The math it took to calculate the number of calories in your favorite snack involved a lot more guesstimation than you may think.

The Food and Drug Administration began requiring standardized nutrition labeling — including the number of calories per serving — on most packaged foods in 1990. Obesity rates skyrocketed in the United States over the next few decades, spurring a 2016 change to the rule to list calorie counts on nutrition labels in a large, bold font.

Just how accurate are these calorie counts?

The popular protein bar brand David is currently fighting a lawsuit alleging the company’s bars contain nearly twice as many calories as the labels claim, based on independent testing. The company’s founder said the lawsuit used an incorrect testing method to measure calories, and that the fat substitute the company uses in the protein bars does not contain as many calories as true fat does (about 9 calories per gram).

Nutrition scientists say there’s wide variation in how many nutrients and calories each person absorbs during digestion.

That’s why the FDA allows a discrepancy of up to 20% when counting calories.

For example, if a frozen meal is labeled as having 500 calories, it could have as many as 600 calories and still be compliant, said Lindsay Moyer, a registered dietitian at the Center for Science in the Public Interest.

Dr. Zhaoping Li, chief of the clinical nutrition division at UCLA, said the amount of energy we get from food is affected by many factors, such as “how well your stomach digests it, how much is absorbed in your small intestine, and what energy can be harvested after the microbiome works on it on the large intestine.”

Together, these variables can cause inconsistencies between how many calories a food actually contains and how many are listed on that food’s nutrition label — even if a company has made an earnest attempt at calculating them.

“It’s guesswork,” Li said.

Or as Marion Nestle, a professor emerita of nutrition, food studies and public health at New York University, puts it: “Calories are kind of eye-rolling.”

How do you measure calories in food?

The most accurate method to measure energy and calories in food is called a bomb calorimeter, Li said. The process works on the theory that when one thing releases heat — in this case, food — it’s absorbed by another, in this case whoever is digesting that food, as energy.

To make this calculation, food scientists seal a piece of food in a pressurized, oxygen-filled steel container — the “bomb.” That container is encased in an insulated box filled with water.

The scientists light the food on fire and let it burn completely. Then, they take the temperature of the water. They use an equation to tell them how much energy — and therefore calories — are in that piece of food, based on how much the burning piece of food raised the temperature of the water surrounding it.

But the bomb calorimeter’s calculations can be flawed, Nestle said.

“Not all the components of foods are absorbed, and the only thing that counts for calories is what gets absorbed across the intestinal wall — fiber calories, for example, don’t count,” she said.

Nestle said it’s much more common for food manufacturers to estimate the number of calories in packaged food using an Atwater factor. This mathematical equation uses a fixed number of calories per gram for macronutrients, specifically fat, carbohydrates and protein.

Every gram of carbohydrates — which include sugar — contains about 4 calories. The same is true for protein. Fat contains about 9 calories per gram. Food scientists can get a basic estimate of the number of calories in a food based on this equation.

“And that is close enough,” Nestle said. “You have to be comfortable with approximations.”

Some companies, particularly restaurants looking to calculate calorie counts for menu items, may use a U.S. Department of Agriculture database that houses calorie estimates for individual foods, such as a roma tomato or a commercially prepared slice of white bread. They may also use a more specific database calculation program that takes into account the restaurant’s suppliers, CSPI’s Moyer, said.

Still, this involves a bit of guesswork.

“Let’s say they get ingredients from a supplier. That supplier may have already done an analysis, but then it may be cooked or processed in a way that changes the nutrients of the ingredient,” Moyer said.

Then there’s the human factor. Restaurants that prepare food in-house likely have more variation in calorie counts than, say, a protein bar.

“If someone makes your sandwich at a place that makes subs, you might see them grab a preweighed slab of meat, so these calorie counts could be pretty accurate. But then if they take a squeeze bottle of sauce and just go for a while, that will vary,” Moyer said. “I wouldn’t treat calorie labels on a menu the same as a prepackaged food such as a bar.”

Few calories from fiber

Certain kinds of food are harder to break down, meaning the body actually absorbs fewer calories from them than a label may state.

For example, accurate calorie counts are particularly difficult to calculate for plant foods, which have cell walls that can be heard for teeth to break through, since chewing is the first phase of digestion, Moyer said.

“People typically absorb slightly fewer calories for these foods than what the label calculates,” she said.

Almonds are another good example, she said. People absorb more calories from almond butter than from whole almonds because “it’s basically chewed for you.”

Several studies conducted by the U.S. Department of Agriculture have found that whole almonds, cashews, lentils and chickpeas all have fewer calories once digested than what labels estimate. Walnuts, which are rich in protein and omega-3 fatty acids, have 21% fewer calories than typically believed, according to USDA estimates.

Insoluble fiber is another nutrient that may not contribute any calories at all, since the body doesn’t have the enzymes necessary to digest its chemical bonds — meaning it passes through the digestive system without getting broken down into energy. Whole grains, nuts, seeds and the skins of fruits and vegetables contain insoluble fiber: While they do have calories from sugars, fats and proteins, the calories that come from insoluble fiber aren’t absorbed.

“Generally, fiber is a carbohydrate that we can’t fully break down and digest, so fiber has few, if any, digestible calories,” Moyer said.

The large intestine can extract some, but not all, calories from another type of fiber, called soluble fiber. Oats, beans, lentils, apples and avocados all have high soluble fiber content. Soluble fiber can also prevent the digestive system from absorbing some fat and cholesterol.

So, should you trust calorie counts?

There’s limited research measuring the accuracy of packaged food labels, and the FDA doesn’t test every product on the market.

“It’s sort of ‘buyer beware’ if the facts on a nutrition label sound too good to be true,” Moyer said. “Compare it to similar products; if you are looking at a row of peanut butter and you see one that is 120 calories per serving when all the rest of them say 180 calories per serving, that would raise a red flag for me.”

Paying attention to the calorie counts on nutrition labels is a good place to start, she said, but no company can claim to calculate calories down to the single digit, for example, that a portion of frozen pizza has 352 calories.

“Folks mean well, and the calculation software will spit a number out to you to the precise calorie, but that’s a joke. No one knows to that degree,” Moyer said.