Spinach contains significantly more iron than kale. A 100-gram serving of raw spinach provides about 2.7 mg of iron, while the same amount of raw kale delivers around 1.5 mg. But the raw numbers don’t tell the whole story, because how much iron your body actually absorbs from each green depends on other compounds in the leaves.
Iron Content Side by Side
By weight, spinach has roughly 80% more iron than kale. That gap holds whether you’re comparing them raw or cooked, though cooking concentrates the nutrients in both greens as water evaporates. A cup of cooked spinach (about 180 grams) provides around 6.4 mg of iron, which covers a large portion of the daily recommended intake: 8 mg for adult men and 18 mg for women of reproductive age, according to the National Institutes of Health.
A cup of cooked kale delivers closer to 1 mg of iron. So if you’re choosing between the two purely to boost your iron intake, spinach wins by a wide margin on paper.
Why Spinach’s Iron Is Harder to Absorb
Spinach is loaded with oxalic acid, a natural plant compound that binds to minerals like iron and calcium and prevents your body from absorbing them efficiently. USDA researchers analyzing hundreds of spinach varieties found oxalate concentrations ranging from 647 to 1,287 mg per 100 grams of fresh spinach. That’s a substantial amount.
Kale contains far less oxalic acid. This means that while spinach has more iron per serving, your body may absorb a higher percentage of the iron in kale. The exact absorption rate varies from person to person and meal to meal, but the difference is real enough that nutrition researchers consistently flag spinach’s oxalate content as a limitation.
Both greens contain only nonheme iron, the type found in all plant foods. Your body absorbs nonheme iron less efficiently than the heme iron in meat and seafood. The NIH notes that vegetarians need almost twice the standard iron recommendation to compensate for this difference.
How to Get More Iron From Either Green
Vitamin C is the simplest tool for boosting iron absorption from plants. Eating spinach or kale alongside citrus fruit, bell peppers, tomatoes, or a squeeze of lemon juice helps convert nonheme iron into a form your body picks up more readily. This is especially useful with spinach, where the oxalate content is working against you.
Cooking also helps. Heat breaks down some of the oxalic acid in spinach, which can modestly improve mineral availability. Boiling is more effective than steaming for reducing oxalates, though you lose some water-soluble vitamins in the process. Sautéing with a splash of lemon juice gives you a reasonable middle ground.
Calcium can interfere with iron absorption when consumed at the same time. Both spinach and kale contain calcium, though kale’s calcium is more bioavailable (spinach’s oxalates block calcium absorption too). If you’re eating these greens specifically for iron, pairing them with high-calcium foods like dairy at the same meal may reduce how much iron you absorb.
Spinach vs. Kale for Overall Nutrition
Iron isn’t the only reason people compare these two greens. Kale is higher in vitamins C and K per serving, and its calcium is actually usable by the body, unlike spinach’s calcium which is largely locked up by oxalates. Spinach, on the other hand, is higher in folate, magnesium, and potassium in addition to iron.
Neither green is objectively better. They serve slightly different nutritional roles. If iron is your primary concern and you eat a plant-based diet, spinach still delivers more total iron even after accounting for lower absorption rates. But rotating between both greens, and pairing them with vitamin C-rich foods, gives you the broadest nutritional benefit.
Practical Amounts That Matter
For context, a woman between 19 and 50 needs 18 mg of iron daily. A cup of cooked spinach gets you about a third of the way there. Reaching that target from greens alone would require eating an impractical volume of either spinach or kale, so these foods work best as one piece of a broader iron strategy that includes beans, lentils, fortified cereals, or (for non-vegetarians) red meat and shellfish.
Adults over 51 and adult men need 8 mg per day, a much easier target. A cup of cooked spinach paired with lentils and some bell pepper at dinner could realistically cover most of a day’s iron needs for these groups. Kale on its own contributes modestly, but its lower oxalate load means the iron it does provide is put to better use.