How the calorie count works
Every food nutrition label, this calculator included, starts from a figure per 100 grams. Raw banana runs at 89 kcal per 100 g, so the calories in one fruit are just that rate scaled to its actual weight:
A medium banana at 118 g works out to 89 × 118 ÷ 100, which comes to about 105 kcal, the figure most nutrition sites quote for "one banana" because medium is the size most people picture. Change the size and the same sum simply runs with a different weight; change the count and the total scales in a straight line, three medium bananas is exactly three times one.
Why banana size changes the number so much
"One banana" is not one weight. USDA FoodData Central, the reference database behind most nutrition labels, defines five standard banana sizes by length, each with its own typical weight: extra small (under 6 inches) at 81 g, small (6 to 6.875 inches) at 101 g, medium (7 to 7.875 inches) at 118 g, large (8 to 8.875 inches) at 136 g, and extra large (9 inches or longer) at 152 g. The gap between smallest and largest is nearly double, 72 kcal against 135 kcal, which is why "how many calories in a banana" only has a useful answer once you say which size you mean. Weighing a banana on kitchen scales and using the gram figure directly is always more accurate than guessing a size by eye.
The ripening carb story
A green banana and a heavily speckled one taste completely different because their carbohydrate is made of different things, not different amounts. Unripe banana starch is largely resistant starch, a form your small intestine cannot break down, so it passes through mostly intact and behaves more like fibre. As a banana ripens, enzymes convert that starch into simple sugars, mainly glucose, fructose and sucrose, which is why a ripe banana tastes sweeter and a green one tastes starchy and bland. Peer-reviewed studies tracking bananas through ripening show resistant starch dropping from the majority of the fruit's starch content in the green stage to almost nothing by full ripeness, with free sugars rising to take its place.
Total calories move only slightly across that change, since resistant starch and sugar both start from the same carbohydrate pool. The practical difference is digestion speed and gut effect rather than energy content: a greener banana releases its carbohydrate more slowly and feeds gut bacteria on the way through, while a ripe one gives quicker, sweeter energy. Neither is "healthier" outright; which one suits you depends on whether you want a slow-release snack or a fast source of sugar before exercise.
Reading the fruit comparison
The table above pulls every fruit in this site's food dataset so a banana's calorie count has real neighbours to sit next to, not just an isolated number. Per 100 g, banana lands solidly in the middle: well ahead of watery fruit such as watermelon and strawberries, and well behind energy-dense fruit such as dates, raisins and avocado, which concentrate calories through low water content and, for avocado, natural fat. Per typical serving the picture shifts a little, since serving sizes themselves vary from a single small fig to a full cup of grapes, which is why both columns are worth checking rather than just one.
Potassium and the pre-workout case
Bananas earned their reputation as a training snack from a genuine, if modest, mix of traits: quick-digesting carbohydrate for fast energy, a small amount of potassium, and a shape that survives a gym bag. A medium banana's 422 mg of potassium is real and useful, but it is not exceptional; a medium potato holds nearly 900 mg, and a cup of cooked spinach around 840 mg. The honest case for a pre-workout banana rests more on convenience and quick sugar than on potassium supremacy.
What it would take to burn one off
The table above turns the calories in your selected bananas into rough activity minutes, using the same MET (metabolic equivalent of task) arithmetic behind this site's Calorie Burn Calculator:
Fixing the reference person at 70 kg and solving for minutes at three common intensities (walking at 3.5 METs, moderate cycling at 6.8 METs, running at 6 mph at 9.8 METs) turns one medium banana's 105 kcal into roughly 24.5 minutes of walking, 12.6 minutes of moderate cycling, or 8.7 minutes of running. Anyone lighter or heavier than 70 kg, or working harder or easier than the stated pace, will see a different real number; the table is a fair starting estimate, not a personal reading.
Where these numbers come from
Calorie, carbohydrate, protein, fat and fibre figures for banana and every comparison fruit come from USDA FoodData Central, the standard reference nutrition database. Banana size weights follow USDA's own standard serving definitions for raw banana by length. Potassium and the MET values behind the burn-off table are added from the same USDA entry and the Compendium of Physical Activities respectively, since they sit outside this site's core food dataset. Real fruit varies by ripeness, growing conditions and exact size, so treat every figure here as a solid typical value rather than a lab measurement of the specific banana in your hand.
Questions people ask
How many calories are in one banana?
It depends on size. A medium banana (about 118 g) is roughly 105 kcal. A small one is closer to 90 kcal, a large one around 121 kcal, and an extra large one around 135 kcal. Weigh it if you want an exact figure rather than an estimate by size.
Do riper bananas have more calories than green ones?
Barely. Ripening converts resistant starch into sugar rather than adding energy, so the calorie count for a given weight of banana stays close to the same from green to fully ripe. What changes is how quickly your body digests the carbohydrate, not how much of it there is.
Is a banana a good pre-workout snack?
Reasonably, yes. It offers quick-digesting carbohydrate and a genuine dose of potassium in a convenient, no-prep package. It is not uniquely rich in potassium compared with foods like potato or spinach, and it works better as a top-up 30 to 60 minutes before training than as your only pre-workout food.
How does a banana compare with other fruit for calories?
It sits in the middle of the pack. Per 100 g, banana has more calories than watery fruit such as watermelon and strawberries, but far fewer than dried or dense fruit such as dates, raisins and avocado. See the comparison table above for the full spread across this site's fruit dataset.
How long would I need to exercise to burn off a banana?
For a 70 kg reference adult, roughly 24 to 25 minutes of walking, around 13 minutes of moderate cycling, or about 9 minutes of running covers one medium banana's 105 kcal. Your own number shifts with your weight and effort; the table above recalculates for however many bananas you have selected.
Are banana carbs bad for blood sugar?
Not inherently. A riper banana raises blood glucose faster than a greener one because more of its carbohydrate is already simple sugar, but a whole banana still carries fibre that slows the rise compared with sugar alone. Anyone managing diabetes should factor in the total carbohydrate figure shown above and their own glucose response, ideally with a dietitian's input.
Why isn't potassium shown in the site's other food comparisons?
This site's core food dataset tracks calories, protein, carbohydrate, fat and fibre consistently across roughly 200 foods. Potassium is added specifically to this page from the same USDA source because it is central to the pre-workout framing above, not because other foods lack a potassium value in reality.
Nutrition figures here are typical published values for raw banana and common fruit, not a measurement of any specific piece of fruit. This tool is for general information, not medical or dietetic advice; anyone managing a medical condition such as diabetes or kidney disease should check dietary choices with a doctor or registered dietitian.