Calorie counting: the maths is right. The numbers you type in are not.

Energy in versus energy out is physics, and physics doesn't care about your feelings. The trouble is that humans are terrible at measuring either side. Here's when counting helps and when it backfires.

What calorie counting actually is

You get a daily calorie number, usually your estimated needs minus a few hundred. Then you weigh, scan or guess everything you eat and log it in an app. Stay under the number and you lose weight. That's the pitch.

Flexible dieting is the gym version. It says no food is banned as long as it fits your daily protein, fat and carb targets. Pizza fits? Pizza's fine. It's popular because it feels less like a diet, and because people can eat Pop-Tarts and post about it.

The modern version lives in your phone. Scan a barcode, pick "chicken breast, grilled" from a list of 40 slightly different chicken breasts, and the app does the sums. It feels precise. It's an estimate built on estimates: the label, the database entry, your guess at the portion, and your memory of what you ate at 11 pm.

None of that makes counting useless. It makes it a rough tool. Rough tools are fine as long as nobody pretends they're a laser.

How it works in your body

Your body weight changes when the energy you take in and the energy you burn are out of balance. That's not a theory, that's the first law of thermodynamics. But the body isn't a bank account. When you eat less, your body burns a bit less and your appetite goes up, which is why weight loss slows down over time and why the old "cut 3,500 calories, lose a pound of fat" rule overestimates long-term results.¹

Labels count energy using fixed values: about 9 kcal per gram of fat, 4 for protein and carbs, and 7 for alcohol.⁹ An average adult needs somewhere around 2,000–2,500 kcal a day, depending on size, age and activity.¹⁰

What you eat, what you don't

Alcohol on calorie counting

50 Club: one glass of dry wine or one light beer, logged.

At 7 kcal per gram,⁹ alcohol is almost as energy-dense as fat, gives you nothing to eat, and lowers your resistance to everything else. Flexible dieters love to "save up" calories for drinks. Your liver doesn't do that kind of accounting. All the details are in our alcohol guide.

What it's good for, with proof

Tracking is linked to weight loss

A systematic review of 22 studies found a consistent link between self-monitoring (food diaries, weighing yourself) and weight loss.² The authors called the evidence weak because of how the studies were done: people who track are also people who are more motivated. Still, it's one of the most reliable habits in weight-loss programmes.

The macro split matters less than you think

In a two-year trial with 811 adults, four diets with very different amounts of fat, protein and carbs gave about the same weight loss.⁵ A meta-analysis of 48 trials of named diets found the differences between them were small.⁶ That's flexible dieting's best argument: the total counts more than the label on the diet.

The uncomfortable truth

And the biggest one: a calorie isn't a calorie for your appetite. In a tightly controlled study, people ate about 500 kcal a day more on an ultra-processed diet than on an unprocessed one, even though both were matched for sugar, fat and fibre on the plate.⁷ Counting doesn't fix food that's built to be overeaten.

The risks

• Eating-disorder risk. In a study of nearly 500 college students, those who used calorie trackers had higher levels of eating concern and dietary restraint.⁸ It can't prove the app caused it, but if logging makes you anxious, stop.
• Food quality goes missing. 300 kcal of crisps and 300 kcal of lentils are the same number and very different meals.
• Burn-out. Most people stop logging within weeks. A method you can't keep up doesn't work.
• Targets that are too low. Very low calorie targets cost muscle and energy, and belong under medical supervision.

Allergies: watch the staples

Calorie counting has no staples of its own. But the "diet foods" it often leans on do:

This article is general information, not medical advice. If you have a medical condition or take medication, talk to your doctor before changing how you eat.

Sources

1. Hall KD et al. Energy balance and its components: implications for body weight regulation. American Journal of Clinical Nutrition 2012;95(4):989–994. doi.org/10.3945/ajcn.112.036350
2. Burke LE, Wang J, Sevick MA. Self-monitoring in weight loss: a systematic review of the literature. Journal of the American Dietetic Association 2011;111(1):92–102. doi.org/10.1016/j.jada.2010.10.008
3. Lichtman SW et al. Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. New England Journal of Medicine 1992;327(27):1893–1898. doi.org/10.1056/NEJM199212313272701
4. Urban LE et al. Accuracy of stated energy contents of restaurant foods. JAMA 2011;306(3):287–293. doi.org/10.1001/jama.2011.993
5. Sacks FM et al. Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates. New England Journal of Medicine 2009;360(9):859–873. doi.org/10.1056/NEJMoa0804748
6. Johnston BC et al. Comparison of weight loss among named diet programs in overweight and obese adults: a meta-analysis. JAMA 2014;312(9):923–933. doi.org/10.1001/jama.2014.10397
7. Hall KD et al. Ultra-processed diets cause excess calorie intake and weight gain: an inpatient randomized controlled trial of ad libitum food intake. Cell Metabolism 2019;30(1):67–77. doi.org/10.1016/j.cmet.2019.05.008
8. Simpson CC, Mazzeo SE. Calorie counting and fitness tracking technology: associations with eating disorder symptomatology. Eating Behaviors 2017;26:89–92. doi.org/10.1016/j.eatbeh.2017.02.002
9. Regulation (EU) No 1169/2011 on food information to consumers, Annex II (the 14 allergens). eur-lex.europa.eu/eli/reg/2011/1169/oj Annex XIV gives the energy conversion factors.
10. NHS. Understanding calories. Accessed November 2026. www.nhs.uk/live-well/healthy-weight/managing-your-weight/understanding-calories