Net carbs are total carbohydrates minus fibre and minus some sugar alcohols. The idea is to count only the carbohydrate that actually reaches your bloodstream as glucose. It is a useful shortcut for people on low-carb diets, and it is also a marketing term with no legal definition, which is where the trouble starts.
The calculation
Take the total carbohydrate on the label, subtract the fibre, subtract sugar alcohols, and what remains is net carbs.
A food with 20 g of total carbohydrate, 8 g of fibre and 2 g of erythritol has 10 g of net carbs. That is the whole arithmetic, and it is the same everywhere.
Outside the US, labels often list carbohydrate with fibre already excluded, in which case the number on the packet is already closer to net carbs and subtracting again will double-count.
Why fibre gets subtracted
Fibre is carbohydrate your small intestine cannot break down. It travels through largely intact, so it does not raise blood glucose the way starch and sugar do.
That is the general rule, and it is not absolute. Soluble fibre ferments in the large intestine and yields a small amount of energy. Insoluble fibre mostly passes through and adds bulk. Treating every gram of fibre as a free subtraction slightly overstates the case, but for most foods it is close enough to be useful.
Sugar alcohols are the messy part
This is where net carb maths gets abused.
Erythritol is barely absorbed and barely affects blood sugar, so subtracting it is fair. Maltitol is a different story. It is absorbed enough to raise blood glucose in many people, and products sweetened with it can carry far more usable carbohydrate than a net carb label implies.
Sugar alcohols also have a digestive cost. Eat enough of them and you get bloating, gas and worse, which is why the boxes carry a small warning that nobody reads until it is too late.
If a protein bar claims 2 g of net carbs and lists 20 g of maltitol, treat the claim with suspicion.
Total carbs or net carbs
Which you count depends on what you are doing.
For general weight management, total carbohydrate is simpler and harder to game, and the calories were never in dispute either way. Fibre still counts toward the calories on your label in many jurisdictions even when it is subtracted from net carbs.
For strict keto or for managing blood sugar, net carbs are the more relevant figure, because the point is the glucose response rather than the calorie total.
Either way, this is a second-order decision. If your overall calorie intake is not roughly right, the carbohydrate accounting will not save you. Start with your maintenance calories and treat the carb split as tuning.
Where people go wrong
Assuming net carbs mean fewer calories. They do not. A low net carb bar can be a high calorie bar, and plenty of them are.
Trusting front-of-pack claims over the nutrition panel. Do the subtraction yourself.
Forgetting that individual responses differ. Two people can eat the same food and see meaningfully different blood glucose responses, which is why anyone managing diabetes should be working from their own readings rather than a label heuristic. [ADD SPECIFIC: guidance from a named diabetes body, if you want a citation here]
Practical eating
The foods that come out well on net carbs are the ones that come out well on most measures: vegetables, leafy greens, berries, nuts, eggs, fish, meat. High fibre, moderate total carbohydrate, minimal processing.
The foods that look best on the front of the packet and worst under examination are the engineered ones. Bars, low-carb breads, keto desserts. Some are fine. Many rely on a generous reading of what counts as subtractable.
Cook more of your own food and the question mostly stops mattering, because whole ingredients do not come with marketing claims attached.