How many calories in a tin of tuna? Brine, oil, drained, and the mayo problem

Tinned tuna is the highest protein-per-calorie food most people can buy without cooking anything, and it routinely gets logged wrong by a factor of three. Not because of the fish. Because of the liquid it sits in and what goes on top of it afterwards.

Here is what tuna actually costs, by tin, by 100g, and by what you mix into it.

The quick answer

Canned light tuna in water, drained, is about 116 kcal per 100g, with 26g of protein and under 1g of fat. USDA reference data is consistent across brands within about 10 kcal.

The number that matters more is the drained weight, not the tin weight. A tin sold as 145g is not 145g of fish.

By the tin, because that is the unit people buy

Tin sizes differ by market and brand, and the drained weight is always printed somewhere smaller than the headline weight.

Pouches have no drained-weight problem, because there is barely any liquid in them. What is printed on the front is what you eat, which makes them the easiest format to log accurately.

(For why oil is so dense that a few millilitres change a meal this much, the olive oil calorie breakdown covers the per-spoon math.)

The mayo problem

Tuna is almost never eaten plain, and almost everything it gets mixed with is heavier than the fish. This is the actual reason "tuna salad" and "tuna mayo" land where they do.

The pattern is stark. The tuna is 130 kcal. A deli scoop of mayonnaise is 380. The most calorie-dense thing in a tuna sandwich is never the tuna.

Swapping full-fat mayo for Greek yogurt with a spoon of mustard saves 155 kcal per tin and adds about 6g of protein. It is one of the few swaps that improves both sides of the ledger without tasting like a compromise.

(For the strained-yogurt side of that swap, how many calories in yogurt covers which tub to buy.)

The lunch you actually build

A tin of tuna is a 130 kcal ingredient that reliably ends up in a 500 to 700 kcal lunch. That is not a criticism of the lunch. It is a reminder that the protein was never the expensive part.

(For the pasta half of those numbers, the pasta calorie breakdown covers dry-versus-cooked weights, which is the other common tracking error in a tuna pasta salad.)

Tinned versus fresh

Tinned tuna is a genuinely good substitute for fresh on protein and calories. Where it loses is omega-3 content, since skipjack is a lean tuna and canning in water washes some of the fat away. Tinned salmon and sardines carry several times more omega-3 than tinned tuna, at a higher calorie cost.

(For the oily-fish comparison in full, the salmon calorie breakdown covers fresh, tinned and smoked.)

How often is too often?

This is the one question about tuna that is not really about calories.

The FDA and EPA joint advice on eating fish places canned light tuna in the Best Choices group at two to three servings a week, and albacore or white tuna in Good Choices at one serving a week. A serving is 4oz, or about 113g cooked, which is roughly one standard tin.

The difference is mercury. Skipjack, which is what light tuna usually is, is a smaller and shorter-lived fish than albacore, so it accumulates less. The advice matters most during pregnancy, when breastfeeding and for young children. For a healthy adult, a tin a day is more than the guidance suggests and a few tins a week is comfortably inside it.

Tuna mistakes to avoid

Logging the tin weight instead of the drained weight

A 145g tin is about 112g of fish. Logging 145g overstates by 38 kcal, which is small. The reverse error is bigger: people who drain an oil tin badly log 130 kcal for something closer to 350.

Treating "in oil" and "in oil, drained" as the same entry

Most tracking apps have both. They differ by 80 kcal per 100g, and if you tip the oil in as well the gap is far wider. Pick the right entry once and save it as a favourite.

Assuming spring water is lighter than brine

They are the same calories. Brine is salt water. If you are watching sodium rather than calories, spring water is the one to buy, and rinsing a brine tin under the tap removes a good share of the salt.

Eyeballing the mayonnaise

A tablespoon of full-fat mayo is 95 kcal and a real-world spoon into a mixing bowl is usually two of them. This is the largest single variable in the entire dish, and it is the one nobody measures.

Buying flavoured tins as a light option

Tuna in tomato sauce is fine at about 105 kcal per 100g. Tuna in mayonnaise tubs, pesto, or oil-based dressings run 190 to 230 kcal per 100g, roughly double plain tuna, and they are usually sold in the same shelf position as the plain tins.

Three tuna plates worth memorising

The desk lunch (155 kcal)

• 1 tin light tuna in spring water, drained (112g, 130 kcal)
• 100g cucumber and tomato (20 kcal)
• Lemon juice, black pepper, a pinch of salt

Adds up to: ~155 kcal, 30g protein. Roughly 19g of protein per 100 kcal, which almost nothing else at a desk can match.

The honest tuna mayo (170 kcal)

• 1 tin drained (130 kcal)
• 3 tbsp plain nonfat Greek yogurt (35 kcal)
• 1 tsp Dijon mustard, chopped red onion, salt

Adds up to: ~170 kcal, 35g protein. The same texture as tuna mayo for a third of the calories, and more protein than the original version.

The full lunch (400 kcal)

• 1 tin drained (130 kcal)
• 150g cooked rice (195 kcal)
• 100g cucumber and edamame (55 kcal)
• 1 tbsp soy sauce and a teaspoon of sesame seeds (20 kcal)

Adds up to: ~400 kcal, 36g protein. A complete lunch that holds until dinner, built on a tin and a bowl of leftovers.

For a version with actual instructions, the Mediterranean tuna salad bowl is the same idea with the numbers worked out.

The verdict

Tuna is one of the cheapest, fastest and leanest protein sources in a supermarket, and it is almost impossible to make it expensive on its own. Everything that makes a tuna lunch heavy arrives afterwards: the oil in the tin, the mayonnaise in the bowl, the cheese on the melt.

Buy it in water, drain it properly, and mix it with something that is not 680 kcal per 100g. The fish was never the problem.

Pairs well with: the salmon calorie breakdown, how much protein you need per day, and high protein snacks under 250 calories.