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Even if you spent the entire day in bed, your body would burn roughly a thousand to two thousand calories keeping your heart beating, lungs working and temperature steady. That resting cost is your basal metabolic rate (BMR). Add movement and digestion and you get your total daily energy expenditure (TDEE), the figure most calorie plans are built on.
Where your energy actually goes
People often assume exercise is the main part of their daily burn. For most people it is not. Staying alive costs the most, followed by day-to-day movement such as walking, standing and fidgeting, then deliberate exercise, and finally the cost of digesting food itself.
- Basal metabolism (BMR)≈ 60–70%
- Daily movement and exercise≈ 20–30%
- Digesting food≈ 10%
These shares are typical ranges, not exact. A very active person has a bigger movement slice; an older or smaller person has a smaller BMR slice.
The formula, with real numbers
The most widely used equation today is Mifflin-St Jeor (1990). It uses weight in kilograms, height in centimetres and age in years.
750 + 1,093.75 − 150 + 5 = 1,699 kcal a day at rest. For a 30-year-old woman of 62 kg and 162 cm the same steps give about 1,322 kcal.
The older Harris-Benedict equation (revised in 1984) gives about 64 kcal more, a gap of roughly 4%. Real people differ from both by a similar margin.
From BMR to TDEE
To include daily movement, multiply BMR by an activity factor. The factors are rough averages, and most people choose a level that is too high.
Same body, different lives: the difference between a desk job and a physical job with training is about 1,200 kcal a day.
Turning TDEE into a plan
Eating about as much as your TDEE keeps weight roughly stable. Eating more leads to gradual gain; eating less to gradual loss. A commonly quoted rule of thumb is that about 7,700 kcal is one kilogram of body weight, so a daily gap of 500 kcal is a bit under half a kilogram a week. Real bodies adapt, so the figure is a guide, not a promise.
Values are the percentage of TDEE eaten. A cut of 10 to 20% is generally considered moderate. Very large cuts are harder to keep up and are best planned with a doctor or dietitian.
- 1Estimate TDEEUse the calculator with an honest activity level.
- 2Track for 2–3 weeksEat roughly that amount and weigh yourself at the same time each week.
- 3Adjust by 5–10%If weight moved the wrong way, change intake slightly and repeat. The trend is the real answer.
Why two people of the same weight can differ
This is why a formula can be 10% off for a particular person even when the inputs are right.
Quick practice
- BMR for a 40-year-old man, 80 kg, 180 cm? 800 + 1,125 − 200 + 5 = 1,730 kcal.
- TDEE with light exercise (×1.375)? 1,730 × 1.375 ≈ 2,379 kcal.
- If someone eats 2,000 kcal against a TDEE of 2,400, what is the daily gap? 400 kcal, or about 17%.
- Why can two people with the same BMR gain and lose weight differently? Different movement, digestion, sleep and measurement error.
Pair the calorie estimate with other numbers: BMI for a rough weight check, heart rate zones for training, and sleep cycles for recovery.
Questions people ask
What is BMR?
Basal metabolic rate is the energy your body needs at complete rest to keep you alive, mostly breathing, circulation, temperature control and cell repair.
What is the difference between BMR and TDEE?
BMR is energy at rest. TDEE adds daily movement, exercise and digesting food, so it is always higher.
Which BMR formula is the best?
Mifflin-St Jeor is widely recommended for the general population. The Katch-McArdle formula can be better when body fat is known. None is accurate for everyone.
How many calories should I eat to lose weight?
A moderate gap of 10 to 20% below your TDEE is common. The best number depends on health, age and goals, so check with a professional for anything larger.
Does muscle really raise BMR?
Yes, but modestly. A kilogram of muscle burns only a few more calories at rest than a kilogram of fat, so the effect is real but small.
Are calorie calculators accurate?
They are estimates, usually within about 10%. Use the result as a starting point and adjust from how your weight changes over a few weeks.