What are net carbs?
Net carbs are an estimate of the carbohydrate in food that your body digests and absorbs as sugar. Not all carbohydrates behave the same way. Dietary fiber passes through the small intestine largely intact and is fermented in the colon without raising blood glucose much. Because fiber does not affect blood sugar the way sugars and starch do, it is subtracted from total carbohydrates. Sugar alcohols are partly subtracted too, depending on the type.
On a ketogenic diet, net carbs are the main number to track for ketosis. Ketosis depends on keeping carbohydrate intake low enough that insulin stays low and the liver turns more fat into ketones. Most adults stay in ketosis at 20 to 50 g of net carbs a day. Total carbs matter less because fiber does not count against that limit, though some people prefer to track total carbs for simplicity.
How to calculate net carbs from a nutrition label
On a US nutrition facts label, Total Carbohydrate appears as a bold line. Beneath it, indented, are Dietary Fiber and Total Sugars. The basic formula is: Net Carbs = Total Carbohydrate − Dietary Fiber. If the product also lists sugar alcohols (optional on US labels, also indented under Total Carbohydrate), subtract erythritol in full and other sugar alcohols at 50%, which is what this calculator does.
Serving size matters. Everything on the label, including the net carb result, applies to one serving as the label defines it. If you eat two servings, double the result. The servings field in this calculator handles that for you.
Net carbs and sugar alcohols - what you need to know
Sugar alcohols are sweeteners that are only partly absorbed, so they raise blood glucose less than sugar. They are common in keto-labeled products, protein bars, sugar-free gum, and specialty foods. Their effect varies a lot by type, which is why this calculator treats erythritol differently from the rest.
Erythritol: subtracted in full
Erythritol is mostly absorbed in the small intestine and passed unchanged in urine, so very little is used for energy. Studies show little or no blood glucose or insulin response at typical amounts, which is why this calculator subtracts erythritol grams in full.
One caution: a 2023 study in Nature Medicine linked higher blood erythritol levels with a higher rate of heart attack and stroke, and found that erythritol made platelets more prone to clotting in lab tests. It was an observational association, not proof that erythritol causes harm, and more research is needed. People with heart disease or a higher clotting risk may prefer to use it sparingly and to ask their doctor.
Maltitol: why it is different
Maltitol has a glycemic index of roughly 35 to 50 depending on the form, well above erythritol and enough to raise blood glucose noticeably in many people. It is common in "sugar-free" chocolate, candy, and keto-labeled snacks because it tastes and behaves much like sugar. Some products subtract maltitol entirely to claim very low net carbs, which can mislead strict keto dieters. This calculator subtracts only 50% of maltitol grams.
Xylitol, sorbitol, and other sugar alcohols
Xylitol, sorbitol, isomalt, and other sugar alcohols generally have a smaller effect on blood glucose than maltitol, with glycemic index values often quoted in the single digits to low teens. They are partly absorbed and partly fermented in the gut, which is also why larger amounts can cause bloating or diarrhea. This calculator subtracts 50% of them, a common middle-ground approach. If a glucose monitor or ketone readings suggest a sweetener affects you more or less, adjust for yourself.
Net carb limits on keto
The usual net carb range for nutritional ketosis is 20 to 50 g a day. The lower end (20 g) is a common starting point for beginners, for people with insulin resistance, and for anyone who wants to reach ketosis sooner. It also leaves room for hidden carbs from condiments, sauces, and foods assumed to be carb-free. If you have type 2 diabetes and take glucose-lowering medication, cut carbs with your doctor, because doses may need to come down.
After several weeks, some people find they stay in ketosis at 30 to 50 g of net carbs a day. The threshold is individual and varies with activity: people who train hard often tolerate more, because exercising muscle uses more glucose. A blood ketone meter, used at a few different carb intakes, is the most reliable way to find your own limit.
High-carb foods that surprise keto dieters
Some foods look keto-friendly but contain more net carbs than expected. Low-carb tortillas often list 3 to 5 g of net carbs each, which adds up at two or three per meal. Some protein bars marketed as keto use maltitol, which affects blood sugar more than the label may suggest. Nuts are commonly underestimated: cashews have about 8 g of net carbs per ounce, close to half of a 20 g daily budget in one snack.
Other common surprises: flavored drinks sweetened with a little fruit juice; dairy, where lactose adds up across several servings; and vegetables like cherry tomatoes, onions, and bell peppers eaten in larger amounts than you think. Weighing food and using reliable nutrition data, rather than estimating, is the surest way to stay within your target.