Net carbs are calculated by subtracting fiber and, in some cases, sugar alcohols from the total carbohydrates listed on a nutrition label. The basic formula is: net carbs = total carbohydrates minus fiber minus sugar alcohols. But the details matter, because not all sugar alcohols are treated equally, and labels in different countries handle fiber differently.
The Basic Formula
Start with the total carbohydrates on the nutrition label. Subtract all the grams of dietary fiber. If the product contains sugar alcohols, subtract half of those grams. What remains is your net carb count.
Here’s a practical example: a protein bar lists 29 grams of total carbohydrates, 3 grams of fiber, and 18 grams of sugar alcohols. First, subtract the fiber: 29 minus 3 gives you 26. Then calculate half the sugar alcohols: 18 divided by 2 equals 9. Subtract that from 26 and you get 17 grams of net carbs.
The logic behind this calculation is straightforward. Your body can’t break fiber down into glucose, so it passes through your digestive system without raising blood sugar. Sugar alcohols are partially absorbed, which is why you only subtract half rather than the full amount.
Why Fiber Gets Fully Subtracted
Fiber is a carbohydrate your body simply cannot digest. Unlike starches and sugars, which get broken down into glucose and enter your bloodstream, fiber travels through your gut intact. This is true for both types of fiber. Insoluble fiber (found in whole grains, nuts, and vegetables) moves through your system promoting regularity. Soluble fiber (found in oats, beans, and fruits) dissolves in water and forms a gel that actually slows digestion, which can help prevent blood sugar spikes after a meal.
Because neither type raises blood sugar in a meaningful way, both are fully subtracted from total carbs when calculating net carbs. Some soluble fibers do get fermented by gut bacteria and produce a small number of calories, but the effect on blood glucose is negligible for the purposes of carb counting.
Why Sugar Alcohols Are Only Half Subtracted
Sugar alcohols are sweeteners commonly found in “sugar-free” or “low-carb” products. They include ingredients like xylitol, sorbitol, maltitol, and erythritol. Unlike fiber, sugar alcohols are partially absorbed by the body, meaning they do contribute some carbohydrate energy, just less than regular sugar.
The standard guideline, used by UCSF’s Diabetes Teaching Center and other clinical resources, is to subtract half of the sugar alcohol grams from total carbohydrates. This rule of thumb works as a reasonable average across the different types of sugar alcohols, since some are absorbed more than others.
Maltitol is one to watch carefully. It’s widely used in sugar-free candy and chocolate, and it has a relatively high glycemic impact compared to other sugar alcohols. The half-subtraction rule still applies, but if you’re tracking carbs closely for blood sugar management, products heavy in maltitol may affect you more than you’d expect from the net carb number alone.
The Erythritol Exception
Erythritol is the one sugar alcohol that many people subtract completely. It’s absorbed into the bloodstream but excreted unchanged in urine, contributing essentially zero calories and having no measurable effect on blood sugar. Many keto and low-carb communities treat erythritol the same as fiber, subtracting 100% of its grams. This isn’t an official clinical guideline, but the logic is sound given erythritol’s unique metabolism. If you’re using the half-subtraction rule for erythritol, you’re being conservative, not wrong.
How Allulose Fits In
Allulose is a newer sweetener that creates a labeling headache. It’s technically a sugar (a “rare sugar” found naturally in small amounts in figs and raisins), so the FDA requires it to be included in total carbohydrates on U.S. nutrition labels. However, the FDA allows manufacturers to exclude allulose from both “Total Sugars” and “Added Sugars” on the label, because it contributes only about 0.4 calories per gram (compared to 4 calories per gram for regular sugar) and has minimal impact on blood sugar.
This means a product sweetened with allulose may look higher in total carbs than it functionally is. If the label breaks out allulose separately, you can subtract its full gram amount when calculating net carbs, just as you would with fiber. Some brands already do this math for you and print a net carb count on the front of the package, but check the nutrition facts panel to verify how they arrived at that number.
A Common Mistake With Imported Products
If you buy food labeled in the European Union or the UK, the carbohydrate number on the label already excludes fiber. Under EU regulations, fiber is listed as a separate line item and is not counted within the carbohydrate total. So if you subtract fiber again, you’re double-counting and will underestimate your actual net carbs.
On a U.S. label, fiber is listed as a subset of total carbohydrates, indented underneath it. On an EU label, the carbohydrate line shows what Americans would already consider “non-fiber carbs,” and fiber appears separately below. The practical takeaway: if you’re reading a European nutrition label, the carbohydrate number is already closer to net carbs. You’d only need to adjust for sugar alcohols (listed as “polyols” on EU labels) if they’re present.
Putting It All Together
For a standard U.S. nutrition label, here’s your calculation:
- Step 1: Find total carbohydrates
- Step 2: Subtract all grams of dietary fiber
- Step 3: Subtract half the grams of sugar alcohols (or all of them if the only sugar alcohol is erythritol)
- Step 4: Subtract all grams of allulose, if listed separately
The result is your net carb count. This number represents the carbohydrates that will meaningfully affect your blood sugar. It’s the figure most people on keto, low-carb, or diabetic diets use to track their intake.
One thing worth noting: “net carbs” is not an FDA-regulated term. There’s no official government definition, and manufacturers who print net carbs on packaging may use slightly different math. Always check the nutrition facts panel and ingredient list so you can run the calculation yourself. The label gives you everything you need.