How to calculate your daily calorie needs — the method a nutritionist would use

Every calorie calculator gives you a number and moves on. Most don't tell you where that number came from, how accurate it is, or when it'll stop being right. This post walks through the exact method a registered nutritionist would use — the Mifflin-St Jeor equation — so you can understand your Total Daily Energy Expenditure (TDEE), not just copy it.

What is TDEE and why does it matter?

Your Total Daily Energy Expenditure is the total number of calories your body burns in a day — everything from keeping your heart beating to climbing stairs to digesting food. It's the one number that anchors every nutrition goal: eat below it to lose fat, above it to gain muscle, at it to maintain weight.

Step 1: Calculate your Basal Metabolic Rate (BMR)

BMR is the number of calories your body burns at complete rest — the energy needed to keep your heart beating, lungs breathing, and brain running. We calculate it using the Mifflin-St Jeor equation, the most accurate formula in routine clinical use. It was derived from a 1990 study and has since been validated as the best-performing predictive equation for most healthy adults.

The two equations differ by a constant (166 calories) because, on average, men carry more lean mass per kilogram of body weight than women — and lean mass burns roughly three times more calories at rest than fat tissue does.

Step 2: Multiply by your activity factor

BMR is what you'd burn lying perfectly still. To get your total daily expenditure, you factor in everything else — walking, working, training, even fidgeting. This is done by multiplying BMR by an activity factor. The result is your TDEE.

“Most people overestimate their activity level. If you're unsure between two levels, always pick the lower one — you can always adjust if your weight isn't responding the way the maths says it should.— Sarah Mitchell, RD

Step 3: Adjust for your goal

TDEE is your maintenance number — eat that and your weight stays put. To change body composition, you shift it. A 500 kcal daily deficit is the standard sustainable rate for fat loss: fast enough to see results, slow enough to preserve muscle. For muscle gain, a smaller surplus of 250 kcal minimises fat gained alongside new muscle tissue.

Step 4: Split into macros

Total calories set the budget; macros decide what you build with it. Protein is calculated from bodyweight rather than as a percentage — because protein needs scale with the amount of tissue you carry, not how much you eat. Fat sits at 30% to support hormone production. Carbohydrates fill whatever is left, fuelling training and recovery.

Protein: 2.0 g × bodyweight (kg)
Fat: 30% of daily calories ÷ 9
Carbs: remaining calories ÷ 4

The number you get today isn't your final answer

The Mifflin-St Jeor equation is accurate to roughly ±10% for most people. The only way to discover your true TDEE is to eat at the calculated number for one to two weeks and watch what your body does. If you're losing weight faster than the plan predicts, your true TDEE was higher than estimated. Slower? It was lower. Adjust accordingly. A 4–5 kg change in body weight, or a significant sustained change in activity level, is enough to warrant a recalculation.