Foods high in oxalates, ranked by milligrams

Spinach is the outlier at roughly 700 mg per cooked half cup, followed by rhubarb, Swiss chard, almonds, and beets. The mg-per-serving ranking, why boiling and a little calcium cut absorption, and who actually needs to care about oxalate.

1. 01

   Spinach: ~700+ mg per cooked half cup, the outlier

   Cooked spinach is the single densest oxalate source by a wide margin, carrying roughly 700 mg or more in a half cup, which is more than ten times a stone-former's entire daily allowance in one side dish (Harvard T.H. Chan School of Public Health; USDA). Raw spinach is nearly as extreme once you account for how much fits in a large salad. This is the one food where the amount genuinely matters for anyone prone to calcium-oxalate stones, and it is why spinach is the first thing a kidney-stone clinic asks about. For everyone else spinach stays an excellent food, dense in iron, folate, and vitamin K, and the sensible move is not to avoid it but to boil it and pair it with a calcium source rather than eating large raw quantities daily.

2. 02

   Rhubarb, Swiss chard, and beet greens: several hundred mg per serving

   The other very-high plants cluster just behind spinach: stewed rhubarb, cooked Swiss chard, and beet greens each land in the several-hundred-milligram range per serving, with rhubarb and chard among the most oxalate-dense foods measured (USDA; Massey oxalate database). Rhubarb is a special case, its leaves contain enough oxalic acid to be toxic and are never eaten, while the stalks people cook are lower but still high. For a stone-former these are foods to treat like spinach, boiled and portioned, not raw and daily. Their nutrient density is real, so the goal is moderation and preparation rather than a ban.

3. 03

   Almonds, peanuts, and cashews: ~110-120 mg per ounce

   Nuts are the densest non-leafy source: an ounce of almonds carries ~122 mg of oxalate, peanuts ~113 mg, and cashews are close behind, so a handful is not trivial and almond flour concentrates it further per serving because so many nuts go into it (USDA). This matters most for anyone who has swapped wheat flour for almond flour on a low-carb or gluten-free plan and is eating it in large daily amounts. The fix for stone-formers is portion size and pairing with calcium, not elimination, since nuts also deliver magnesium, healthy fats, and plant protein that most diets are short on.

4. 04

   Beets, potatoes, and sweet potatoes: ~30-100 mg per serving

   The dense root and tuber group runs from a baked potato with skin at ~97 mg, through beets at ~75 mg per half cup, to sweet potatoes lower at ~28-40 mg per cup (USDA; Harvard). These are the foods that quietly add up because portions are large and they appear at many meals. Most of a potato's oxalate sits in and just under the skin, so peeling lowers it, though at the cost of fiber and potassium. For the average eater these numbers are unremarkable; for a stone-former they are worth counting alongside the leafy greens rather than assuming only spinach matters.

5. 05

   Cocoa, dark chocolate, wheat bran, and buckwheat

   Cocoa is genuinely oxalate-rich, so cocoa powder, dark chocolate, and unsweetened baking chocolate all contribute meaningfully per serving, and among grains wheat bran and buckwheat are the standouts while refined grains are low (USDA; Massey database). This is the band that surprises people, because chocolate and bran cereal rarely come to mind as oxalate foods the way spinach does. The amounts are moderate rather than spinach-level, so for most people they are a background contribution, but for a stone-former on a strict count they belong on the list, especially if dark chocolate or a bran cereal is a daily habit.

6. 06

   Soy foods and legumes: tofu, tempeh, and some beans

   Soy foods carry a fair amount of oxalate, with tofu and tempeh in the moderate-to-high band and soy milk lower, and among legumes navy beans and other dried beans contribute usefully (USDA). This is worth knowing for plant-based eaters who lean on tofu and beans as protein staples, because several servings a day add up. Calcium-set tofu is a helpful case, since the added calcium partly binds its own oxalate in the gut. As with the rest of the list, the practical answer for stone-formers is pairing legumes with a calcium source and keeping portions reasonable rather than dropping a whole protein category.

7. 07

   The real fix: calcium, boiling, and water, not avoidance

   The most important item on this list is not a food but a method, because oxalate only causes stones when it reaches the kidney unbound to calcium. Eating a calcium source in the same meal lets the calcium trap the oxalate in the gut so the pair leaves in the stool, which is why the National Kidney Foundation tells stone-formers to keep calcium intake normal at 1,000-1,200 mg a day and specifically not to cut it. Boiling greens and discarding the water removes 30 to almost 90 percent of their soluble oxalate, far more than steaming (Chai and Liebman 2005). Drinking enough water to keep urine dilute matters more than any single food. Done together, these let most people keep high-oxalate foods on the menu.