Banana calories and why size matters

Between 72 and 135 calories, depending which banana you picked up#

USDA puts raw banana at 89 calories per 100 grams and then, unusually for a fruit, names five separate sizes for it: extra small at 81 g of pulp, small at 101 g, medium at 118 g, large at 136 g and extra large at 152 g¹. Run each weight through USDA's own density and "a banana" costs anywhere from 72 to 135 calories — the big one is 88% more than the small one, and both are the thing people mean when they say they had a banana with breakfast. The densities and gram weights are USDA's; the multiplication is mine.

That is the headline, and it is only the first of three places a banana leaks precision. The second is that a good portion of what you weigh in the produce aisle is peel. The third is that a banana's carbohydrate changes identity while it sits on your counter, in ways that move its blood-sugar behavior and its measured fiber a great deal, and its calorie total barely at all. None of the three is a database error. They are all consequences of the fruit being a living thing you buy by the piece.

The size classes are defined by length; the calories live in mass#

Here is the whole range in one place. Every gram weight and the per-100-gram energy are USDA's; the final column is that density applied to that weight.

Notice what the size names are keyed to: length, in bands under an inch wide. Calories are keyed to mass. A banana is a curved cylinder whose girth varies independently of its length, so two fruits that both measure 7½ inches can differ by a comfortable 15 grams — and nobody is holding a ruler in the fruit aisle anyway. The classification is doing what it can with a fruit sold by the piece, which is the general problem catalogued in the common-foods reference: the unit people use and the unit calories live in are not the same unit.

The two cup entries are worth their own glance. A cup of sliced banana is 150 g and about 134 calories; a cup of mashed banana is 225 g and about 200. Same fruit, same cup, 50% more calories, because mashing removes the air between the slices. If you bake with banana, that difference is the recipe.

A sizeable share of what you are holding is peel#

USDA's 118 g medium is pulp — the fruit after you have thrown the wrapper away. The wrapper is not a rounding error.

Across ten cultivars at two maturation stages, Aquino and colleagues measured pulp-to-peel ratios of 1.18 to 2.26 in unripe fruit, rising by an average of 53% on ripening to a range of 1.43 to 4.18³. Take those ripe ratios at face value and a banana carrying 118 g of pulp weighs somewhere between about 146 g and 200 g in your hand. Weigh that whole fruit, apply USDA's 89 calories per 100 grams — which describes pulp only — and you log 130 to 178 calories where the food is 105. That is an overstatement of roughly a quarter to two-thirds, and the arithmetic is mine, resting on Aquino's ratios and USDA's density.

One caveat that matters: those cultivars are Brazilian — Ouro, Maçã, Mysore, Caru-Roxa, Marmelo among them — and not the Cavendish that dominates most Western supermarkets, so treat the range as a range and not as a number for your specific banana. The direction is not in doubt. The practical instruction follows immediately: peel first, then weigh, or skip the scale entirely and use the size class, which is what it exists for. Fruit sold by the piece is one of the few places where estimating without a scale costs you almost nothing, because the size classes already carry the uncertainty for you.

Ripening rearranges the carbohydrate and leaves the calorie total alone#

A green banana and a brown-spotted one taste like different foods, and chemically they nearly are. In the most thorough recent accounting — USDA's own laboratory, working through a controlled single lot and a separate retail sample — starch falls from about 21 g per 100 g in unripe fruit to roughly 0.5 g in overripe, while glucose and fructose climb and total sugars in retail bananas run 15.0 g per 100 g slightly ripe, 17.1 g ripe and 16.7 g overripe². Moisture rises too, from 74.8 to 80.3 g per 100 g across the controlled ripening series.

Nothing enters a banana after it leaves the tree. Ripening is a rearrangement, not a delivery — the fruit gets sweeter by taking itself apart.

So does the calorie total move? Barely, and the study is precise about it. Comparing energy calculated with specific Atwater factors against the general ones, the authors report that the two "did not differ for ripe and overripe bananas and was only 13 kcal per banana different for the slightly ripe (with values derived from specific Atwater factors being higher)." Thirteen calories, on the one stage where the convention matters at all — that is the size of the effect that gets described online as a ripe banana being "full of sugar." It is sweeter, it digests faster, and it is not meaningfully more food. Why starch and sugar carry slightly different energy values in the first place is worked through in the macro conversion factors.

What does move is the starch's fate. The starch in a green banana is largely the granular, enzyme-resistant kind, which is why unripe banana is one of the canonical dietary sources of resistant starch and why it behaves less like a carbohydrate and more like fiber on the way through. Ripening dismantles that, which is the honest version of the fast-versus-slow carbohydrate distinction: the same fruit sits on both sides of it depending on the week.

The fiber number is an artifact of which assay the lab ran#

This is the part of the banana literature that has not filtered into food databases, and it is startling. Phillips and colleagues measured dietary fiber in the same bananas two ways. The traditional enzymatic-gravimetric method (AOAC 991.43) returned about 2 g per 100 g regardless of ripeness. The newer modified method (AOAC 2011.25) returned about 18 g per 100 g in unripe fruit, falling to 4–5 g in ripe and about 2 g in overripe².

A ninefold gap, same fruit, same laboratory, same day. The explanation is mostly resistant starch: the older method includes a cooking step that gelatinizes some resistant starch and hands it back to the enzymes, so the assay digests away the very thing it is supposed to count. USDA's published 2.6 g per 100 g comes from the traditional lineage, which is why a green banana's fiber line looks unremarkable on a label. It is one more reason the printed figure is a convention rather than a reading, alongside everything else in how accurate nutrition labels are.

What to log, and what a banana is actually good at#

Log the size class, not the fruit. Extra small 72, small 90, medium 105, large 121, extra large 135 — five numbers, and picking the right one is a better use of attention than a scale you will use on the peel by accident. If you genuinely cannot tell, 105 for a medium is the middle of the distribution and wrong by at most about 30 calories in either direction, which is small next to the errors that dominate a real day of eating.

On potassium, the reputation is bigger than the fruit. A medium banana carries about 422 mg (USDA's 358 mg per 100 g applied to 118 g — my arithmetic), against a labeling Daily Value of 4,700 mg⁴. That is roughly 9% of a day, which is a real contribution and not a special one; plenty of foods do better per gram. What a banana is genuinely unmatched at is delivery. It arrives sterile in its own packaging, needs no knife, no plate and no refrigeration, costs under 150 calories at its largest, and carries a couple of grams of fiber and 9% of a mineral most people under-eat. That is a good deal. It is just not a nutritional event, and treating it as either a superfood or a sugar bomb overshoots in both directions.

FAQ#

How many calories are in a medium banana?#

About 105. USDA defines a medium banana as 118 g of pulp and raw banana as 89 calories per 100 g, which multiplies out to 105¹. The same source's other size classes run from 81 g (about 72 calories) to 152 g (about 135), so "medium" is a real choice and not a default — a large banana costs 16 more calories than a medium and 49 more than an extra small.

Do ripe bananas have more calories than green ones?#

Essentially no. Ripening converts starch to sugar rather than adding anything: starch falls from roughly 21 g per 100 g to about 0.5 g while sugars rise, and calculated energy differed between Atwater conventions by only 13 kcal per banana at the slightly ripe stage and not at all for ripe or overripe fruit². A ripe banana is sweeter and faster to digest, not more caloric.

Should I weigh a banana with the peel on?#

No — USDA's figures describe pulp only. Measured pulp-to-peel ratios in ripe fruit span 1.43 to 4.18 across cultivars³, so a banana holding 118 g of edible pulp can weigh 146 g to 200 g whole. Logging that whole weight against a per-100-gram pulp figure overstates the food by roughly a quarter to two-thirds. Peel it, then weigh it, or just use the size class.

Sources#

1. USDA FoodData Central. Bananas, raw (FDC 173944, SR Legacy).
2. Phillips KM, McGinty RC, Couture G, Pehrsson PR, McKillop K, Fukagawa NK. Dietary fiber, starch, and sugars in bananas at different stages of ripeness in the retail market. PLoS One. 2021;16(7):e0253366.
3. Aquino CF, Salomão LCC, Cecon PR, Siqueira DL, Ribeiro SMR. Physical, chemical and morphological characteristics of banana cultivars depending on maturation stages. Revista Caatinga. 2017;30(1):87-102.
4. 21 CFR 101.9 — Nutrition labeling of food, including Daily Reference Values for adults and children 4 or more years of age. US Code of Federal Regulations (Cornell LII).