How to calculate GI formula: Understanding the Glycemic Index and Load

December 28, 2025 •

3 min read

The Glycemic Index (GI) was first introduced in 1981 by David J. Jenkins and his co-workers to provide a new way to classify carbohydrates based on their physiological effects on blood glucose. To truly understand its impact on your health, it's essential to know how to calculate GI formula, or more accurately, how the values are determined and applied to your diet.

Quick Summary

Explaining the scientific method for experimentally determining a food's Glycemic Index (GI) and the more practical Glycemic Load (GL) formula. This guide covers the lab process, calculation formulas, influencing factors, and meal calculations.

Key Points

GI Formula: The official GI is calculated experimentally by comparing the incremental area under the curve (iAUC) of a test food's blood glucose response to that of a reference food.
Experimental Process: Determining a food's GI requires feeding a standard amount of available carbohydrate to subjects and measuring blood glucose levels in a lab setting.
GL Formula: Glycemic Load (GL) is a practical measure for a real-world serving, calculated as $GL = (GI \times \text{grams of available carbohydrate per serving}) / 100$.
Influencing Factors: A food's GI value can be altered by cooking methods, ripeness, processing, and the combination of foods in a meal.
Meal Calculation: The overall GI of a meal is a weighted average based on the GI and carbohydrate content of its individual components.
Practical Application: For daily diet management, focusing on Glycemic Load (GL) is often more useful than just the Glycemic Index (GI), as GL accounts for portion size.

What is the GI formula? The scientific method explained

The Glycemic Index (GI) of a food is not calculated with a simple home formula, but rather determined through a standardized scientific experiment. This involves measuring how the body's blood glucose responds after consuming a fixed amount of carbohydrates from a test food compared to a reference food. The final GI value is an average of the results from multiple test subjects.

The Experimental Process

The process for determining GI involves feeding healthy volunteers a test food portion containing 50 grams of available carbohydrates after an overnight fast. Their blood glucose is monitored over two hours. The same subjects repeat the process with a reference food (glucose or white bread) which is assigned a GI of 100. The incremental area under the blood glucose response curve (iAUC) is calculated for both the test food and the reference food.

The GI Calculation Formula

The GI for an individual is calculated as:

$GI{\text{Individual}} = (\text{iAUC}{\text{Test Food}} / \text{iAUC}_{\text{Reference Food}}) \times 100$

The reported GI value for the food is the average of individual GI results.

The practical GI formula: Glycemic Load (GL)

For practical dietary management, the Glycemic Index has limitations because it doesn't consider portion size. The Glycemic Load (GL) formula offers a more realistic measure of how a typical serving affects blood sugar.

The GL Calculation Formula

The GL is calculated with the following formula:

$GL = (\text{GI} \times \text{grams of available carbohydrate per serving}) / 100$

This shows that a high-GI food in a small portion may have a low GL, while a low-GI food in a large portion could have a higher GL.

What factors influence the GI and GL formulas?

Several factors can influence a food's glycemic response:

Cooking and Processing: More processed or longer-cooked foods generally have a higher GI. For example, mashed potatoes have a higher GI than baked potatoes.
Ripeness: The ripeness of fruits, such as bananas, affects their GI.
Combining Foods: Adding protein, fat, or fiber to a meal can slow digestion and reduce the overall glycemic response.
Individual Variation: A person's glycemic response can vary daily due to factors like insulin sensitivity.

Combining GI values for a full meal

To estimate a meal's GI, calculate a weighted average. Determine the proportion of total carbohydrates each food contributes, multiply each food's GI by its carbohydrate proportion, and sum these values.

GI vs. GL: A comparison

Understanding the differences between GI and GL is key for making informed dietary choices.


Feature	Glycemic Index (GI)	Glycemic Load (GL)
Measurement	Glycemic potential relative to a reference food.	Overall glycemic impact of a specific portion size.
Scale	0 to 100, where 100 is pure glucose.	Calculated value, not limited to 100.
Key Factors	Type of carbohydrate, cooking, processing.	Combines a food's GI and carbohydrate amount per serving.
Practical Use	Comparing glycemic quality of different carb foods.	Planning meals and managing overall blood sugar.
Example	Watermelon has a high GI (~76).	Typical serving of watermelon has a low GL (~8) due to low carb density.

Conclusion: How to best use GI and GL

While the scientific GI formula explains the research, the practical Glycemic Load is more useful for daily health. Understanding both GI and GL allows for informed carbohydrate choices. Prioritizing lower GI options, combining them with healthy fats, proteins, and fiber, and managing portion sizes are effective strategies. These are tools for a balanced diet, not strict rules. For comprehensive GI information, refer to databases like the University of Sydney's.

Sources

: Omni Calculator. (n.d.). Glycemic Index Calculator. https://www.omnicalculator.com/health/gi
: Aspedan. (n.d.). Glycemic Index Calculator. https://aspedan.com/glycemic-index-calculator/
: Glycemic Index Guide. (n.d.). Glycemic Load Explained: Definition, Formula, Benefits, and.... https://glycemic-index.net/glycemic-load/
: Wikipedia. (n.d.). Glycemic index. https://en.wikipedia.org/wiki/Glycemic_index
: Better Health Channel. (n.d.). Carbohydrates and the glycaemic index. https://www.betterhealth.vic.gov.au/health/healthyliving/carbohydrates-and-the-glycaemic-index

Frequently Asked Questions

The official GI is determined through a scientific experiment. A test food containing 50 grams of digestible carbohydrates is fed to healthy subjects, and their blood glucose response over two hours is measured. This response is compared to that of a reference food (pure glucose) to calculate the GI.

The scientific formula for GI is $GI = (\text{iAUC of test food} / \text{iAUC of reference food}) \times 100$, where iAUC stands for incremental area under the blood glucose response curve.

Glycemic Load (GL) is a practical measure that considers both the food's GI and the amount of carbohydrate in a typical serving. The formula is $GL = (\text{GI} \times \text{grams of available carbohydrate per serving}) / 100$.

You can estimate the GI of a meal by calculating the weighted average of the GI values for each carbohydrate-containing food item. This requires knowing the GI and available carbohydrate content of each component in your meal.

The GI value can be affected by several factors, including the food's ripeness, cooking method (e.g., al dente vs. soft pasta), processing (e.g., whole grain vs. refined), and the presence of other nutrients like fat, protein, or fiber.

GL is often more practical because it accounts for portion size. A food with a high GI but low carbohydrate content (like watermelon) can have a low GL, indicating a minimal effect on blood sugar in a typical serving.

No, it is not possible to accurately calculate a food's GI at home. The process is a standardized scientific experiment involving blood glucose measurements in multiple subjects. For diet planning, it is best to rely on published GI tables or online databases.

No, different varieties of the same food can have different GI values. For example, the GI of rice varies depending on whether it is long-grain, short-grain, or Basmati. This is also true for other carbohydrate-rich foods like potatoes and bananas.

The way a food is cooked directly influences its GI. Cooking time breaks down the food's cellular structure, making the carbohydrates easier to digest and leading to a higher GI. For example, softer pasta has a higher GI than al dente pasta.