Watermelon has a high glycemic index, around 80, but a cup of diced watermelon has a low glycemic load of about 9. That combination sounds contradictory, but it isn't: watermelon is mostly water, so a normal cup of cubes carries very little actual carbohydrate, and the real-world impact on blood sugar is mild despite the high index number.
The numbers
Harvard Health cites watermelon at a glycemic index of about 80, the same figure used across this site. A one-cup serving of diced watermelon (about 152 grams) has only about 11 grams of available carbohydrate, because watermelon is roughly 92 percent water.
| Food | GI | Available carbs (g) | Glycemic load |
|---|---|---|---|
| Watermelon, 1 cup diced (152 g) | 80 (high) | 11 | 9 (low) |
| Watermelon, 2 cups diced | 80 (high) | 22 | 18 (medium) |
| White rice, ½ cup cooked | 73 (high) | 22 | 16 (medium) |
Harvard Health puts the glycemic load of a serving at about 5. A full cup of diced watermelon has a little more carbohydrate, about 11 grams, which works out to a glycemic load of about 9. Either way, it's low. Run your own portion in the glycemic load calculator, or see watermelon next to other everyday foods in our GI vs. GL chart.
Sources: Harvard Health, The lowdown on glycemic index and glycemic load (GI 80; glycemic load of about 5 per serving); Atkinson, Foster-Powell and Brand-Miller, International tables of glycemic index and glycemic load values: 2008, Diabetes Care (raw watermelon 76 ± 4); USDA FoodData Central (carbohydrate and fiber per cup). GL bands per serving: low 10 or under, medium 11 to 19, high 20 or more.
Why the GI number alone is misleading here
Glycemic index measures how fast a food's carbohydrate raises blood sugar, compared gram for gram with pure glucose. It says nothing about how much carbohydrate is actually in a real serving. Watermelon is the textbook example: its carbs act fast, but there simply aren't many of them in a normal portion, so the real effect is mild. If you judged watermelon by its index number alone, you'd avoid it for no reason. We go deeper into this exact distinction, using watermelon as the lead example, in our full glycemic index vs. glycemic load guide.
How ripeness and juice change it
Juice is where watermelon stops being gentle. Juicing removes what little fiber the fruit has, and a glass goes down far faster than a bowl of cubes, so it's easy to drink the carbohydrate of two or three cups at once. Two cups' worth of juice carries far more carbohydrate than the single cup of cubes most people would eat.
Ripeness matters less. No reliable published test compares the GI of riper and less ripe watermelon, so the fair statement is only that sweeter, riper fruit carries somewhat more sugar per cup.
A cup of watermelon is a low-glycemic-load treat on its own; two cups reach the medium band, about the same as half a cup of white rice. The load adds up the way any food's does if you eat several servings in one sitting or pair it with other high-carb foods at the same meal, so it's the total plate that matters, not watermelon in isolation. For another fruit where ripeness changes the picture a lot, see our bananas page.
If you're managing gestational diabetes, our gestational diabetes grocery guide covers fruit alongside the rest of the cart; your care team's plan comes first.
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