Ask the internet how many calories you should eat and you will get a hundred different numbers. The honest answer: it depends on your body and your goal, but the math behind it is simpler than it looks.
Start with your BMR
Your basal metabolic rate (BMR) is the energy your body burns just staying alive: breathing, pumping blood, keeping warm. The most reliable everyday formula is Mifflin-St Jeor:
- 10 × weight (kg) + 6.25 × height (cm) − 5 × age, then +5 for men or −161 for women.
That gives you a baseline before you have moved a muscle.
Add your activity (TDEE)
Multiply your BMR by an activity factor, roughly 1.2 if you are mostly sitting, up to about 1.7 if you are very active, and you get your Total Daily Energy Expenditure: the calories you burn on a normal day.
Then adjust for your goal
- Fat loss: subtract around 450 kcal for a steady, sustainable deficit.
- Muscle gain: add around 350 kcal so the surplus fuels muscle, not just fat.
- Maintain: eat at your TDEE.
One rule worth keeping: never crash below about 1,200 kcal a day without medical supervision. Very low targets backfire; they tank your energy, your muscle, and your ability to stick with the plan.
TailorFork’s questionnaire runs this exact calculation for you in about 30 seconds, then builds a week of meals to hit it.