How to Calculate Net Carbs on Keto — and Why They Matter

If you’re new to the keto diet, you’re probably asking yourself, what are net carbs, and why should I care? When you cut down on carbs or start a keto diet like Bulletproof, carbohydrates are pretty much the first thing you check when looking at a food’s nutrition label. And knowing how to calculate net carbs is key.

Usually, there’s more to the label than just total carbohydrates. Fiber, sugar, sugar alcohols — they’re all listed under carbohydrates. What do they mean, and which ones should you pay attention to so you don’t go over your daily carb limit?

The key to figuring out carb count is calculating a food’s net carbs: the carbs in food that actually impact your blood sugar. Manufacturers came up with the term “net carbs” in the early 2000s, when low-carb diets began to catch on. But there’s no official definition of net carbs, and the net carb count on labels can be deceiving.

When you’re on a keto or low-carb diet, every gram of carbohydrate you eat counts, so it’s essential to know how to track net carbs accurately. This guide explains what net carbs are, why net carbs matter, and how to calculate net carbs for yourself.

What are net carbs?

Net carbs are the carbohydrates in food that you can digest and use for energy. On a keto diet, starches and sugars go into the net carb calculation; a lot of other types of carbohydrates like insoluble fiber have no energy value or impact on your blood sugar, so you don’t count them towards your daily keto carbohydrate limit.

To calculate net carbs for keto, take a food’s total carbs and subtract:

• Fiber, a type of carbohydrate from plants that the human digestive system can’t process. Your body doesn’t have the enzymes to break down fiber, so it passes through your digestion unchanged. That means, for keto at least, it has zero net carbs and zero calories.
• Sugar alcohols like xylitol and erythritol. Sugar alcohols taste sweet, but their molecular structure is slightly different from that of sugar molecules, which leaves a lot of sugar alcohols either partially or entirely indigestible by humans.

Note that certain sugar alcohols do impact your blood sugar, and you should factor them into your keto carb count if you eat a large amount. For details, see the “How to calculate net carbs (including sugar alcohols)” section below.

Related: Ketosis and the Ketogenic Diet Explained – A Complete Beginners Guide

Why net carbs matter on keto

Carbs are fine in moderation. In fact, most people do better with some carbs, which is why Bulletproof suggests a cyclical keto diet with one carb refeed day a week. Excess carbs, however, and especially refined carbs like starches and sugars, may:

• Spike your blood sugar
• Contribute to inflammation^[1][2]
• Trigger food cravings^[3]
• Disrupt your hormones^[4]
• Mess with your gut bacteria
• Play a part in obesity, insulin resistance, diabetes, and metabolic syndrome^[5]

So you want the right amount of carbs for you. On keto, you keep carbs lower and eat more fat. Just be sure to get your fat from smart sources like grass-fed animals, wild-caught fish, avocados, olive oil, and pastured egg yolks (Here’s a one-page guide to the best high-quality fat sources).

When you keep net carbs low enough — under about 50g/day for most variations of keto — your body may go into ketosis: a state where you shift from burning glucose, or carbs, for energy to burning fat (including body fat). A keto diet may suppress your appetite,^[6][7]  may decrease inflammation,^[8][9] and may help you lose weight.^[10][11]

On a keto diet, a keto cycling diet like the Bulletproof Diet, or any other kind of lower carb diet, people limit the carbs they eat with the goal of maximizing fat-burning and minimizing inflammation. That’s why it’s so important to know how to figure out net carbs. Go over your carb limit, and you’ll fall out of ketosis, and lose out on all the benefits.

How to calculate net carbs for keto (including sugar alcohols)

The basic formula to calculate net carbs is:

Net carbs for keto = Total carbs – fiber – sugar alcohols

However, not all sugar alcohols are truly carb-free, and some manufacturers selling “low-carb” foods will list carb-significant sugar alcohols as if they’re exempt from total carb count, to make products appear lower-carb than they actually are.

On the other side of things, manufacturers will sometimes list sugar alcohols that don’t affect your blood sugar as if they were normal carbs, making net carb counts seem higher than they actually are.

The solution is to know your sugar alcohols. The following sugar alcohols do not count toward net carbs for keto purposes:^[12][13][14]

• Erythritol
• Xylitol
• Mannitol
• Lactitol

If you’re eating something with the above sugar alcohols, you don’t need to include them in your carb count. The sugar alcohols below, however, do count (at least partially) toward net carbs:^[15]

• Maltitol
• Sorbitol
• Isomalt
• Glycerin

Each gram of maltitol, sorbitol, isomalt, or glycerin counts as about half a gram of carbs for keto, so take the number of grams of the sugar alcohol, divide by 2, and add it to your carb count. For example:

Net carbs = total carbs – fiber – sugar alcohols + (maltitol / 2)

Use the above formula to calculate your net carbs for keto. And while we’re talking about sugar alcohols, it’s worth mentioning that while you can’t digest them, your gut bacteria sometimes ferments sugar alcohols to various degrees, creating gas and bloating in your small intestine.

As a ballpark estimate, if you’re on a keto diet, don’t eat more than roughly 15 grams of sugar alcohols at a time, and be especially wary of mannitol, maltitol, and sorbitol — you may end up with digestive discomfort.

While it’s good to know how to calculate net carbs, you don’t have to track them too carefully unless you’re on a ketogenic diet. Barring that, the occasional extra few grams now and then won’t make a meaningful difference in your performance long-term.

And if you find you get dry eyes or unstable energy on a keto or low-carb diet, consider a weekly carb refeed day where you eat more carbs than usual. Everyone is slightly different; what matters is finding the net carb intake that works best for you.