Calories in Watermelon: Per Slice and Per Cup

One cup of diced watermelon (154 g) contains 46 calories, according to USDA FoodData Central — making it one of the lowest-calorie fruits you can eat by volume.

Watermelon earns its reputation as a summer staple on the numbers alone: 92% water, a glycemic load well below the threshold of concern at typical serving sizes, and a flavour that makes it easy to swap for higher-calorie desserts. Understanding exactly how the calories stack up by portion helps you fit it confidently into any eating pattern.

Calories and Macros by Portion

The table below uses USDA FoodData Central data (FDC ID 167765, raw watermelon) for all values.

Serving	Weight	Calories	Carbs	Sugar	Fiber	Protein	Fat
100 g	100 g	30 kcal	7.6 g	6.2 g	0.4 g	0.6 g	0.2 g
1 cup diced	154 g	46 kcal	11.5 g	9.4 g	0.6 g	0.9 g	0.2 g
1 wedge (1/16 of melon)	~286 g	86 kcal	21.6 g	17.7 g	1.1 g	1.7 g	0.4 g
2 cups diced	308 g	93 kcal	23.2 g	18.9 g	1.2 g	1.9 g	0.4 g
Half a small melon (~3 kg)	~1,500 g	450 kcal	113 g	92 g	6 g	9 g	2.2 g

A standard restaurant-style thick wedge (about 1/16 of a full melon) comes in at roughly 86 calories. At a backyard cookout, two of those wedges puts you near 172 calories — still modest, but worth knowing if you are actively tracking.

What the Macros Tell You

Watermelon is almost entirely carbohydrate and water. Fat is negligible (under 0.5 g per cup) and protein is minimal (under 1 g per cup), so the caloric contribution is driven almost entirely by sugar — specifically fructose, glucose, and sucrose in roughly equal parts.

The fiber content is low at 0.6 g per cup, which is relevant for blood sugar response. Because there is little fiber to slow digestion, the natural sugars absorb relatively quickly, giving watermelon a high glycemic index (GI of approximately 72 on a 0–100 scale). However, the glycemic load per cup is only about 5 — well within the low range (under 10) — because the serving contains relatively few total carbohydrates. Use our net carbs calculator to see how a specific portion fits your daily carb target.

Watermelon also provides a meaningful dose of lycopene (6–7 mg per cup), citrulline (amino acid supporting nitric oxide production), and vitamin C (approximately 21% of the daily value per cup). These are genuine nutritional bonuses, not marketing claims.

Does It Fit Your Goals?

Weight loss. Watermelon is an excellent choice on a calorie deficit. Its 92% water content means a two-cup serving weighs over 300 g but delivers fewer than 95 calories, providing physical volume and mild sweetness that can displace higher-calorie dessert options. It sits well within any calorie deficit plan. The main caveat: because it digests quickly and has limited fiber and protein, it may not sustain fullness as long as fruit with more fiber (berries, pears, apples). Pairing it with a protein source — Greek yogurt, cottage cheese — extends satiety.

Blood sugar management. The low glycemic load per cup makes moderate watermelon portions generally manageable for most people monitoring blood sugar. The key word is moderate: a one-cup serving (46 calories, 9.4 g sugar) is very different from three to four wedges eaten quickly on an empty stomach, which can deliver 60-plus grams of rapidly absorbed sugar. If you are tracking carbohydrates for diabetes or prediabetes, use the glycemic load calculator to check how your planned serving scores against your daily targets. People using continuous glucose monitors often find that eating watermelon after a protein-containing meal blunts the glucose response significantly compared with eating it alone.

Hydration. At 92% water by weight, watermelon contributes meaningfully to daily fluid intake — a cup provides roughly 140 mL of water alongside its electrolytes (potassium: 170 mg per cup). This makes it a practical choice before or after exercise in warm weather.

Photograph your plate before you eat and let CalEye log the calories in seconds — no measuring cups required.