The glycemic index (GI) of a food is calculated by comparing the blood sugar response it produces to the response from pure glucose. Specifically, you divide the incremental area under the blood glucose curve after eating a test food by the incremental area under the curve after eating pure glucose, then multiply by 100. The result is a score from 0 to 100, where pure glucose sits at the top.
That formula sounds simple, but actually producing a reliable GI value requires controlled lab testing with human participants, blood draws at precise intervals, and careful math. Here’s how it all works.
The Core Formula
The glycemic index equation looks like this:
GI = (iAUC of test food ÷ iAUC of glucose) × 100
The “iAUC” stands for incremental area under the curve. It represents how much your blood sugar rises above your fasting (baseline) level over a two-hour window after eating. Only the rise above baseline counts. Any dips below your starting blood sugar level are ignored.
So if a food produces a blood sugar curve that covers half the area of the glucose curve, its GI is 50. If it produces almost the same spike as glucose, its GI approaches 100. Foods are then classified into three ranges: low GI (1 to 55), medium GI (56 to 69), and high GI (70 or above).
How the Blood Sugar Curve Is Measured
To generate the data that goes into that formula, researchers run a standardized test. A minimum of 10 healthy participants fast for at least 12 hours overnight. On one visit, each person eats a portion of pure glucose containing 50 grams of available carbohydrate. On a separate visit, they eat a portion of the test food that also contains 50 grams of available carbohydrate. The amount of food varies depending on carb density: 50 grams of carbs from white bread looks very different from 50 grams of carbs from watermelon.
Blood samples are taken at fasting and then at regular intervals, typically every 15 minutes during the first hour and at 90 and 120 minutes. Some protocols sample every 10 minutes for more precision. Each blood draw gives a glucose concentration reading. When you plot all those readings on a graph with time on the horizontal axis and blood sugar on the vertical axis, you get a curve showing how blood sugar rose and then fell back toward baseline.
Calculating the Area Under the Curve
The area under that blood sugar curve is what makes the GI calculation possible, but you can’t just eyeball it. Researchers use a mathematical method called the trapezoidal rule, which breaks the curve into a series of trapezoids (shapes with two parallel sides) between each pair of time points. You calculate the area of each trapezoid, then add them all together.
For each time interval, the formula is straightforward: take the average of the two blood sugar readings (both measured as the rise above fasting level), then multiply by the number of minutes between them. Do this for every consecutive pair of readings from time zero to 120 minutes, sum the results, and you have the incremental area under the curve. The key detail is that only values above the fasting baseline are included. If blood sugar dips below baseline at any point, that segment counts as zero, not as a negative area.
Each participant ends up with two iAUC values: one for glucose, one for the test food. You calculate each person’s individual GI ratio, then average all the individual ratios across the group. This approach (averaging individual ratios rather than averaging group curves) is the recommended method because it accounts for the natural variation between people.
Why You Can’t Calculate GI at Home
Even with a continuous glucose monitor or a fingerstick meter, calculating a meaningful GI at home is unreliable. Research from Tufts University found that the GI of the same food eaten by the same person can vary by an average of 20 percent from one occasion to the next. Between different people, variability averages 25 percent. In some cases, the same individual’s GI reading for the same food differed by more than 60 points between trials.
That level of inconsistency means a food that registers as low GI on Monday could register as high GI on Wednesday, even under identical conditions. Factors like your insulin sensitivity, recent sleep, stress, and longer-term blood sugar control all influence the result. Insulin response and longer-term glucose control (measured by HbA1c) accounted for the largest chunks of this variability, roughly 15 to 16 percent each. This is why published GI values come from averaging results across multiple people on multiple test days.
Factors That Change a Food’s GI
Even in a lab, the GI of a food is not fixed. How that food is prepared, cooked, and served can shift its score dramatically.
Cooking Method
Heat transforms starch granules from a tightly packed crystalline structure into a swollen gel, a process called gelatinization. Gelatinized starch is far easier for your digestive enzymes to break down, which means faster absorption and a higher blood sugar spike. Different cooking methods produce different degrees of this transformation. Extrusion (the industrial process used to make puffed snacks and some cereals) damages and gelatinizes starch more thoroughly than boiling or baking, which is one reason many processed grain products have high GI values.
Cooling and Reheating
When cooked starchy foods cool down, some of the gelatinized starch molecules reassemble into a more resistant crystalline structure. This is called retrogradation, and it creates resistant starch that your enzymes can’t break down as easily. The practical result: cold potato salad has a lower GI than a freshly boiled hot potato. Repeated cycles of cooling and reheating form progressively more resistant starch, further lowering the glycemic response.
Acidity
Adding acidic ingredients slows stomach emptying, which slows the rate at which glucose enters your bloodstream. Adding vinegar and olive oil as a dressing to cooked and cooled potatoes reduced the GI by 43 percent compared to the same potatoes served hot without dressing. Refrigeration alone reduced it by 26 percent, so the vinegar contributed a meaningful additional reduction on top of the cooling effect.
Particle Size and Physical Structure
Whole, intact grains have a lower GI than finely ground flour made from the same grain. Larger particles have less surface area exposed to digestive enzymes, so they break down more slowly. This is why steel-cut oats produce a different blood sugar response than instant oats, and why whole fruit has a lower GI than fruit juice.
GI vs. Glycemic Load
One important limitation of the glycemic index is that it’s based on a fixed amount of carbohydrate (50 grams), not a realistic serving. Watermelon has a high GI of around 76, but a typical slice contains only a small amount of carbohydrate, so its real-world impact on blood sugar is modest. Glycemic load (GL) accounts for this by combining GI with actual serving size.
The formula is: GL = (GI × grams of carbohydrate per serving) ÷ 100. A GL of 10 or below is considered low, 11 to 19 is medium, and 20 or above is high. So that slice of watermelon, despite its high GI, typically lands in the low GL range. If your goal is to understand how a specific portion of food will affect your blood sugar, glycemic load is the more practical number.