Food carbohydrates are measured primarily through a method called “calculation by difference,” where a lab determines the weight of everything else in the food (protein, fat, water, and mineral ash) and subtracts it all from the total weight. Whatever is left over is counted as carbohydrate. This is the standard approach behind the numbers on nutrition labels, though more precise techniques exist for measuring specific types of carbs like sugars, fiber, and sugar alcohols.
The “By Difference” Method
The official formula works like this: total carbohydrate = total weight of the food minus protein, minus fat, minus moisture, minus ash. Ash is the mineral residue left after burning the food at high temperatures. Each of those four components is measured directly using established lab techniques, and the carbohydrate value is simply what remains.
This approach is practical but imperfect. Because carbohydrate is the only nutrient not measured directly, any small errors in measuring the other four components get rolled into the carbohydrate number. If a lab slightly overestimates moisture content, the carbohydrate value drops. If fat is slightly underestimated, the carbohydrate value goes up. The result is a reasonable estimate, not a precise measurement, which is one reason nutrition labels carry some built-in wiggle room.
How Labs Measure Individual Sugars
When food scientists need to know exactly how much glucose, fructose, sucrose, or maltose a food contains, they turn to a technique called high-performance liquid chromatography (HPLC). The process involves dissolving the food sample in a solvent (often an 80% ethanol solution), then pushing the liquid through a column packed with material that separates different sugar molecules based on their chemical properties. A detector at the end identifies and quantifies each sugar individually.
This method is far more precise than calculation by difference, but it’s also more expensive and time-consuming. It’s used in product development, quality control, and research rather than routine nutrition labeling. One study comparing different extraction methods found that sugar levels in the same wheat flour sample varied by as much as 1.8% of dry weight depending on which preparation technique the lab used, highlighting how sensitive these measurements can be to methodology.
For liquids like fruit juice, soft drinks, and tomato concentrate, a simpler tool called a refractometer can estimate sugar content. It works by measuring how much a beam of light bends as it passes through the liquid. Sugar dissolved in water changes the angle of light refraction in a predictable way, and this reading is converted to a Brix value: one degree Brix equals one gram of sucrose per 100 grams of solution. It’s fast and inexpensive, though it only gives a total sugar estimate rather than breaking down individual sugars.
Measuring Sugar Alcohols and Complex Carbs
Sugar alcohols like erythritol, xylitol, and sorbitol are common in sugar-free products, and measuring them requires a different approach. Gas chromatography paired with mass spectrometry (GC-MS) can identify and quantify sugar alcohols alongside regular sugars and organic acids in a single test. The food sample is chemically modified to make these compounds volatile enough to travel through a gas chromatography column, where they separate by weight and chemical structure before a mass spectrometer identifies each one.
This technique is powerful enough to simultaneously measure sugars, sugar alcohols, and carboxylic acids from a single injection of the sample. It’s particularly useful for fruits and processed foods where multiple types of carbohydrates coexist.
What Counts as Fiber on the Label
Dietary fiber has a specific regulatory definition that affects how carbohydrates are reported. The FDA defines dietary fiber as non-digestible carbohydrates with three or more linked sugar units that are either naturally present (“intrinsic and intact”) in plants or are isolated and synthetic fibers that the FDA has determined provide a measurable health benefit.
Those health benefits must be demonstrated through scientific evidence. Qualifying effects include lowering blood sugar after meals, reducing LDL cholesterol, lowering blood pressure, improving bowel regularity, or increasing mineral absorption. A manufacturer can petition the FDA to add a new fiber to the approved list, but it must come with data showing a real physiological effect. This means not every non-digestible carbohydrate automatically qualifies as fiber on a label.
How Accurate Nutrition Labels Are
Nutrition labels aren’t expected to be perfectly exact. The FDA classifies total carbohydrate as a “Class II nutrient” that must be present at 80% or more of the declared value. So if a label says 30 grams of carbohydrate per serving, lab testing should find at least 24 grams.
There’s a notable exception for foods where sugars make up 90% or more of the total carbohydrate, like fruit drinks, juices, and candy. For those products, carbohydrate is treated differently: the lab-tested amount must be no more than 120% of what the label claims. In other words, if the label says 25 grams, the actual content can’t exceed 30 grams. This rule exists because rounding conventions can create misleading discrepancies in sugar-heavy foods.
Net Carbs and What They Leave Out
The “net carbs” number you see on many food packages isn’t an official FDA measurement. It’s calculated by taking total carbohydrates and subtracting fiber and sugar alcohols. The logic is that fiber passes through your digestive system without being absorbed and most sugar alcohols have minimal effect on blood sugar, so neither should “count” the same way that starch or table sugar does.
This is why a protein bar with 24 grams of total carbohydrate can claim only 6 net carbs if it contains enough fiber and sugar alcohols. The calculation is straightforward, but it oversimplifies things. Different sugar alcohols affect blood sugar to different degrees, and some types of fiber are partially fermented in the gut, providing a small amount of energy. Net carbs is a useful shorthand, not a lab measurement.
Measuring Carbs by Their Effect on Blood Sugar
All of the methods above measure the physical quantity of carbohydrate in a food. But the same number of grams from different foods can have wildly different effects on your blood sugar. The glycemic index (GI) captures this by scoring foods on a scale based on how quickly they raise blood glucose after eating. White bread spikes blood sugar fast and scores high; lentils release glucose slowly and score low.
The glycemic index has a blind spot, though. It doesn’t account for how much carbohydrate a typical serving actually contains. Watermelon has a high GI but relatively few carbs per slice. A related measure called glycemic load combines both factors: the speed of blood sugar rise and the total carbohydrate in a real-world portion. That said, Harvard Health notes that the total amount of carbohydrate in a food is still a stronger predictor of blood sugar response than either the glycemic index or glycemic load alone. For most practical purposes, knowing how many grams of carbohydrate you’re eating matters more than where those carbs fall on any index.