To calculate net carbs, start with total carbohydrates on the nutrition label, subtract all of the dietary fiber, and subtract half of any sugar alcohols. Formula: Net carbs = Total carbs – Fiber – (Sugar alcohols / 2). The idea is that fiber and many sugar alcohols pass through digestion without significantly raising blood glucose, so subtracting them gives a more accurate picture of the carbs that will hit your bloodstream.
The Net Carb Formula
In the US, the Nutrition Facts label lists:
- Total Carbohydrate
- Dietary Fiber
- Total Sugars (including Added Sugars)
- Sugar Alcohols (only if the manufacturer chooses, or the product makes a claim about them)
Start with total carbs and subtract the components that do not meaningfully raise glucose:
- Fiber: subtract 100% (fiber is not digested to glucose)
- Sugar alcohols (most): subtract 50%
- Erythritol and allulose: subtract 100%
- Maltitol: subtract only 25% (it raises glucose more than others)
Worked Examples
Example 1: A Plain Apple
1 medium apple: Total carbs 25 g, Fiber 4 g, Sugar alcohols 0 g.
Net carbs = 25 – 4 – 0 = 21 g
Example 2: A Cup of Cooked Lentils
Total carbs 40 g, Fiber 16 g, Sugar alcohols 0 g.
Net carbs = 40 – 16 – 0 = 24 g
Example 3: A Keto Bar
Label reads: Total carbs 22 g, Fiber 10 g, Sugar alcohols 8 g (erythritol).
Because erythritol is zero-glucose, subtract all 8 g.
Net carbs = 22 – 10 – 8 = 4 g
Example 4: A Sugar-Free Chocolate With Maltitol
Total carbs 18 g, Fiber 1 g, Sugar alcohols 10 g (maltitol).
Maltitol conservatively counts as 75% of a regular carb, so subtract 25% of 10 g = 2.5 g.
Net carbs = 18 – 1 – 2.5 = 14.5 g
Example 5: A Slice of 100% Whole-Grain Bread
Total carbs 20 g, Fiber 3 g, Sugar alcohols 0 g.
Net carbs = 20 – 3 – 0 = 17 g
Sugar Alcohols: Not All Equal
| Sugar Alcohol | Glycemic Index | Subtract | Notes |
|---|---|---|---|
| Erythritol | 0 | 100% | Nearly zero absorbed; some GI tolerance issues in large doses |
| Allulose | 0 | 100% | Technically a rare sugar; FDA excludes from added-sugar count |
| Xylitol | 12 | 50% | Moderate glucose effect; toxic to dogs |
| Sorbitol | 9 | 50% | Laxative at high doses |
| Mannitol | 0-10 | 50% | Often in sugar-free gum |
| Isomalt | 2 | 50% | Hard candies, gums |
| Lactitol | 3-6 | 50% | Chocolate, ice cream |
| Maltitol | 35-52 | 25% | Most glycemic of common sugar alcohols |
| Hydrogenated starch hydrolysates (HSH) | varies | 50% | Mix of maltitol, sorbitol, etc. |
Net Carbs vs Total Carbs: Which Should You Use?
Diabetes Management (ADA)
The American Diabetes Association generally recommends counting total carbohydrates. Reasons:
- Simpler and more forgiving for insulin dosing
- High-fiber foods already have their effect accounted for implicitly in glycemic response
- Avoids over-subtracting on foods where only 2-3 g of fiber are listed
See our complex carbohydrates guide for how different carb types affect glucose.
Keto and Low-Carb Diets
Keto followers almost universally track net carbs (usually capping at 20-50 g per day). The distinction matters more when fiber or sugar alcohols are a large share of the total.
A Middle Path
If a food has 5+ g of fiber, it is reasonable to subtract fiber to avoid over-dosing insulin or unnecessarily restricting nutrient-dense foods like beans and lentils. If fiber is under 5 g per serving, the difference is small enough to use total carbs.
How to Read a Label for Net Carbs
- Check the serving size. Multiply or divide if you are eating a different amount.
- Note Total Carbohydrate.
- Find Dietary Fiber. Subtract fully.
- Find Sugar Alcohols if listed. Subtract per the table above.
- The remainder is net carbs – the carbs likely to raise blood glucose.
Label Walkthrough: A Can of Black Beans
Serving size: 1/2 cup (130 g). Total carbs: 22 g. Dietary fiber: 7 g. Sugar alcohols: 0 g.
Net carbs per serving = 22 – 7 = 15 g.
If you eat a full cup, double to 30 g net carbs.
Label Walkthrough: Chocolate Keto Ice Cream
Serving size: 2/3 cup. Total carbs: 17 g. Fiber: 8 g. Sugar alcohols: 5 g (erythritol).
Net carbs = 17 – 8 – 5 = 4 g per serving. However, eating a whole pint (3 servings) = 12 g net carbs, and erythritol in 10+ g doses can cause GI upset.
Common Pitfalls
- Forgetting serving size. A 4 g net-carb keto bar may be 8 g if you eat two.
- Using net carbs for insulin dosing without clinician input. Many people need to dose on total carbs.
- Treating all sugar alcohols the same. Maltitol is close to regular sugar metabolically.
- Assuming fiber is always 0 glycemic. Most is; a few processed fibers (soluble corn fiber, IMO) affect glucose in sensitive individuals.
- Trusting the front-of-package “net carbs” claim. Recalculate from the Nutrition Facts panel yourself.
Test Your Own Response
If you have a glucose meter or CGM, run a simple test: eat a standard portion of the food alone, check glucose at 1 and 2 hours, and compare to a rise of 10 mg/dL per gram of digestible carb on average. A keto bar labeled 4 g net carbs should cause about a 10-15 mg/dL rise in a small person. If you see 40+, the label is misleading for your body. Our diet and nutrition hub has more monitoring tips.
The Bottom Line
Net carbs = Total carbs – Fiber – (Sugar alcohols / 2), with adjustments for specific sugar alcohols. Use net carbs for keto and low-carb tracking; stick with total carbs for diabetes insulin dosing unless your clinician advises otherwise. Always read the actual Nutrition Facts panel and test your own response when a label seems too good to be true.
