Net carbs, and how to work them out

The formula

Net carbs are the carbohydrates your body actually digests into glucose. On a US nutrition label the arithmetic is total carbohydrate − fibre − sugar alcohols.

Neither subtraction is as clean as it looks, and on a British or European label the starting number is not even the same thing. This page is about the parts that go wrong.

Why fibre comes out

Fibre is a carbohydrate by chemistry but not by behaviour. Your small intestine has no enzyme for most of it, so it passes through without raising blood glucose the way starch or sugar does. Some is fermented in the colon, which yields a little energy — which is why the EU counts fibre at 2 kcal/g rather than 4, and why subtracting it is a useful convention rather than a physical law.

For practical low-carb purposes, subtracting fibre in full is the accepted approach and the one nearly every keto resource uses.

The mistake that doubles the subtraction

This is the one that catches people, and remarkably little is written about it.

On a US label, “Total Carbohydrate” includes fibre. You subtract, and you get net carbs.

On a UK or EU label, “Carbohydrate” already excludes fibre — fibre is declared on its own line and was never part of the number above it. Subtract again and you have undercounted, sometimes badly.

The tell is the indentation. “Carbohydrate” with “of which sugars” underneath is the European format, and the figure is already net of fibre. “Total Carbohydrate” with “Dietary Fiber” underneath is the American format, and you have the subtraction to do.

So if you shop in Britain and follow an American keto guide, you have probably been writing down fewer carbs than you ate.

Sugar alcohols, where it stops being clean

Sugar alcohols are the second subtraction, and they do not behave alike:

• Erythritol is largely absorbed and excreted unchanged, with almost no effect on blood glucose. Subtracting it in full is defensible.
• Maltitol is roughly half as glycaemic as sugar. Subtracting it in full flatters a “low net carb” bar considerably.
• Xylitol and sorbitol sit somewhere between the two.

The common convention is to subtract erythritol in full and half of everything else. It is a rule of thumb rather than a measurement, and it is where most “2 g net carbs” bars come from.

A worked example

A US label on 100 g of almonds: total carbohydrate 21 g, dietary fibre 10 g, no sugar alcohols. Net carbs = 21 − 10 = 11 g.

The same almonds on a UK label: carbohydrate 11 g, with fibre 10 g listed separately. Net carbs = 11 g. Same nuts, same answer, different arithmetic — and had you subtracted on the second one you would have written down 1 g.

How many net carbs a day?

Most ketogenic guidance puts the threshold between 20 and 50 g of net carbs a day, with 20 g the usual starting point. Where you fall depends on your size, your activity and what you are trying to do — which makes it a question for a professional rather than for an app.

What slotho actually shows you

Worth being exact, because apps tend to be vague here. slotho shows Net carbs as its own row on a food whenever the label declares both carbohydrate and fibre: total carbohydrate minus fibre, never below zero. It logs total carbohydrate and fibre as separate figures and writes both into Apple Health under the standard types, so the underlying numbers stay yours.

That row assumes a US-style label. On a UK or EU label, where carbohydrate already excludes fibre, it subtracts fibre a second time (the trap described above), so read it as a floor there. Sugar alcohols are not subtracted.

Its keto rating works from total carbohydrate, not net: the stars come from what share of a food’s calories is carbohydrate, and how slotho rates a food for keto sets out the thresholds.

Treat published per-food figures with suspicion

Net carb tables disagree with each other constantly, and the cause is almost always the source data rather than the arithmetic. Public food databases have real gaps: entries with no fibre value at all, dry weights presented as cooked, and branded products standing in for the generic food.

A missing fibre field silently becomes a zero, and a zero makes net carbs equal total carbs. Avocado is the classic casualty — it carries around 7 g of fibre per 100 g, so an entry without it overstates the net carbs several times over. Before trusting a figure, check the entry actually has a fibre number in it.

slotho supports general wellness and education. It is not a medical device and does not provide medical advice. Talk to a qualified professional before making changes to your diet, fasting or exercise.

More

How slotho rates a food for keto covers the keto stars and their thresholds. Can Apple Health track food? explains which nutrition fields are written to your Health store. The slotho FAQ answers the shorter questions.