Net Carb Calculator

The term on the packet is not a real thing

Start with the fact that reframes everything else on this page.

"Net carbs" has no legal definition. The American Diabetes Association does not recognise it. The US Food and Drug Administration does not use it, and its labelling rules make no provision for it. The only carbohydrate figure that is actually regulated is the Total Carbohydrate line on the nutrition panel, and the FDA's position is that this is the number you should use.

So when a wrapper announces "only 3g net carbs!" in large friendly letters, that is not a regulated measurement. It is a marketing claim, calculated by the manufacturer using whatever arithmetic they preferred that morning, with nobody checking. Different brands use different sums. There is no standard holding any of them to anything.

Which is precisely why a tool like this one exists. The raw ingredients of the calculation, total carbohydrate and fibre, are regulated and printed on the back. You can do the sum yourself, honestly, and find out what the front of the packet decided not to tell you.

Why fibre comes off in full

The idea behind net carbs is sound even if the label is not. Not all carbohydrate is carbohydrate in any way that matters to you.

Fibre is the clearest case. It is chemically a carbohydrate, and legally it is counted as one, but you do not have the enzymes to break it into glucose. It travels through your small intestine largely intact. Some of it gets fermented by bacteria in your colon, which yields a trickle of energy, but for the purposes of blood sugar it is close enough to nothing.

So this tool subtracts fibre in full, and that is the near-universal convention. The 9 grams of fibre in a bran-heavy bar are on the label because the law says they must be. They are not going to raise your blood sugar.

Why sugar alcohols only come off by half

Now the part where this calculator quietly does something more careful than most, and where you might otherwise think it had gone wrong.

Enter 10 grams of sugar alcohol and it does not subtract 10. It subtracts 5.

That is deliberate. Sugar alcohols, the xylitol and sorbitol and maltitol in every sugar-free sweet, are not like fibre. They are partially absorbed. They do carry energy, generally somewhere between nothing and 2 calories a gram against sugar's 4, and they do move your blood sugar, just less than sugar would. Treating them as free is simply wrong.

The trouble is that they do not behave alike. Erythritol is almost entirely excreted unchanged and barely registers. Maltitol, which is what most sugar-free chocolate is built on, has a glycaemic index somewhere around 35 to 52 and can move your blood sugar enough to matter. Halving is the standard clinical rule of thumb for exactly this reason: it is a reasonable average across a family of ingredients that range from nearly free to nearly sugar.

So the tool is being conservative on purpose. It would rather tell you a slightly higher number than let a bar of maltitol quietly ruin your day.

This tool will disagree with the wrapper, and it is right

Here is what all of that looks like in practice, using a real example that has been picked over in the nutrition literature.

Take a maltitol-sweetened low-carb bar. Its nutrition panel reads 23 g of total carbohydrate, 9 g of fibre and 11 g of sugar alcohol. The front of the wrapper says 3 g net carbs.

You can see where the 3 came from. Subtract everything: 23 − 9 − 11 = 3. All the fibre, and every gram of the sugar alcohol, treated as though none of it counted at all.

Now type those same three numbers into this calculator:

23 − 9 − (11 ÷ 2)
= 23 − 9 − 5.5
= 8.5 g

Eight and a half, not three. Nearly triple what the packet claims, from the packet's own numbers. Nothing has been invented. The only difference is that the manufacturer counted 11 grams of maltitol as nothing, and this tool counts it as what it is, which is about half a carbohydrate.

If you are eating 20 net carbs a day to hold ketosis, that single bar is not the 15 percent of your budget the wrapper implies. It is over 40 percent of it. Our Keto Calculator shows how little room there is to play with.

The one exception worth knowing

The halving rule is a good average, and averages have edges. The main one is erythritol.

Erythritol is absorbed but then excreted essentially unchanged, contributing almost no calories and almost no glucose response. If the only sugar alcohol in a product is erythritol, subtracting all of it rather than half is defensible, and most low-carb communities do exactly that. This tool will halve it, which makes your number slightly conservative. That is a safe direction to be wrong in, but it is worth knowing why the figures differ.

The reverse case is more useful. If the ingredients list maltitol or maltitol syrup, halving may actually be generous, because maltitol behaves closer to sugar than the rule assumes. Check the ingredient list rather than the summary line: "sugar alcohols" tells you a category, and the category ranges from nearly free to nearly sugar.

One last thing worth saying plainly. If you have diabetes and are dosing insulin against your carbohydrate intake, the ADA's guidance is to count total carbohydrate rather than net, and that guidance exists because getting this wrong has consequences well beyond an interrupted diet. This tool is for people managing a low-carb diet, not for calculating a dose.

Questions people ask

What is the formula?

Total carbohydrate, minus all the fibre, minus half the sugar alcohol. If the answer would go below zero, the tool reports zero.

Why does this give a different answer from the label?

Because most labels subtract every gram of sugar alcohol and this tool subtracts half. Half is the more honest figure, and "net carbs" is not regulated, so the label is under no obligation to agree with anyone.

Should I count net carbs or total carbs?

If you are managing a low-carb or ketogenic diet, net carbs are the more useful working number. If you have diabetes and dose insulin, the ADA recommends total carbohydrate, and that recommendation is worth following.

What if the label has no fibre or sugar alcohol?

Leave them blank or enter zero. The tool treats blanks as zero, so your net carbs will simply equal your total carbohydrate, which for most whole foods is roughly true anyway.

References

Where this comes from. The American Diabetes Association states plainly that "net carbs" has no legal definition, is not used by the FDA and is not recognised by the ADA, and that the assumption behind the calculation, that fibre and sugar alcohols are neither absorbed nor metabolised, is not always true. The convention of subtracting half the sugar alcohol, and the worked example of a maltitol-sweetened bar whose label understates its own arithmetic, are set out by Healthline's review of the subject.

1. American Diabetes Association. Get to know carbs. https://diabetes.org/food-nutrition/understanding-carbs/get-to-know-carbs
2. Healthline. Net carbs: what they are, how to calculate them, and the limits of the label. https://www.healthline.com/nutrition/net-carbs
3. National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Energy. Washington DC: The National Academies Press. https://www.ncbi.nlm.nih.gov/books/NBK591020/