Hong Life CalculatorsUnderstanding the net carbs calculator — concept and principle
Net carbs are the carbohydrates that are digested and absorbed as sugar, as opposed to fibre and sugar alcohols that are only partly absorbed or not at all. People on low-carb and keto diets, and some people managing blood sugar, use net carbs to judge how a food will affect them.
How it is calculated
- Net carbs = total carbohydrate − fibre − sugar alcohols × factor.
- The factor is 1 when the sugar alcohol is erythritol (subtracted fully) and 0.5 for sugar alcohols such as maltitol (half subtracted).
- The result cannot go below 0 g.
The principle
Fibre is not broken down by human digestive enzymes, so it passes into the large intestine, where gut bacteria ferment part of it. It does not raise blood glucose directly. Sugar alcohols (polyols) are absorbed to different degrees. Erythritol is largely absorbed but then excreted unchanged in urine, so it supplies almost no energy and has very little effect on blood sugar. Maltitol, sorbitol and xylitol are partly absorbed and partly fermented; maltitol in particular raises blood sugar noticeably, which is why only about half is subtracted. "Net carbs" is a practical convention rather than a regulated label term in the United States.
Worked example
- Inputs: 30 g total carbohydrate, 8 g fibre, 6 g sugar alcohols, erythritol counted as zero (factor 1).
- Net carbs = 30 − 8 − 6 × 1 = 16 g.
- If the 6 g were maltitol (factor 0.5), the result would be 30 − 8 − 3 = 19 g.
| Total carbohydrate (g) | Net carbs (g) |
|---|---|
| 7.5 | 0 |
| 15 | 1 |
| 22.5 | 8.5 |
| 30 | 16 |
| 37.5 | 23.5 |
| 45 | 31 |
| 60 | 46 |
How to read the result
Ketogenic diets usually limit net carbs to roughly 20–50 g a day, so a 16 g snack would use a large part of the budget. If you monitor blood glucose, your own meter readings after a food are more reliable than any label arithmetic, because individual responses differ.
Common mistakes
- Subtracting fibre from labels that already exclude it. In the EU, UK and Australia "carbohydrate" on the label already excludes fibre, so subtracting it again double-counts. US labels include fibre in total carbohydrate.
- Treating all sugar alcohols like erythritol.
- Forgetting that large amounts of sugar alcohols can cause bloating or diarrhoea.
Frequently asked questions
Do I subtract allulose?
Allulose is a rare sugar that is barely metabolised, so many low-carb dieters subtract it, but check how your label lists it.
Is soluble fibre different from insoluble fibre?
Both are subtracted here; soluble fibre may provide a small amount of energy through fermentation, but it does not raise blood glucose directly.
Why do brands show different net carbs for similar products?
They may use different rules for sugar alcohols and fibre types, since there is no single legal definition.
Sources: Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005). · Regulation (EU) No 1169/2011 on the provision of food information to consumers.