What Are Net Carbs? The Subtraction and Where It Breaks

Net carbs are what is left of a food’s carbohydrate after you take off the fiber, and by some counts the sugar alcohols as well. A food with 20 g of total carbohydrate and 5 g of fiber has 15 g of net carbs. No US rule defines the term, and the American Diabetes Association doesn’t recommend using it. That’s why two bars with similar ingredients can print different “net” numbers. I ran the subtraction on a made-up label and on a bowl of chia pudding in grams, then hunted for the places where it undercounts.

What are net carbs, and does any law define them?

A net carb count is arithmetic that shoppers do themselves. You start with total carbohydrate, take off dietary fiber, and sometimes take off sugar alcohols too. The logic is that fiber isn’t easily digested in the small intestine, so it doesn’t arrive as glucose the way starch and sugar do. FDA’s label guide to dietary fiber adds a wrinkle. Soluble fiber is broken down by bacteria in the large intestine and provides some calories. Insoluble fiber passes through largely intact and doesn’t.

That leaves two common formulas. The strict one is total carbohydrate minus fiber. The loose one also subtracts sugar alcohols. A brand picks whichever it likes, and the law doesn’t stop it. In a 2014 proposal to revise the Nutrition Facts label, FDA described a petition noting that manufacturers define “net carbs” in different ways. FDA wrote that a request to define the term was outside the scope of that rulemaking.

The federal label rule works differently. It gets total carbohydrate by taking protein, fat, moisture and ash off the weight of the food (21 CFR 101.9). That leaves fiber inside the total. A US label has no net line to read. The ADA page above says terms like “net carbs” haven’t been defined by FDA and tells readers to check total carbohydrates first.

Elsewhere the word “carbohydrate” already does part of the job. The UK and EU labeling regulation defines carbohydrate as any carbohydrate that humans metabolize, polyols included. Its nutrition declaration lists carbohydrate as sugars, polyols and starch, with fiber as a separate line (Annex I). Australia’s Food Standards Code defines carbohydrate as available carbohydrate. Its energy table separates carbohydrate from unavailable carbohydrate, a category that includes dietary fiber. I read these as labels whose carbohydrate figure has no fiber in it. Take fiber off again and the same grams are removed twice, so the result reads lower than the food really is. The three-market table is in the keto diet tracker post.

How do I work out net carbs from a US label?

Read total carbohydrate, subtract dietary fiber, then decide what to do with any sugar alcohol line. Here is a protein bar I made up, with the lines a label built to 21 CFR 101.9 would show.

Label line	Per bar
Protein	20 g
Total fat	8 g
Total carbohydrate	24 g
Dietary fiber	8 g
Total sugars	2 g
Sugar alcohol (all maltitol)	10 g
Calories	210

The strict count is 24 - 8 = 16 g of net carbs, and the loose count is 24 - 8 - 10 = 6 g. Same bar, two answers. Neither is printed on the label. A “6 g net carbs” badge on the front is the maker’s choice of formula, and it flatters the bar. The last 4 g of the 24 g are starch and other carbohydrate, a number I picked so the lines add up.

Pick one formula, write it at the top of your food log and use it everywhere. Two formulas inside one week make the totals impossible to compare.

Do fiber and sugar alcohols have calories?

Yes, some. How many depends on the rule and on the ingredient. Fiber isn’t a zero. The FAO’s chapter on energy conversion factors puts non-fermentable fiber at 0 kcal per gram and fermentable fiber anywhere from 0 to 4.0. For ordinary mixed foods it gives 0 to 1.9. Regulators have to pick one number for something that varies by food, and they don’t all pick the same one.

Substance	US label rule (kcal/g)	UK and EU rule (kcal/g)	Australia, Schedule 11
Fiber	2 for soluble, none for insoluble	2	8 kJ/g, about 1.9 kcal/g
Maltitol	2.1	2.4 (all polyols)	13 kJ/g, about 3.1 kcal/g
Erythritol	0	0	1 kJ/g, about 0.2 kcal/g
Allulose	0.4	not listed	2 kJ/g, about 0.5 kcal/g

I converted the Australian kilojoule factors to kilocalories myself. The US allulose figure is a policy: FDA says it will not object to 0.4. So the same maltitol carries 2.1, 2.4 or about 3.1 kcal per gram depending on the country that printed the label. Erythritol is the only sugar alcohol near zero everywhere.

Back to the made-up bar. I worked its calories three ways, and the 4-4-9 factors in the first two rows are the ones explained in calories per gram.

Method	Sum	kcal
Every carbohydrate at 4 kcal/g	20 × 4 + 8 × 9 + 24 × 4	248
Only the 6 g of net carbs at 4 kcal/g	20 × 4 + 8 × 9 + 6 × 4	176
US label method: digestible carbs at 4, soluble fiber at 2, maltitol at 2.1	80 + 72 + 24 + 16 + 21	213

The US method, one of several the rule allows, gives 213 kcal. A label prints 210, since labels round to the nearest 10 calories above 50. The net-only tally leaves out 37 kcal, all of it fiber (16) and maltitol (21). That’s small for one bar. It repeats every day you eat one. I assumed all 8 g of the fiber is soluble. If it were insoluble, its 16 kcal would drop out and the same method would give 197.

What does the subtraction do to real food?

It removes grams from fiber-rich foods and no calories. I worked three of them from USDA FoodData Central: chia seeds, flaxseed and raspberries. The last column counts only protein, fat and net carbs, the way a net-only tally would.

Food	Grams	Total carbs (g)	Fiber (g)	Net carbs (g)	Energy in the record (kcal)	Net-only tally (kcal)
Chia seeds, dried	30	12.6	10.3	2.3	146	112
Flaxseed	30	8.7	8.2	0.5	160	138
Raspberries, raw	100	12.9	6.5	6.4	57	31

Chia is the extreme case. Of its 12.6 g of carbohydrate, 10.3 g is fiber. Raspberries are milder. Half of their carbohydrate is fiber and half still counts. In every row the record sits above the tally, by 22 to 34 kcal. The records don’t treat fiber alike, either. Chia’s 486 kcal per 100 g falls below the 4-4-9 sum on its total carbohydrate (511 kcal). The raspberry record’s 57 kcal matches its own sum (57.4).

Now a recipe. Stir 30 g of chia seeds into 200 g of unsweetened almond milk, add 60 g of raspberries and leave it overnight. Rows are rounded, and the totals come from the unrounded values.

Ingredient	Grams	Total carbs (g)	Fiber (g)	Net carbs (g)	Energy (kcal)
Chia seeds	30	12.6	10.3	2.3	146
Almond milk, unsweetened	200	2.6	0.4	2.2	30
Raspberries	60	7.7	3.9	3.8	34
Bowl	290	23.0	14.6	8.4	210

Fiber makes up 64% of this bowl’s carbohydrate, so 23 g on a label-style count becomes 8.4 g net. The bowl also holds 6.4 g of protein and 11.2 g of fat. Count only protein, fat and net carbs and you get 160 kcal. That is 50 kcal short of the records’ 210. The EU rule’s 2 kcal per gram would put 14.6 g of fiber at about 29 of those. I don’t know how each record handled the other 21. So I show the gap and claim no correction.

To turn a calorie goal into grams of protein, fat and carbs before you subtract anything, use the macro calculator. Its carb figure is a plain gram target, not a net one, so the fiber comes off in a second step that you do yourself.

Where does the net carb count mislead?

In four places, and one of them is a question about safety rather than arithmetic. The trouble sits in packaged low-carb food, in its sweeteners and added fibers.

Is maltitol really close to zero?

No. Livesey’s review of polyols as sugar replacers analyzed the glucose responses in the literature. It put the glycemic index of maltitol at 35, against 65 for sucrose and 100 for glucose. Erythritol came out at 0. A 35 is a real response, about a third of glucose’s. Read loosely as a weight, 10 g of maltitol acts like 3.5 g of glucose. That conversion is mine and it is crude, because an index compares responses and does not weigh sugar. Count it that way and the bar has 24 - 8 - 6.5 = 9.5 g of net carbs, not 6 g. Three bars a day widen the gap to 10.5 g. Of the three numbers for this bar, 16, 6 and 9.5, I’d lean on 9.5. It rests on one review. It is no dosing rule. The same review lists polyols among the osmotic, laxative carbohydrates, which is a reason to watch how much you eat.

What about allulose?

FDA says it will not object if a maker leaves allulose out of the sugar lines and counts it at 0.4 kcal per gram. It must still sit inside total carbohydrate, per the FDA announcement. FDA describes allulose as metabolized differently from traditional sugars. So on a US label the carbohydrate line is complete, and what looks shorter is the sugar line. Whether you subtract it is your rule to set. Just set it.

Can an added fiber still raise blood sugar?

Some can. FDA’s list of isolated fibers that count on a label includes inulin, polydextrose and resistant maltodextrin. Isomaltooligosaccharides aren’t on its October 2021 list. In a small double-blind crossover study, Lowery and colleagues gave nine healthy adults 25 g of isomaltooligosaccharide syrup, soluble corn fiber syrup or a placebo. At 30 minutes the isomaltooligosaccharide syrup gave higher glucose than the corn fiber syrup and higher insulin than both other arms. Corn fiber matched placebo. The authors conclude that isomaltooligosaccharide is not a dietary fiber. Nine people is a small study. A maker of corn fiber products paid for the blood analysis. I treat it as a warning and not a verdict. If an ingredient list says isomaltooligosaccharide, I wouldn’t subtract that fiber.

Does a zero for erythritol settle it?

For blood sugar, yes. Livesey’s index for it is 0, so it earns a full subtraction. Safety over years is a separate question. A 2023 study by Witkowski and colleagues followed patients having a cardiac risk assessment. Higher blood erythritol went with more major cardiovascular events over three years. The adjusted hazard ratios for the top quarter against the bottom were 1.80 and 2.21 in two validation cohorts. Eight healthy volunteers who ingested erythritol kept raised blood levels for more than 2 days. It’s an association plus a pilot. The authors themselves call for long-term safety studies. If you have heart disease or its risk factors, ask your doctor before leaning on it.

What should I do with the number today?

Take one packaged food from your cupboard. A bar is ideal. Copy total carbohydrate, fiber, sugar alcohol and calories off the label, work both formulas, then read the ingredient list for maltitol, allulose and isomaltooligosaccharide. Five minutes. You’ll see how far the maker’s “net” figure sits from yours.

My rule is short: subtract fiber that comes from whole foods, and subtract erythritol. Everything else stays in. That means any other sugar alcohol and any added fiber I can’t place. The published numbers for them are too uneven to trust a ratio. Calories always come from the food record. For protein grams, what should my macros be has the numbers by body weight and calorie level. High protein low carb foods ranks foods that carry protein with few carbs. The high protein low carb diet guide shows how a whole day fits together. If you plan to move carbs between days, carb cycling tests whether that beats a flat target.

Ask your diabetes team how to treat fiber and sugar alcohols on a label if you take insulin or another glucose-lowering medicine. The ADA’s advice is to start from total carbohydrate, and the same goes for anyone on a medically supervised ketogenic diet.

For chia and berries the shorthand holds up. For a bar with a sweetener and a long ingredient list it doesn’t. The calories were never subtracted.