Calories in Watermelon UK: The Lowest Calorie Fruit Explained (2026)

Watermelon contains 30 kcal per 100g — the lowest calorie mainstream fruit available in UK supermarkets. A standard 300g slice contains 90 kcal. Watermelon is Free Food on Slimming World with no Swips cost at any portion.

Watermelon's low calorie count comes from its water content — it is 92% water by weight. A large 500g portion contains just 150 kcal. For anyone looking for high-volume, low-calorie snacks that genuinely fill a plate, watermelon is the most efficient option available.

This guide covers calories for every watermelon portion size, seedless vs seeded, watermelon juice, and how watermelon compares to other fruit for weight loss.

How Many Calories in Watermelon?

Watermelon contains 30 kcal per 100g of flesh. Seedless and seeded varieties have identical calorie counts. A standard supermarket pre-cut slice (approximately 300g) contains 90 kcal.

Pre-cut watermelon chunks from supermarket deli sections (Tesco, Sainsbury's, M&S) are typically sold in 200–400g bags and cost 30–120 kcal per bag — one of the lowest calorie ready-to-eat snack options available.

Is Watermelon Free on Slimming World?

Watermelon is Free Food and a speed food on Slimming World — no Swips cost at any portion size. All fresh watermelon qualifies. Watermelon juice carries a Swips cost because pressing removes the fibre that makes whole fruit Free.

At 30 kcal per 100g, watermelon is among the lowest calorie foods on the Free Food list — lower than most vegetables. A 500g portion costs 150 kcal, occupies a large bowl, and provides genuine fullness from the bulk and water content.

Is Watermelon the Lowest Calorie Fruit?

Watermelon at 30 kcal per 100g is the lowest calorie mainstream fruit in UK supermarkets, equal to strawberries (also 30–32 kcal per 100g). Blackcurrants at 28 kcal per 100g are marginally lower but not widely available as a fresh fruit year-round.

Compared to other popular fruits: bananas are 89 kcal per 100g, grapes are 67 kcal, and blueberries are 57 kcal. Watermelon delivers the most volume per calorie of any commonly eaten UK fruit — a 300g bowl is 90 kcal where the equivalent weight of banana would be 267 kcal.

How Many Calories in Watermelon Juice?

Watermelon juice contains approximately 25–30 kcal per 100ml — similar to the whole fruit per gram, but without the fibre. A 250ml glass of fresh watermelon juice contains 63–75 kcal and costs around 3–4 Swips on Slimming World because juice is not Free Food regardless of calorie content.

The Swips cost of juice reflects the absence of fibre rather than the calorie count. 300g of whole watermelon (90 kcal, 0 Swips) satisfies hunger more effectively than 250ml of watermelon juice (70 kcal, 3–4 Swips) because the fibre and chewing time of whole fruit slows digestion.

Is Watermelon Good for Weight Loss?

Watermelon is effective for weight loss because of the combination of low calorie density (30 kcal per 100g) and high water content (92%). A 400g bowl contains 120 kcal and is genuinely filling because of the bulk, chewing time, and water load.

Watermelon also contains citrulline, an amino acid that the body converts to arginine. A 2019 study in the journal Nutrients (23 participants) found that watermelon consumption increased satiety and reduced post-meal appetite compared to a low-fat biscuit snack of equal calories. The practical outcome: watermelon as an afternoon snack reduces the likelihood of reaching for higher-calorie options before dinner.

How Does Watermelon Compare to Other Fruit?

Watermelon and strawberries are the lowest calorie fruits in this table and both are Free speed foods on Slimming World. For maximum volume and satiety per calorie, watermelon is the most effective fruit snack available.

Blueberries are another low-calorie Free Food speed fruit. See Calories in Blueberries UK for a full portion table and frozen blueberry comparison.