A calorie is a unit of energy, and the one printed on a food label is really a kilocalorie: a thousand of the calories you met in physics class. One calorie is the heat needed to raise a gram of water by one degree Celsius. Nutrition labels have used the shorthand for so long that nobody bothers correcting it.
What the number on a label represents
Food is chemical energy, and your body is the machine that extracts it. Protein and carbohydrate give you about 4 calories per gram. Fat gives you 9. Alcohol sits at 7, which explains a great deal about how quickly drinking adds up.
That is why fat-heavy foods carry so many calories in so little volume. A tablespoon of oil and a large apple are not remotely the same size, and the oil has more energy in it. Neither is good or bad on its own, but if you are trying to eat less while staying full, the difference is the whole game. There is more detail in our breakdown of calories per gram.
Calories and kilojoules
Outside the US you will often see kilojoules instead. One calorie is about 4.184 kilojoules, so a 500 calorie meal is a little over 2,000 kJ.
If you are using an app set to the wrong unit, this is where confusion starts. A daily target of 2,000 means something very different depending on which unit is in the box.
Energy in, energy out
Eat more than you burn and you gain weight. Eat less and you lose it. This is not controversial, and it is also not the whole picture, because both sides of that equation move.
What you burn depends on your size, your age, your sex, how much muscle you carry and how much you move, including all the fidgeting and pacing you never think about. What you absorb depends on the food itself, since fibre passes through and protein costs energy to process.
So the arithmetic holds while the inputs wobble. Anyone who tells you calorie balance is a myth is wrong, and anyone who tells you it makes weight management simple has not tried it. Our piece on whether calories in versus calories out holds up goes through the objections one by one.
Not all calories behave the same
The energy is the energy. 200 calories of chicken and 200 calories of biscuits are the same 200 calories on the ledger.
What differs is everything around it. Protein is the most filling of the three macronutrients and costs the most energy to digest. Fibre slows things down and keeps you satisfied. Highly processed food is engineered to be easy to eat quickly, which is exactly the property you do not want when you are trying to eat less of it.
So the calorie count tells you the cost, and the food tells you how full you will be for that price. Both matter, and treating either as the only thing that counts leads people astray.
Working out how many you need
Start with an estimate. The TDEE calculator here uses the Mifflin-St Jeor equation to work out what you burn at rest, then multiplies by an activity factor. The older Harris-Benedict equation does a similar job and tends to be less accurate for modern populations.
Then stop trusting the estimate and start testing it. Eat at that number for two or three weeks and watch the trend on the scale. If your weight holds, the estimate was right. If it drifts, adjust by 100 to 200 calories and check again. No formula knows your job, your sleep, or how much you actually move.
Making the number easier to hit
Eat more protein than feels necessary. It keeps you full and it protects muscle when you are in a deficit.
Put volume on the plate: vegetables, fruit, anything with water and fibre in it. You can eat a satisfying amount of food for very few calories that way.
Watch what you drink, because liquid calories do almost nothing for fullness. And weigh the handful of foods you eat every day, at least for a couple of weeks, because oil and nut butter are where most people's arithmetic quietly falls apart. Our guide to tracking accurately covers the rest.