Calories in, calories out — and why it is harder than that
Eat less than you spend and you lose weight. That part is not in dispute, and it is also not the difficult part.
The difficulty is that both sides of the equation are estimates. What you spend comes from a formula fitted to other people, what you eat comes from labels with legal tolerances on them, and the conversion between calories and kilograms is an average of something that varies. The arithmetic is fine. The inputs are the problem.
Most of what you burn happens while doing nothing
Basal metabolic rate is what the body spends breathing, circulating blood and keeping warm. For a 35-year-old man of 80kg and 180cm it is around 1,755 calories a day by the Mifflin-St Jeor formula — and that is roughly two thirds of a typical daily total.
This is why exercise moves the number less than people expect. Adding an hour of moderate activity does not double anything; it adjusts the smaller third. Someone moderately active on those figures spends about 2,720 a day, so the activity is worth a few hundred rather than a thousand.
Two formulas are usually shown because both are in wide use and they disagree by a few per cent. Mifflin-St Jeor is the modern default and generally the closer; Harris-Benedict is older and reads a little higher. Neither knows anything about your body composition, so both understate a very muscular person.
The activity multiplier is the number to adjust
Maintenance is basal rate times an activity multiplier — 1.2 for sedentary up to 1.9 for an athlete — and those multipliers are broad brackets covering very different lives. Most of the error in a maintenance estimate lives here rather than in the basal formula.
The practical method is to treat the calculated figure as a hypothesis. Eat at it for two or three weeks, weigh consistently, and see what happens. If the weight is not doing what the multiplier predicted, the multiplier is the input to change, not the formula.
Two or three weeks is the minimum, because weight moves day to day with water, food in transit and hormones by considerably more than a week of fat loss weighs. Weighing daily and reading each number is a reliable way to make yourself miserable about noise.
Work it out: TDEE Calculator →
Where the timeline comes from, and why it is optimistic
A kilogram of body fat holds roughly 7,700 calories. That is the number every weight-loss projection runs on: total the deficit, divide, and you have a timeline.
Going from 80kg to 72kg on a 500 calorie daily deficit needs 61,600 calories of deficit, which at 500 a day is 123 days — about 17.7 weeks, losing 0.45kg a week.
Real loss does not follow that line, and it slopes the wrong way. A lighter body burns less doing the same things, so maintenance falls as you lose. Adaptive thermogenesis reduces it further. Non-exercise movement quietly declines when you are eating at a deficit. All three push in the same direction, so the last few kilograms take longer than the projection says.
This is not a reason to distrust the calculation — it is a reason to treat it as a floor on the time rather than a schedule, and to recalculate as the weight comes off.
80 kg to 72 kg = 8 kg
8 kg x 7,700 cal = 61,600 calories of deficit
At 500 cal a day = 123 days (17.7 weeks)
Eating 2,100 against a 2,600 maintenance
Work it out: Calorie Deficit Calculator →
Protein is the macro worth getting right
The recommended daily allowance of 0.8g per kilogram is a floor for avoiding deficiency, not a target for anyone training. Reading it as a target is probably the most common mistake in this whole area.
For building or holding muscle the useful range is 1.6 to 2.2g per kilogram — for an 80kg person, 128 to 176g a day, or around 152g in the middle. That is 608 calories from protein alone, which shapes everything else on the plate.
Spreading it across the day works better than one large serving, because there is a limit to how much can be used at once for tissue repair. Four meals of similar size — about 38g each here — is a reasonable default.
If you are carrying a lot of fat, work from lean mass or a target weight rather than current weight. Protein needs scale with the tissue that uses it, and fat tissue does not.
Work it out: Protein Intake Calculator →
Macro splits matter less than the total
Once protein is set and the calorie total is set, the division of the rest between carbohydrate and fat is largely a question of what you find easy to stick to. The commonly quoted splits — 40/30/30 and its relatives — are conventions rather than findings.
What they are useful for is turning a percentage into grams, which is the form food labels come in. At 2,500 calories a 30/40/30 split is 188g of protein, 250g of carbohydrate and 83g of fat.
The order of importance is worth stating plainly: total calories first, protein second, everything else a long way behind. A plan that gets the first two right and ignores the rest will outperform a meticulously balanced one that misses the total.
Work it out: Macro Calculator →
Questions
How many calories should I eat to lose weight?
Roughly 500 a day below maintenance, which produces about half a kilogram a week. Maintenance itself is an estimate — treat the calculated figure as a starting point and adjust it after a few weeks of real data.
How many calories in a kilogram of fat?
About 7,700. That is where every weight-loss timeline comes from: total the deficit and divide. It is an average of something that varies between people.
Why am I losing weight more slowly than the calculator said?
Because a lighter body burns less, metabolic rate adapts downward under a deficit, and unconscious movement falls off. All three slow the loss, so the later stages always run behind a straight-line projection.
Does exercise burn as much as I think?
Usually less. Basal rate is about two thirds of daily expenditure, so activity is adjusting the smaller third. An hour of moderate exercise is worth a few hundred calories, not a thousand.
How much protein do I need?
1.6 to 2.2g per kilogram if you are training — around 128 to 176g for an 80kg person. The 0.8g recommended allowance is a deficiency floor, not a target.
Do macro ratios matter?
Far less than total calories and protein. Once those two are set, the split of the remainder between carbohydrate and fat is mostly about what you can sustain.
How often should I weigh myself?
Weekly, and watch the trend. Day-to-day swings from water, food in transit and hormones are larger than a week of actual fat loss.