How to read nutrition data (and compare foods fairly)

Nutrition numbers are everywhere — on packaging, in apps, in search results — but they are easy to misread. This guide explains how to read them and, just as importantly, how to compare two foods without being fooled.

The answer first

To compare foods fairly, put them on the same basis (we use per 100 g), and always check the preparation state (raw, cooked, drained, dry). Understand that calories are just the sum of the macronutrients: protein and carbohydrate at about 4 kcal per gram, fat at 9 kcal per gram. Treat “per serving” figures with suspicion, because the serving size is chosen by the manufacturer.

Per 100 g vs per serving

A “serving” is not a fixed quantity — it is whatever the label says. One cereal might use a 30 g serving and another 45 g, making a direct comparison meaningless. Per 100 g removes that distortion: it is the same weight for every food, so the numbers line up. That is why every page on this site reports values per 100 g, and why our comparison pages compute differences on that basis.

The trade-off is that per 100 g does not reflect how much you actually eat. You would never eat 100 g of a calorie-dense food like peanut butter (588 kcal/100 g) in one go, whereas 100 g of broccoli (34 kcal) is a small side. Use per 100 g to compare foods, then use our calorie and macro tool to scale to a realistic portion.

What the macronutrients are

Three macronutrients supply energy, plus alcohol:

Because fat carries more than twice the calories per gram, a small change in fat moves calories a lot. This is why nuts and oils are so energy-dense and why “low-fat” foods are often lower in calories. For a deeper look, read understanding macronutrients.

How calories are built from macros

You can roughly reconstruct a food’s calories from its macros. Take a whole egg: 12.6 g protein, 0.7 g carbohydrate, 9.5 g fat per 100 g.

(12.6 × 4) + (0.7 × 4) + (9.5 × 9)
= 50.4 + 2.8 + 85.5
≈ 139 kcal

That is close to the USDA figure of 143 kcal (small differences come from fiber, rounding and the energy of organic acids). If a label’s calories are wildly different from this calculation, treat it with caution.

Raw vs cooked: the biggest trap

Cooking changes water content, and water has no calories but adds weight. Two examples:

• Boiling adds water. Dry rolled oats are 389 kcal per 100 g, but a bowl of cooked porridge is far lower per 100 g because it is mostly water. The same applies to dry vs cooked rice, pasta and lentils.
• Roasting and frying remove water and can add fat. Roasted chicken breast is more calorie-dense than raw, and fried foods more so again.

So “100 g of oats” can mean 389 kcal or about 70 kcal depending on whether it is dry or cooked. We label the preparation state on every food page for exactly this reason.

Sugar, fiber and “net carbs”

Sugar is a subset of total carbohydrate, not an addition to it — a food with 20 g carbohydrate and 5 g sugar has 5 g of that 20 g as sugar. Fiber is also counted within total carbohydrate in USDA data, but it is largely indigestible, which is the basis of the “net carbs” idea (total carbs minus fiber). Whether net carbs are useful depends on your goals; for most people, total fiber intake matters more.

A quick checklist for fair comparisons

1. Are both foods on the same basis (per 100 g, or the same serving)?
2. Are both in the same state (both raw, or both cooked)?
3. Are you comparing the metric you care about — total calories, protein per calorie, fiber?
4. What is the source, and how recent is it?

Follow those four steps and you will avoid most nutrition-label mistakes.

A note on accuracy

The figures on this site are USDA FoodData Central reference values per 100 g of edible portion. They are population averages; real foods vary by variety, brand and cooking method. This is general information, not medical or dietary advice. See our methodology for how we source and compute everything.