The numbers, and one qualifier that has to travel with them

USDA's food composition data gives cooked lentils, per 100 grams: 116 calories, 9.0 g protein, 7.9 g fiber, 3.3 mg iron, 181 µg folate, 369 mg potassium, and 2 mg sodium. A cup of cooked lentils weighs 198 grams.

Multiplying that out (these per-cup figures are calculated from the USDA per-100-gram values rather than published by USDA directly), a cup of cooked lentils gives roughly:

To put the fiber figure in context, average fiber intake among American adults runs about 17.7 grams a day for men and 15.0 for women. One cup of cooked lentils is roughly a typical adult's entire current daily intake. Against the Adequate Intakes (38 grams for men under 50, 25 for women), it is a large single contribution from one cheap ingredient.

Now the iron, and the qualifier that must come with it. Six and a half milligrams looks like most of an adult man's RDA of 8 mg, or a third of the 18 mg for women aged 19 to 50. That comparison is misleading on its own.

The NIH Office of Dietary Supplements is clear about the difference between iron in plants and iron in meat:

"The bioavailability of iron is approximately 14% to 18% from mixed diets that include substantial amounts of meat, seafood, and vitamin C… and 5% to 12% from vegetarian diets."

"The requirement for iron is 1.8 times higher for people who follow vegetarian diets than those for people who include animal products in their diet."

And: "Phytate (which is present in grains and beans)… [has] the opposite effect." That is, it inhibits absorption.

So lentils contain a lot of iron, and a smaller fraction of it reaches you than the number implies. Vitamin C enhances the absorption of non-heme iron, and that part is established. It is the honest basis for squeezing lemon over a lentil dish or eating it alongside peppers or tomatoes. What is not established is that doing so produces a meaningful change in anybody's iron status. Treat it as sensible practice with a mechanism behind it, not as an intervention.

If you are working from the dry bag, USDA gives raw lentils as 352 calories, 24.6 g protein, 10.7 g fiber and 6.5 mg iron per 100 grams, with a cup of dry lentils weighing 192 grams. Dry roughly triples in volume when cooked, so a cup of dry lentils makes about three cups cooked.

Cooking them

No soaking. That is the whole advantage over dried beans: lentils go from the bag to the pot.

Rinse and check. Tip them into a sieve, rinse, and glance for small stones or debris. Modern processing is good and this is usually unnecessary, but it costs ten seconds and a stone costs a tooth.

Simmer 15 to 20 minutes for most lentils, in unsalted or lightly salted water or stock, with the pot uncovered or partly covered. Start tasting at 15 minutes.

Use about three parts liquid to one part lentils if you want them drained, or more if you want soup. Unlike pasta, you do not need a full pot of water.

Salt at the end if you want them to hold their shape, or from the start if you do not mind softer lentils. The old rule that salt prevents legumes from softening is more folklore than physics, but acid does, so add tomatoes, vinegar or lemon after the lentils are tender, not before.

On varieties: brown and green lentils are generally treated as the ones that hold their shape, red and yellow as the ones that break down into a purée, and small dark ones as firmest of all. This is cooking experience rather than a sourced fact: no authoritative reference for lentil variety behavior could be located, and USDA publishes a single generic lentil record rather than separate data by type. Buy whichever is in front of you, notice what it does, and adjust.

Five things to do with a bag

1. A pot of them, plain, for the week. Cook two cups of dry lentils in stock with a bay leaf, an onion and a couple of garlic cloves. Drain if needed. That is roughly six cups cooked, which goes into salads, soups, bowls and eggs for several days. Refrigerated, treat it like any leftover: 3 to 4 days. Frozen in single-meal portions, it keeps well.

2. Soup, which is what everyone does, done better. The improvement is finishing acid and finishing fat. A soup that tastes flat almost always needs a squeeze of lemon or a splash of vinegar at the end, and a spoonful of olive oil stirred in off the heat. Both go in after cooking, not during.

3. A lentil salad, which is the underrated one. Cooked lentils dressed while still warm with olive oil, vinegar or lemon, mustard, salt and plenty of pepper, then loaded with whatever is raw and crunchy (celery, red onion, cucumber, radish) and a large handful of parsley. Warm lentils absorb dressing in a way cold ones do not. This keeps for days and improves overnight.

4. Red lentils as a thickener. Half a cup of red lentils dropped into almost any vegetable soup breaks down completely and gives it body, without anyone identifying them. This is the easiest way to add substantial fiber to a dish somebody else is eating without a conversation about it.

5. Stretching something else. A cup of cooked brown lentils stirred into a meat sauce, a chili, or a filling replaces some of the meat, adds fiber, and costs almost nothing. The texture is close enough that it goes unremarked.

Where lentils sit in current dietary guidance

Something changed recently that is easy to miss. In the Dietary Guidelines for Americans 2025 to 2030, released in January 2026, beans, peas and lentils are listed among the Protein Foods: "a variety of plant-sourced protein foods, including beans, peas, lentils, legumes, nuts, seeds, and soy." The accompanying serving guide counts half a cup of cooked beans, peas or lentils as one Protein Foods serving, against a Protein Foods target running from one to one and a half servings a day at 1,000 calories up to four or five at 3,000 calories or more. At a typical adult intake that puts a single cup of cooked lentils at roughly half a day's protein servings.

They have not always been grouped this way, and the older placement is still in circulation. See the caution below.

This is a classification change, not a nutritional upgrade. The lentils are the same lentils. But it has a practical consequence worth knowing: if you are consciously working down a vegetable list, legumes have quietly stopped appearing on it, and it is easy to eat fewer of them by accident.

One caution if you go looking: MyPlate.gov still describes beans, peas and lentils as part of the Vegetable Group and uses a different unit (a quarter cup as one ounce-equivalent) from the new serving guide (half a cup as one serving). The two federal sources do not currently agree, so do not try to reconcile them. Use one or the other.

Storage

Dry. An airtight container, cool and dark. They keep for a year or more. Older lentils take longer to cook and may never fully soften, which is the usual explanation when a bag from the back of the cupboard refuses to cooperate.

Cooked, refrigerated. Three to four days. That is the general federal leftovers figure, and the federal cold storage charts contain no row for cooked legumes. The 3-to-4-day rule is the answer, and any more specific number you see elsewhere did not come from an agency.

Cooked, frozen. They freeze very well. Portion into single meals, cool before packing, press the air out, and label with the date. Reheat to 165°F.

A small aside about leftovers. Cooking and then cooling starchy foods converts a portion of the starch into a form that resists digestion in the small intestine. The direction of that change is real and has been measured in white rice and in potatoes. The size of it is modest and disputed, reheating does not fully preserve it, and it has not been measured in lentils at all. Whether starch formed this way in a home kitchen counts as dietary fiber for labeling purposes is unresolved. So it is an interesting thing that happens in your refrigerator, not a reason to change what you cook or how you store it. A companion article on cooked and cooled starch covers what is and is not established.

Gas, briefly

Lentils contain the same family of carbohydrates that makes beans famous: oligosaccharides your enzymes cannot break down and your gut bacteria can. They are widely reported as gentler than large beans, though the reason is more likely quantity and cooking than anything special about the lentil.

The pooled trial evidence on added fiber is worth knowing here, and the numbers are more precise than the usual warnings. Across the trials, flatulence rose by a standardized mean difference of 0.80 (95% CI 0.47 to 1.13), a large, consistent effect. Bloating came in at 0.07 (−0.38 to 0.51) and abdominal pain at −0.14 (−0.36 to 0.09), with no detectable effect in either case: more wind, not more discomfort. The certainty behind those figures was rated low. A companion article on beans and gas covers what reduces the oligosaccharides, and how much of the "you adapt in two weeks" reassurance is supported. Adaptation has not been demonstrated on any timeline. The trial that looked hardest found only partial adaptation: 15 to 23 percent of people were still reporting gas at six to twelve weeks. The pooled evidence describes the flatulence effect as persistent rather than fading, a real result and not a failure of nerve.

If lentils reliably cause you real pain rather than ordinary gas, that is not the expected pattern and is worth raising with a clinician rather than pushing through.