Net carbs is a subtraction the manufacturer performed and printed on the front of the box. The FDA does not define it, does not regulate it, and does not put it on the Nutrition Facts panel. Total Carbohydrate is the regulated line, and it is sitting a few rows above the claim, usually in smaller type.
That does not make the front number worthless. It makes it an estimate whose accuracy depends entirely on what got subtracted and how much of the work that subtraction was asked to do. Our guide to reading a sugar-free label walks the panel from the top. This one is about the single number that is not on it.
The formula, and who decides it
The usual version is total carbohydrate, minus fiber, minus sugar alcohols. Whatever is left gets called net carbs, sometimes impact carbs, sometimes active carbs.
The reasoning behind it is sound in outline. Fiber and most polyols are not absorbed and burned the way starch and table sugar are, so counting them identically overstates what a food does to you. Nutrition science has a version of this idea already: available carbohydrate, which is total carbohydrate minus fiber, and which is what glycemic testing is standardised against.
Where it goes wrong is that nothing obliges a brand to use the same formula as the brand next to it. Some subtract every gram of polyol. Some subtract half. Some subtract allulose, which is not a polyol at all. Two bars can print 2 grams of net carbs on the front, carry different ingredients, and behave differently, and both labels are legal.
Fiber: the half that mostly holds up
Fiber is the part of the subtraction with the strongest case behind it. It reaches the large intestine largely intact instead of being broken down into glucose in the small intestine, which is why it is excluded from available carbohydrate in the first place.
The caveat is that it is not energy-free. Fermentable fibers get worked on by gut bacteria, and some of that energy comes back to you as short-chain fatty acids. A gram of fiber is not a gram of nothing. It is a gram of something that arrives slowly, partially, and by a different route.
For a home-baked dessert this matters less than it sounds, because the fiber in one is usually attached to a real ingredient rather than added from a bag. Our black bean brownies print 14 grams of carbohydrate and 3 grams of fiber per brownie, and the fiber is there because the base is a can of blended beans. Nobody engineered that number. It came with the ingredient.
Sugar alcohols: the half that does not hold up
This is where the formula does most of its lying, and it lies in one direction.
Polyols are not interchangeable. Erythritol is absorbed in the small intestine and leaves in urine without being used for energy, so it tests at or near zero on glycemic index and contributes almost no calories. Maltitol is a disaccharide, a real share of it is broken down and absorbed as glucose, and published glycemic values for it land far above erythritol, closer to half the response of table sugar than to nothing.
Both get grouped under the same heading on a panel. Both get subtracted at the same rate in the usual formula. So a bar built on maltitol and a bar built on erythritol can arrive at the same net carb figure while one of them is doing considerably more than the other.
The ingredient list is more reliable than the arithmetic here. Most polyols end in -itol, and the order tells you roughly how much is in there, because ingredients are listed heaviest first. If maltitol is in the first three, the front number is carrying a lot of weight.
Allulose, which the formula has no slot for
Allulose is the case that breaks the tidy version of this in the other direction.
It is a rare sugar rather than a polyol, so the standard subtraction does not remove it. FDA guidance lets manufacturers leave it out of the total sugars and added sugars lines while still counting it inside total carbohydrate. So a product sweetened with it can show grams of carbohydrate, zero grams of sugar, and a net carb figure that looks high next to how the food actually behaves. The full explanation of why allulose is not a sugar alcohol covers what it does once it is in you.
Our sugar-free marshmallows are the clearest example I can hand you. One marshmallow prints 10 calories and 10 grams of carbohydrate. Run the usual net carb formula on that and you get 10 grams of net carbs, from something with 10 calories in it. The arithmetic and the food disagree, and the food is right.
Where the panel rounds
Two rounding rules worth carrying around, because they are per serving and the panel does not do the multiplication for you.
Under 0.5 grams of sugar per serving prints as 0. Under 5 calories per serving can print as 0 as well. Neither is a lie. Both stop being useful the moment you eat three servings of something, which is the normal way a person eats a drink mix, a syrup or a sugar-free candy.
Serving size and servings per container are the first two lines for a reason. Every number under them is scaled to the first and multiplied by the second.
What this looks like in a dessert you made yourself
The good news about cooking is that nobody is selling you the front of the box. You have the ingredient list because you bought the ingredients.
Every recipe here prints total carbohydrate and fiber separately, so you can run whatever subtraction your own plan uses and see how big it is. The molten chocolate mug cake carries 24 grams of carbohydrate and 7 grams of fiber, so the subtraction is doing real work and you can see exactly how much. Protein peanut butter cups come to 22 grams of carbohydrate with 9 grams of fiber for a serving of two.
What we do not print is a net carb number, and that is deliberate. Printing one would mean picking a formula on your behalf and hiding the inputs, which is the thing that makes the number unreliable on packaged food in the first place. You get the lines. You do the subtraction your dietitian told you to do.
The number to count
If a carbohydrate figure matters to you for a medical reason, count total carbohydrate. It is defined, it is regulated, and it is the one your care team is almost certainly talking about when they give you a target.
Use the fiber line if your plan subtracts fiber, because plenty do. Treat the sugar alcohol line as a question rather than an answer, and go look at which polyol it is before you decide what to remove.
I develop dessert recipes and none of this is medical advice. Carbohydrate targets are set against your labs, your medication and your situation, and a recipe site is not in a position to have an opinion about any of the three. Take the mechanics from me and the plan from the person who has seen your numbers.
The short version
Total carbohydrate is measured and regulated. Net carbs is a claim built on top of it, using a formula the brand chose, applied to ingredients the formula treats as identical when they are not.
Read the polyol before you accept the subtraction. And where a dessert can get there on fiber, protein and fruit instead of a sweetener you have to research, the question mostly goes away. Our no added sugar collection is built that way, with the carbohydrate and fiber printed on every one so you can check the maths yourself. A handful of them are in the free sampler further down this page if you would rather start in a kitchen than in an aisle.
