How Many Calories Do I Need a Day? BMR and TDEE Explained

The 2,000 Calorie Myth

The figures printed on food packaging — 2,000 calories a day for women, 2,500 for men — are population averages designed for labelling, not personal targets. A 25-year-old builder and a 60-year-old office worker do not need the same amount of energy, and neither of them is likely to need exactly 2,000.

Your own number comes from two calculations, and both are simple enough to do on paper.

BMR and TDEE: The Two Numbers That Matter

BMR (Basal Metabolic Rate) is what your body burns doing absolutely nothing — keeping your heart beating, your lungs working, and your temperature stable. If you stayed in bed all day, this is roughly what you would use.
TDEE (Total Daily Energy Expenditure) is your BMR plus everything else: walking to the kitchen, typing, fidgeting, training, and digesting your food. This is your actual maintenance level.

BMR typically accounts for around 60–70% of the total, which surprises people who assume exercise dominates. It does not.

Step 1: Calculate Your BMR

The most widely used equation is Mifflin-St Jeor, published in 1990 and generally more accurate for modern populations than the older Harris-Benedict formula. It takes weight in kilograms, height in centimetres, and age in years:

Men: BMR = (10 × weight) + (6.25 × height) − (5 × age) + 5
Women: BMR = (10 × weight) + (6.25 × height) − (5 × age) − 161

The only difference between the two is the constant on the end, which accounts for average differences in body composition.

Step 2: Multiply by Your Activity Level

BMR alone is not a target — eating at it would leave you well short. Multiply it by the factor that matches how you actually live:

Sedentary (× 1.2): little or no exercise, desk job
Lightly active (× 1.375): exercise 1–3 days a week
Moderately active (× 1.55): exercise 3–5 days a week
Very active (× 1.725): exercise 6–7 days a week
Extra active (× 1.9): intense daily training, or a physically demanding job on top of exercise

TDEE = BMR × Activity Multiplier

Two Worked Examples

A woman, 35, 68 kg, 165 cm, training three to five times a week:

BMR = (10 × 68) + (6.25 × 165) − (5 × 35) − 161 = 1,375 kcal
TDEE = 1,375 × 1.55 = 2,132 kcal to maintain

A man, 40, 85 kg, 180 cm, at the gym twice a week:

BMR = (10 × 85) + (6.25 × 180) − (5 × 40) + 5 = 1,780 kcal
TDEE = 1,780 × 1.375 = 2,448 kcal to maintain

The Multiplier Is Where People Go Wrong

Getting your weight and height slightly wrong barely moves the answer. Getting the activity level wrong wrecks it.

Take the man above. At lightly active he needs 2,448 calories. If he selected extra active instead, the calculator would tell him 3,382 — a difference of 935 calories a day, which is a large meal, every day, in the wrong direction.

The common error is counting workouts twice: an hour in the gym feels significant, but if the other twenty-three hours are spent sitting, you are lightly active, not very active. A useful rule of thumb is to pick the level you think applies and then drop down one. If your weight holds steady after a few weeks, you chose correctly.

Turning TDEE Into a Goal

TDEE is maintenance. Adjust from there:

To lose weight: TDEE − 500 calories a day
To maintain: eat at TDEE
To gain: TDEE + 500 calories a day

The 500 comes from the rough convention that a pound of body fat stores about 3,500 calories, so a 500 daily deficit equals roughly 0.45 kg (1 lb) a week. Our 35-year-old woman would eat about 1,632 calories a day to lose at that rate.

Bigger deficits are not better. Aggressive restriction costs you muscle alongside fat, is much harder to sustain, and tends to end in the rebound that undoes it. As a general floor, daily intakes below roughly 1,500 calories for men or 1,200 for women should only be undertaken with medical supervision.

Why the Scales Argue With the Maths

People abandon a plan in week two because the numbers say one thing and the bathroom scale says another. Usually the maths is fine:

Water weight moves far faster than fat. Salt, carbohydrate intake, hormones, and glycogen stores can swing your reading by a kilo or more within a day. Fat loss cannot.
Your TDEE falls as you lose weight. A lighter body costs less to run, so the deficit that worked at the start gradually becomes maintenance. Recalculate every 4–5 kg.
Calorie labels are approximate. Packaging is permitted a margin of error, and portion estimates at home are usually generous.

Judge progress on a weekly average rather than any single morning, and give any change at least two to three weeks before deciding it has failed.

What These Formulas Cannot Know

Mifflin-St Jeor is an equation fitted to population data, and individual metabolic rates vary by roughly ±10% around the prediction — a few hundred calories either way. It also cannot see your body composition, which matters because muscle is more metabolically active than fat. If you know your body fat percentage, the Katch-McArdle formula will usually serve you better.

Treat the output as a well-informed starting point to be adjusted against real-world results, not a prescription. If you are pregnant or breastfeeding, under 18, managing a medical condition, taking medication that affects weight or appetite, or recovering from an eating disorder, your requirements sit outside what any general formula models — speak to a GP or a registered dietitian.

Conclusion

Work out your BMR, multiply by an activity level you can defend honestly, and adjust by 500 calories in whichever direction you are heading. That gives you a starting figure grounded in your own body rather than a number from the side of a cereal box. Run your details through a TDEE Calculator to get your BMR, maintenance level, and goal targets in one step — then let a few weeks of real data fine-tune it.

Try our TDEE Calculator

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