How to Read Nutrition Labels and Use a Food Barcode Scanner

By the DietX team · Updated November 4, 2026 · 6 min read

To read a nutrition label, start with the serving size, because every number below it refers to that amount, then scale the calories to what you actually ate. A food barcode scanner saves you the typing, but check its result against the package. Labels themselves are close estimates: US rules treat food as misbranded if calories run more than 20% over what the label states.

How to read a nutrition label

The US Nutrition Facts label puts serving information first, and that's the right order to read it in.

1. Serving size. This is the amount all the other numbers refer to.
2. Servings per container. A package that looks like one portion may hold two or three.
3. Calories and macros. Calories per serving, plus protein, carbs and fat. Protein and carbs carry about 4 kcal per gram and fat about 9 (FAO).
4. Your actual amount. Scale everything to what you ate.

Here is a worked example with a made-up label. A bag of granola says: serving size 30 g, 150 kcal per serving, 3 servings per bag.

• A 45 g bowl is 150 × 45 ÷ 30 = 225 kcal.
• The whole bag is 150 × 3 = 450 kcal.

What is a serving size?

A serving size is the amount the label's figures describe. It doesn't tell you how much to eat, and it may be smaller than what you'd normally pour. Log that 45 g bowl of granola as one serving and you'd record 150 kcal instead of 225.

To see how your usual portion compares, weigh it once on a kitchen scale.

Per 100 g versus per serving

Many labels, especially outside the US, show values per 100 g, sometimes next to a per-serving column. It's easy to log the wrong one.

Per 100 g is also the fair way to compare two products, because brands choose their own serving sizes. The granola above works out to 150 ÷ 30 × 100 = 500 kcal per 100 g.

A real example from USDA data: plain nonfat Greek yogurt has 59 kcal per 100 g, so a 170 g container is about 100 kcal.

Cooked versus raw weight

Cooking changes weight, mostly through water, while the calories stay with the food. Packaged rice, pasta and oats are often labeled for the dry product, so check whether the figures say dry or as prepared.

Match the value to how you weighed the food.

• 75 g of dry rice is about 274 kcal. Logged by mistake with the cooked value, it reads about 98 kcal, so 176 kcal too low.
• 200 g of cooked rice is 260 kcal. Logged by mistake with the raw value, it reads 730 kcal, so 470 kcal too high.

This mix-up is on our list of common calorie counting mistakes, and it's easy to fix once you know about it.

How accurate are nutrition labels?

Close, with some room for error. Under US rules, food counts as misbranded if its calories run more than 20% over the label, so a 400 kcal label could hold up to 480 kcal without crossing that line. That's the line for misbranding and doesn't tell you what a typical product contains. A study of 10 frozen meals measured them 8% above the stated calories on average, which would make a 400 kcal meal 432 kcal. It's a small sample, so take it as a rough signal.

In practice, labels are good enough for daily tracking. Misreading the serving can easily cost more than the label's own error, as the granola example shows, so put your effort there.

How a food barcode scanner helps, and when it goes wrong

A barcode scanner reads the code on the package and pulls the matching entry from the app's food database. It's a great fit when:

• The food comes in a package with a barcode.
• You eat the same products often.
• It's a single-serve item, so there's no portion to work out.

It helps less with loose food, restaurant meals and homemade dishes. And you still need to say how much you ate: a scan tells the app what the product is, and the bowl you poured is up to you.

How to check a wrong barcode result

Entries can be out of date, match the wrong pack size or assume the wrong serving, so run a quick check.

1. Match the name and size. Is it the same product, flavor and pack?
2. Compare the calories. Hold the package next to your phone and compare the figure per serving or per 100 g.
3. Check the serving it assumed. One serving and one package are very different numbers.
4. Run the macro sum. Protein × 4 + carbs × 4 + fat × 9 should land near the calories (FAO). For plain nonfat Greek yogurt (USDA), 10.2 × 4 + 3.6 × 4 + 0.4 × 9 = 58.8, close to the listed 59 kcal per 100 g. Fiber, alcohol and rounding can shift this a little, but a big gap suggests the entry is wrong.
5. Fix it. Use the numbers on the package. In DietX, you can edit any item before you save it.

Which food barcode scanner apps charge for it?

This is where apps differ most. Prices below are from the US App Store.

We couldn't confirm a barcode scanner for SnapCalorie, which does list a nutrition label scanner.

If a free barcode scanner is what you're after, Cronometer and YAZIO are better picks than DietX, which has no free subscription tier. Our guide to free calorie counter apps goes through what each free plan includes.

DietX makes more sense if you'd like barcode scanning in the same place as photo, voice and text logging, since not everything you eat comes with a code. For food without a package, see how to log food by voice, and if you're just starting out, read our beginner's guide to counting calories.

Frequently asked questions

Sources

1. FDA: How to understand and use the Nutrition Facts label
2. US Code of Federal Regulations: 21 CFR 101.9, nutrition labeling
3. FAO: Food energy, methods of analysis and conversion factors
4. USDA FoodData Central (SR Legacy)
5. Urban et al. (2010), J Am Diet Assoc: Stated energy contents of reduced-energy foods
6. Cronometer: Gold
7. YAZIO
8. fatsecret on the App Store
9. Lifesum
10. MyFitnessPal: Premium
11. Lose It! on the App Store
12. MacroFactor on the App Store
13. Cal AI on the App Store
14. DietX on the App Store

General information, not medical advice. Talk to a doctor or dietitian before changing your diet, especially if you are pregnant, under 18 or have a health condition. If tracking food makes you anxious or you have a history of disordered eating, please skip calorie counting and ask a professional for support.