Home›Glossary›Glycemic Index

Glycemic Index

General

Glycemic Index (GI)

A 0–100 scale measuring how quickly a food raises blood sugar per gram of carbohydrate, relative to a reference food like pure glucose.

Definition

Glycemic Index (GI) ranks how quickly a specific food raises blood sugar relative to a reference food (pure glucose or white bread), based on a standardised 50-gram carbohydrate portion. It's scored on a 0–100 scale, with higher values indicating a faster, sharper blood sugar rise.

GI is useful for comparing the inherent carbohydrate quality of different foods on equal footing, but it doesn't account for how much of a food you actually eat — that's what glycemic load adds. The Glycemic Index Calculator looks up or estimates a food's GI value for reference.

Formula

GI is determined experimentally, not calculated from a formula — it's measured by tracking blood glucose response in test subjects after eating a 50g carbohydrate portion of a food, compared to the same response after eating pure glucose:

GI = (Blood glucose response to test food ÷ Blood glucose response to glucose) × 100

Worked Example

White bread has a GI of approximately 75, in the "high" category, meaning it raises blood sugar quickly and sharply. Lentils have a GI of approximately 32, in the "low" category, producing a much more gradual rise for the same 50g carbohydrate portion.

Key Things to Know

  • GI alone doesn't account for portion size: pair it with glycemic load for a more realistic picture of a specific meal.
  • Ripeness and preparation affect GI: a ripe banana has a higher GI than an unripe one; cooking method affects starchy foods significantly.
  • Useful for comparing foods, not planning exact meals: GI works best as a general food-ranking tool rather than a precise clinical measurement.
  • Relevant beyond diabetes management: GI also matters for general energy level management and athletic performance nutrition.

Related Calculators

Glycemic Index Calculator →Look up the glycemic index of common foods and see whether it falls in the low, medium, or high GI category, plus an estimated glycemic load per serving.
Glycemic Load Calculator →Calculate glycemic load from a food's glycemic index and carbohydrate content per serving, and see whether it falls in the low, medium, or high GL range.
Diabetes Risk Calculator →Estimate your Type 2 diabetes risk score using the standard ADA-style risk test based on age, weight, activity, and family history in under a minute.

Related Terms

Glycemic LoadGlycemic Load (GL)HOMA-IRHomeostatic Model Assessment of Insulin Resistance

Frequently Asked Questions

A GI of 55 or below is considered low, 56–69 medium, and 70 or above high. Foods lower on the scale generally produce a slower, more gradual blood sugar rise.
Most non-starchy vegetables, legumes, and many whole grains tend to have a low GI, while white bread, white rice, and many refined or highly processed carbohydrates tend to have a high GI.
GI is measured through actual human testing, and results can vary based on ripeness, preparation method, and the specific study's methodology, which is why different databases sometimes list slightly different values for the same food.
No — GI measures blood sugar impact per gram of carbohydrate at a standardised reference portion, while [glycemic load](/glossary/glycemic-load/) factors in your actual serving size, giving a more realistic picture of a specific meal's effect.
Yes — cooking time and preparation can meaningfully change a food's GI; for example, pasta cooked al dente typically has a lower GI than the same pasta cooked until very soft.