Why the Calories on a Label Never Match What You Actually Eat

Pick up almost any packaged food and the nutrition panel will tell you the calories “per 100g”, sometimes with a second, smaller figure for “per serving” next to it. The problem: you almost never eat exactly 100g of anything, and the manufacturer’s own idea of a “serving” often isn’t what you’d actually put on your plate. The label isn’t wrong, it’s just describing a different amount than the one in front of you, and getting from one to the other is a real, one-step calculation, not a guess.

Why Labels Use 100g, Not Your Real Portion

UK food labelling law, based on retained EU Regulation (EU) No 1169/2011 on the provision of food information to consumers, requires the mandatory nutrition declaration on prepacked food to be given per 100g or per 100ml. That’s a fixed, comparable reference amount, the whole point is that you can put two products side by side and compare them fairly, regardless of pack size. A per-portion figure is genuinely useful too, but it’s additional, voluntary information under the same rules, and the manufacturer picks what counts as “a portion” for their own product, not a universal standard everyone uses the same way.

The Real Maths: How to Scale It Yourself

The calculation itself is a plain ratio, the same one behind every “per 100g” conversion:

• Your calories = label calories × (your amount ÷ label amount)

Apply the exact same ratio to protein, carbs and fat too, if the label gives them for the same reference amount, since each one scales independently and at the same rate.

A Real Worked Example

Our own database lists Baked Beans (canned) at 84kcal, 5.2g protein, 13.5g carbs and 0.6g fat per 100g, the exact shape of figure a real tin’s label would give you. Say you’re having a real 150g portion on toast, not 100g. The ratio is 150 ÷ 100 = 1.5, so:

• Calories: 84 × 1.5 = 126kcal
• Protein: 5.2 × 1.5 = 7.8g
• Carbs: 13.5 × 1.5 = 20.3g
• Fat: 0.6 × 1.5 = 0.9g

That’s the real difference a genuine portion makes, 42 more calories than the bare 100g figure most people glance at and stop reading.

When the Label’s Own “Serving Size” Isn’t Yours

Since the per-portion figure on a label is voluntary and manufacturer-chosen, it’s worth checking what that reference amount actually is before trusting it, it doesn’t have to match how much you’d genuinely serve yourself. The same underlying per-100g convention is also what the UK’s red, amber and green traffic-light system is built on, see How the UK’s Traffic Light Food Labels Actually Work for how those colours get assigned.

Skip the Maths: Use a Real Gram to Calorie Calculator

The ratio itself is simple, but it’s easy to slip a decimal place doing it in your head, especially with an awkward portion like 145g or 235g. Our own Gram to Calorie Calculator runs this exact calculation instantly, calories and macros together, from any known amount to your own real portion. If you’d rather estimate a portion by eye instead of weighing it, Tracking Macros Without an App: the Hand-Portion Method covers that different approach.

*This article is general information, not medical or dietary advice. If you’re making dietary changes for a specific health reason, speak to a doctor or registered dietitian.