumulative recurrence was 14% at one year, 35% at five years, and 52% at ten years.
How To Eat A Low Oxalate Diet — Jill Harris, LPN — University of Chicago Kidney Stone Program, University of Chicago Kidney Stone Program, 2015. The daily budget this whole app is built around. The page says: "Less than 100 mg of diet oxalate is good; less than 50 mg is ideal." This is the source of the 100 mg figure — no urology guideline sets one. It is worth knowing that the program is not unanimous on the number: its nephrologist, Dr Fredric Coe, has written that 100 mg/day or less is good and that below 150 mg/day is fine provided dietary calcium is at least 800 mg — the target depends on how much calcium you eat with it. Ask your care team for your number.

Independence and affiliation

OxalWise is an independent app, designed and published by Thomas Hornig, who owns the OxalWise name, the oxalwise.com website, and this app.

OxalWise is not affiliated with, endorsed by, or sponsored by any institution cited on this page, including the Harvard T.H. Chan School of Public Health, the American Urological Association, the European Association of Urology, the University of Chicago Kidney Stone Program, and the University of Alabama at Birmingham. Their published work is cited here as a source, with attribution and a link. Institution names are used only to identify the origin of a figure or a recommendation. No institutional logo, branding, or proprietary material is used anywhere in this app.

This is a reference, not medical advice

OxalWise is educational. It does not diagnose, treat, or prescribe, and it is not a substitute for care from a physician or registered dietitian. Oxalate content varies with variety, soil, season, maturity, and preparation, and published values for the same food can differ by a factor of two — which is why this app shows bands rather than decimals. Your daily target is a clinical decision.

The Science guide · The Food Atlas · FAQ