Glycemic Index vs. Glycemic Load: How to Calculate the Blood Sugar Impact of a Serving
October 29, 2026
The glycemic index (GI) ranks carbohydrate-containing foods by how quickly they raise blood glucose compared with a reference food, usually glucose, which scores 100. Foods are commonly classed as low GI (55 or less), medium (56 to 69), or high (70 or more). But the GI is measured for a portion containing 50 grams of available carbohydrate, which may be a lot of food (about 1 kg of watermelon) or little (a few tablespoons of sugar). The glycemic load (GL) corrects for this by accounting for the amount of carbohydrate in a realistic serving.
The formula is GL = GI × grams of available carbohydrate in the serving ÷ 100. A typical serving of watermelon (120 g) has about 6 g of carbohydrate and a GI around 72, so GL = 72 × 6 ÷ 100 = 4.3, which is low. A cup of cooked white rice (150 g) with about 36 g of carbohydrate and a GI around 73 has GL = 26, which is high.
Skip the math and get your answer instantly:
Open the Macro Calculator →Examples
GL ranges: low is 10 or less, medium 11 to 19, high 20 or more per serving. Values vary by variety, ripeness, cooking, and the testing laboratory, so the numbers below are approximate. A baked potato has a high GI but, depending on size, a high GL; lentils have a low GI and moderate GL; an apple has a low GI and a low GL.
| Food (serving) | GI (approx.) | Carbs (g) | GL |
|---|---|---|---|
| Watermelon (120 g) | 72 | 6 | 4 |
| Apple (medium) | 38 | 25 | 10 |
| White rice, cooked (150 g) | 73 | 36 | 26 |
| Spaghetti, cooked (180 g) | 49 | 44 | 22 |
| Lentils, cooked (150 g) | 32 | 20 | 6 |
| Whole-grain bread (1 slice) | 55 | 14 | 8 |
| Glucose (10 g) | 100 | 10 | 10 |
Using GI and GL in Meals
Combining foods changes the impact: protein, fat, fiber, and acid (vinegar, lemon juice) slow digestion and lower the glycemic response of a meal. Eating rice with vegetables, beans, and chicken produces a smaller blood sugar rise than rice alone. Cooking methods matter: al dente pasta has a lower GI than overcooked pasta, and cooled-then-reheated potatoes and rice have a lower response due to resistant starch. Fruit juice has a higher GI than whole fruit. Portion size has a bigger effect than GI for most foods, which is why GL is more useful.
Who Might Benefit
People with diabetes or prediabetes may use GI and GL as one tool to manage blood glucose, alongside total carbohydrate counting, portion control, physical activity, and medical advice. Research suggests that low-GI diets can modestly improve blood sugar control and cholesterol, with weight loss benefits that are mixed. For healthy people, overall diet quality (whole grains, vegetables, legumes, fruit) matters more than precise numbers. Athletes may deliberately use high-GI foods for quick recovery during or after exercise.
Limitations
GI varies between individuals: studies find that the same food can produce very different glucose responses in different people, depending on gut bacteria, insulin sensitivity, and sleep. Continuous glucose monitors reveal this variation. GI values are tested on single foods in fasting people, while real meals are mixed. The GI says nothing about nutrition, so low-GI foods, like ice cream and chocolate, are not necessarily healthy. Use it as a guide, not a rule, and consult a dietitian for individual advice.
Frequently Asked Questions
What is the glycemic load formula?
GI × grams of carbohydrate in the serving ÷ 100.
What is considered a low glycemic load?
A load of 10 or less per serving.
Why is glycemic load more useful than glycemic index?
Because it accounts for the amount of carbohydrate in a realistic serving.
Does adding protein or fat lower the glycemic response?
Yes, they slow digestion and reduce the rise in blood glucose.
Is a low GI food always healthy?
No. Some low-GI foods are high in fat or sugar alcohols and poor in nutrients.