Anyone who follows a low-carbohydrate or ketogenic diet soon learns to read food labels differently. Total carbohydrate includes sugars and starch that raise blood glucose, but also fibre, which passes through largely undigested, and sometimes sugar alcohols, which are only partly absorbed. “Net carbs” is the practical figure low-carb dieters use: the carbohydrate that counts. This calculator subtracts fibre and, if you choose, some or all sugar alcohols from the total, and compares the result with the usual daily limits for keto.
The term net carbs has no official definition, so the answer depends on the rule you apply. That is why the calculator makes you choose one.
How to use the net carb calculator
- Enter the total carbohydrate in grams per serving from the nutrition label.
- Enter dietary fibre and, if listed, sugar alcohols.
- Choose how to treat sugar alcohols: none, half or all.
- Enter the number of servings you will eat.
- Read the net carbohydrate and what share of a 20, 30 or 50 g daily limit it uses.
Net carb formula
Share subtracted: 0 (none), 0.5 (half) or 1 (all)
Calories ≈ digestible carbohydrate × 4 + fibre × 2 + sugar alcohols × 2.4
| Option in the calculator | Calculation | When it is used |
|---|---|---|
| No adjustment | Net = total carbohydrate − fibre | Standard approach |
| Half of sugar alcohols | Net = total − fibre − ½ × sugar alcohols | Common convention for polyols such as maltitol and sorbitol |
| All sugar alcohols | Net = total − fibre − sugar alcohols | Used for erythritol, which is largely unabsorbed |
There is no regulatory definition: the US Food and Drug Administration does not recognise “net carbs” or “impact carbs” as labelled terms. The energy factor of about 2 kcal per gram for dietary fibre is from FAO Food and Nutrition Paper 77 (2003), and 2.4 kcal per gram is the factor applied to polyols in European Union food labelling (Regulation (EU) No 1169/2011). The ketogenic limit of under 50 g of carbohydrate a day is from Bueno et al., Br J Nutr 2013;110:1178–1187.
Worked example
A snack bar lists 25 g total carbohydrate, 8 g fibre and 10 g sugar alcohols.
- With half of the sugar alcohols subtracted: 25 − 8 − 5 = 12 g net carbs.
- Subtracting none: 25 − 8 = 17 g. Subtracting all: 25 − 8 − 10 = 7 g.
- That 12 g is 60% of a 20 g daily limit, 40% of 30 g and 24% of 50 g.
- Estimated energy from carbohydrate: 68 kcal.
Why the rule matters
- Fibre is the safe subtraction. It does not meaningfully raise blood glucose and is the one most agree on.
- Sugar alcohols differ. Erythritol is mostly unabsorbed; maltitol and sorbitol are partly absorbed and can raise blood glucose, so subtracting all of them can understate the effect.
- Labels are not always exact. Fibre content is determined by lab methods that differ, and manufacturers may add isolated fibres.
- If you have diabetes, test your own response and follow your care team’s counting method, since insulin dosing may use total carbohydrate.
Next steps
Use the Keto Calculator to set daily calories and macros, the Carb Calculator for the full range of carbohydrate targets, and the Fiber Intake Calculator to check you eat enough fibre. The Calorie Density Calculator helps compare foods by energy per gram.
This page is for information only and is not medical or dietary advice. Speak to a doctor or registered dietitian before changing your diet, especially if you have diabetes, kidney disease, are pregnant or take medicines.
Sources
- US Food and Drug Administration. Nutrition labelling guidance: the agency does not define “net carbs”.
- FAO. Food energy — methods of analysis and conversion factors. FAO Food and Nutrition Paper 77, 2003.
- Regulation (EU) No 1169/2011 on the provision of food information to consumers (energy conversion factors, Annex XIV).
- Bueno NB, et al. Br J Nutr 2013;110:1178–1187.
Frequently asked questions
What are net carbs?
Net carbs are the carbohydrates in a food that the body digests and absorbs, usually estimated as total carbohydrate minus dietary fibre (and sometimes sugar alcohols). People on very low-carbohydrate diets count them because fibre does not raise blood glucose the way starch and sugar do.
Is “net carbs” an official term?
No. In the United States the FDA does not define “net carbs” or “impact carbs” on food labels, and the term is used by manufacturers and diet books in different ways. Nutrition labels show total carbohydrate, dietary fibre, total sugars and added sugars.
Should I subtract sugar alcohols?
Practice varies because sugar alcohols are absorbed to different degrees. Erythritol is largely unabsorbed and yields almost no calories, while maltitol and sorbitol deliver a good share of their energy and raise blood glucose. Subtracting half is a cautious convention; if you manage diabetes, follow your clinician’s advice and check blood glucose responses.
Do fibre calories count?
Fibre supplies roughly 2 kcal per gram in the FAO energy conversion factors, less than the 4 kcal per gram of digestible carbohydrate, because it is only partly fermented. The calculator includes a rough energy estimate on that basis.
How low should net carbs be on keto?
Ketogenic diets are generally defined as under 50 g of carbohydrate a day; many people aim for 20 to 30 g. See the Keto Calculator for calories and macros.