What BMI, BMR and TDEE Can and Cannot Tell You

BMI: a screening number for groups

Body mass index is weight in kilograms divided by height in metres squared. A woman of 165 cm and 68 kg has a BMI of 68 ÷ 1.65² = 24.98. Under the WHO categories that is the top of the normal range (18.5–24.9), one step below overweight at 25.

BMI was designed to describe populations, and it says nothing about what the weight is made of. A muscular athlete can score as overweight, and an older adult who has lost muscle can score normal while carrying a lot of fat. Cut-offs also differ by region: because health risks rise at lower BMI in many Asian populations, the WHO Asia-Pacific guidance and the Korean and Japanese obesity societies treat 23 as the start of overweight and 25 as obesity. The example above is normal by the WHO table and already in the higher band by those.

Waist and body fat add what BMI misses

Where fat sits matters more than how much a person weighs. Fat around the abdomen is linked more closely with heart disease and diabetes than fat elsewhere, so a tape measure adds real information. A simple rule of thumb is to keep the waist below half of your height: for 165 cm, under 82.5 cm.

Tape-based body fat estimates such as the US Navy method use neck, waist and, for women, hip measurements. They land within a few percentage points of a DEXA scan for most adults, which is close enough to follow a trend over months, though not to compare two people precisely. Bathroom scales that read body fat through the feet vary much more with hydration and time of day.

BMR: the energy you burn at rest

Basal metabolic rate is the energy the body uses lying still: breathing, circulation, keeping warm. The Mifflin–St Jeor equation, the usual choice today, is 10 × weight in kg + 6.25 × height in cm − 5 × age, plus 5 for men or minus 161 for women. For the same woman aged 35 that is 680 + 1,031.25 − 175 − 161 = 1,375 kcal a day.

This is an estimate from a regression, not a measurement. In validation studies it fell within about 10% of measured resting energy for roughly four people in five, which means one in five is further off. Very muscular people tend to burn more than it predicts, and it was not built for children or for people with conditions that change metabolism.

TDEE and calorie targets

Total daily energy expenditure multiplies BMR by an activity factor: 1.2 for mostly sitting, 1.375 for light exercise one to three days a week, 1.55 for moderate exercise three to five days, 1.725 for hard exercise most days. With moderate activity the example comes to 1,375 × 1.55 ≈ 2,132 kcal. Most people overestimate their activity, so starting one level lower is a sensible check.

A common rule says a deficit of about 7,700 kcal loses one kilogram of fat, so eating 500 kcal a day below TDEE would lose about 0.45 kg a week. In practice weight loss slows: the body gets lighter and burns less, and some adaptation follows. Treat the target as a starting point, keep it for two or three weeks, and adjust from what the scale and the tape measure actually do.

Protein and the limits of the numbers

Once the calorie figure is set, the split between protein, carbohydrate and fat matters for how you feel and what weight you lose. Protein and carbohydrate give about 4 kcal per gram and fat about 9. For people who exercise, 1.6 g of protein per kilogram of body weight is a commonly cited target, about 109 g a day for the example.

None of these formulas replaces a doctor's view. Pregnancy, breastfeeding, diabetes, kidney disease, eating disorders and many medicines change what is appropriate. The numbers are best used to notice a trend and to plan, not to judge one body against a single cut-off.

• Read BMI together with waist size, not on its own
• Check which BMI cut-offs apply where you live
• Pick the activity level you actually keep, not the one you plan
• Hold a calorie target for two to three weeks before changing it
• Expect weight loss to slow over time
• Ask a doctor first if a health condition, pregnancy or medicine is involved