How many calories in watermelon? Per slice, kilo and whole

In summer, watermelon gets eaten by the slice, standing over the sink or at the end of a barbecue, and nobody weighs it. Yet it is one of the easiest fruits to count: it is 91% water and contains almost no fat or protein. According to Ciqual, the French national food composition table run by the food safety agency ANSES, 100 g of watermelon flesh provides 35.4 kcal, 7.9 g of sugar, 0.5 g of fibre and 0.7 g of protein.

If you have seen a different number, that is normal. The USDA, the US Department of Agriculture, lists 30 kcal per 100 g and Norway’s Matvaretabellen 36 kcal. Two things explain the gap. First, the fruits analysed were not equally sweet: 6.2 g of sugar per 100 g in the USDA sample, 7.9 g in the Ciqual one. Second, the calculation methods differ. Ciqual uses the factors from the EU regulation on food information to consumers, 4 kcal per gram of carbohydrate, while the USDA uses food-specific factors and counts only 3.6 kcal per gram of carbohydrate for watermelon. Recalculated with the EU factors, the USDA watermelon would come to about 33 kcal.

The figure per 100 g is not much use until you know how much your portion weighs. With watermelon, the trap is the rind: a slice served with its skin weighs a lot more than what you actually eat. The weights in the table are therefore for the flesh only, without the rind.

For common portions, we used the reference weights from the food tables. Matvaretabellen counts 80 g for a slice, which is also one portion of fruit in the UK NHS “5 A Day” guidance. The USDA uses 152 g for a cup of diced watermelon and 286 g for a wedge cut from a large watermelon, one sixteenth of the fruit.

At the shop, watermelon is sold by weight, rind included, and that is often the number that sticks in your head. But the green rind and the white layer make up roughly half of the fruit. According to Matvaretabellen, the edible part is only 46% of the total weight; the USDA counts 48% waste (rind, seeds and cutting loss), which leaves 52% flesh. A 6 kg watermelon therefore gives only about 3 kg of flesh.

To give you a sense of scale, the USDA describes a large watermelon 38 cm long and 19 cm across that yields 4.5 kg of flesh: about 1,355 kcal by its own values, 1,600 kcal with Ciqual’s, and more than 4 litres of water.

Its very sweet taste gives watermelon a reputation as a treat, but the numbers are more nuanced. With 7.9 g of sugar per 100 g according to Ciqual (3.6 g of fructose, 1.8 g of glucose and 2.5 g of sucrose), watermelon has about as much as an orange, less than an apple and half as much as a banana or grapes. What really sets it apart is its fibre: barely 0.5 g per 100 g, against 2.7 g for an orange or a banana.

Its glycaemic index (GI), on the other hand, is high. The international tables published in 2008 list two measurements, both made in Australia: 72 and 80, on a scale where glucose scores 100. That figure describes how fast the carbohydrates raise blood sugar, not how much carbohydrate there is. The glycaemic load, which takes the portion into account, better reflects the real effect: it is only 4 to 5 for a 120 g serving, which contains just 6 g of available carbohydrate. A large 300 g plate comes to about 11 to 12 by our calculation. To keep the two ideas apart, read our guide to the glycaemic index.

Watermelon and melon often end up in the same fruit salad, but they are not twins. In all three tables, orange-fleshed melon has more calories: only slightly at the USDA (34 kcal per 100 g against 30 for watermelon) and in Matvaretabellen (38 against 36), much more in Ciqual, which analyses the very sweet French Charentais melon (59.8 kcal and 10.6 g of sugar per 100 g, against 35.4 kcal and 7.9 g for watermelon). Honeydew melon, for its part, is lighter than watermelon: 22.2 kcal in Ciqual and 31 kcal in the Norwegian table. In return, orange-fleshed melon provides far more vitamin C (36.7 mg per 100 g against 8.1 mg according to the USDA). All the figures by variety and by slice are in our guide to melon calories.

The name says it all: 91% of a watermelon’s weight is water, according to all three tables. A 300 g plate of flesh therefore provides about 270 ml of water, the equivalent of a big glass. EFSA, the European Food Safety Authority, considers an intake of 2 litres of water a day adequate for women and 2.5 litres for men: on a hot day, watermelon genuinely helps you get there. Our guide on hydration covers your needs and other water-rich foods.

Watermelon is also a notable source of lycopene, the pigment from the carotenoid family that turns its flesh red. The USDA measures 4.5 mg per 100 g, against 2.6 mg in raw tomato. The amount varies a lot from one variety to another: a US Department of Agriculture study of 50 red-fleshed varieties found 3.3 to 10 mg per 100 g, and 16 of the 33 seedless varieties fell into the high or very high categories.

For the rest, watermelon is modest. It provides 4 to 8 mg of vitamin C per 100 g depending on the table, six to eleven times less than an orange (47.5 mg according to Ciqual), a little potassium (100 to 112 mg) and almost no protein (0.6 to 0.8 g). It is a fruit for cooling down and enjoying, not a vitamin powerhouse.

What about yellow watermelon? None of the three tables has a separate entry for it. In the absence of data, at Ember we count it like red watermelon, 30 to 36 kcal per 100 g, without expecting it to supply lycopene, since that is the pigment that makes the flesh red.

Blended, watermelon becomes juice in seconds, with no added sugar. The calories do not change: a homemade juice made from 300 g of flesh provides the same 90 to 106 kcal as the plate of fruit. What changes is how its sugar is classified. The World Health Organization (WHO) counts the sugars naturally present in fruit juices as free sugars, like table sugar, while its recommendation does not apply to the sugars in fresh fruit. It advises keeping free sugars below 10% of energy intake, ideally below 5%, or about 25 g a day. That juice from 300 g of flesh already contains 19 to 24 g of sugar. The UK’s NHS, for its part, limits fruit juice and smoothies to 150 ml a day in total.

Watch out for watermelon-flavoured drinks too. In Norway’s food table, a watermelon slush has 53 kcal per 100 g, with 4.9 g of added sugar: more than the fruit itself. As for dried, shelled watermelon seeds sold as a snack, they are a different food altogether: 557 kcal per 100 g according to the USDA, with 28 g of protein and 47 g of fat.

With 30 to 36 kcal per 100 g, watermelon is one of the foods with the lowest energy density: you can eat a generous plate for very few calories. A 2019 study tested this with 33 adults with overweight or obesity. For four weeks, they ate two cups of fresh watermelon (92 kcal) every day, then, after a break, the same calories in the form of low-fat cookies. Watermelon was more filling: less hunger, less desire to eat and a stronger feeling of fullness. After four weeks of watermelon, average weight had dropped by 0.5 kg (from 89.4 to 88.9 kg), while it rose by 0.6 kg with the cookies. The study is small, every participant started with watermelon, and it was funded by the US National Watermelon Promotion Board: it mainly shows that a filling fruit can usefully replace a sugary snack, not that watermelon makes you lose weight.

You will sometimes read that watermelon has “negative calories”, because digesting it would burn more energy than it provides. That is not true. Digesting and storing food, known as diet-induced thermogenesis, accounts for only 5 to 15% of daily energy expenditure on a mixed diet, according to a reference review. Most of the calories in 100 g of watermelon therefore still count. Our guide to zero-calorie foods looks at this myth in detail.

The real trap lies elsewhere: the mono diet. A “watermelon only” day provides almost no protein, since 1 kg of flesh contains only 6 to 8 g, and hardly any fat. Better to keep it as a dessert or a snack.