Two numbers come up in almost every conversation about weight: BMI, which is a ratio of weight to height, and TDEE, the calories you burn in a day. Both are estimates from simple formulas. Both are useful as starting points. Neither is a diagnosis, and it helps to know exactly what each one is measuring so you know when to trust it.
BMI: a ratio, not a verdict
Body mass index is weight in kilograms divided by height in metres squared. For someone 5 ft 9 in (1.753 m) and 165 lb (74.8 kg):
BMI = 74.8 ÷ (1.753 × 1.753) = 24.4
The standard categories for adults:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity, class 1 |
| 35 to 39.9 | Obesity, class 2 |
| 40 and above | Obesity, class 3 |
The BMI calculator also turns the healthy range back into a weight range for your height, which is often the more useful number: at 5 ft 9 in, a BMI of 18.5 to 24.9 corresponds to about 125 to 169 lb.
Where BMI is wrong
BMI was designed for populations, not individuals. It doesn’t know what your weight is made of, so it fails predictably for:
- Muscular people. A 5 ft 10 in athlete at 200 lb has a BMI of 28.7, “overweight”, with perhaps 10% body fat.
- Older adults, who tend to lose muscle and gain fat at the same weight, so a “healthy” BMI can hide a lot of fat.
- Very tall and very short people. The formula squares height, which slightly overstates BMI for tall people and understates it for short ones.
- Different populations. Health risk at a given BMI differs by ancestry; some clinical guidelines use lower cut-offs for people of South and East Asian descent.
- Pregnancy, where it simply doesn’t apply.
If your BMI is in the healthy range, that’s reassuring but not proof. If it’s high, waist circumference and how you feel and perform are better next questions than the number alone. A clinician looks at BMI alongside blood pressure, blood sugar and lipids, which is the right way to use it.
TDEE: calories in a day
Total daily energy expenditure is the calories you burn in a typical day. It has two parts.
Basal metabolic rate (BMR) is what you’d burn lying still all day: keeping warm, breathing, running your organs. The TDEE calculator uses the Mifflin-St Jeor equation, the one most dietitians use because it’s the most accurate of the simple formulas for most adults:
Men: 10 × kg + 6.25 × cm − 5 × age + 5 Women: 10 × kg + 6.25 × cm − 5 × age − 161
For a 35-year-old woman, 5 ft 5 in (165 cm), 150 lb (68 kg): 680 + 1,031 − 175 − 161 = 1,375 calories at rest.
Activity multiplies that:
| Lifestyle | Multiplier |
|---|---|
| Sedentary: desk job, little exercise | 1.2 |
| Lightly active: exercise 1 to 3 days a week | 1.375 |
| Moderately active: 3 to 5 days a week | 1.55 |
| Very active: hard exercise 6 to 7 days | 1.725 |
| Athlete or physical job | 1.9 |
Lightly active, she burns about 1,375 × 1.375 = 1,890 calories a day. Eat that and weight stays put; eat 500 fewer and, all else equal, lose roughly a pound a week, since a pound of fat is about 3,500 calories.
Where TDEE is wrong
People overestimate activity. Three gym sessions a week plus a desk job is “lightly active”, not “moderately active”. Picking one level too high adds 200 to 300 calories to the estimate, which is the whole deficit most people are trying to create.
The formula has a spread. Mifflin-St Jeor is within 10% for most people, which on 1,900 calories is ±190. Two people with identical stats can differ by that much.
The body adapts. As you lose weight, BMR falls with it, and most people also move a little less without noticing. A deficit that produced a pound a week in month one produces less by month three. Recalculate every ten pounds or so.
Trackers are optimistic. Watches and treadmills tend to overstate exercise calories, sometimes by a lot. Treat the TDEE from the formula as the baseline and the tracker’s number as a rough guide.
Using the two together
The practical approach: find your maintenance calories from the calculator, pick a modest deficit (250 to 500 calories, never below about 1,200 total without medical supervision), track your weight over three or four weeks rather than day to day, and adjust the number to what the scale actually does. The formula gets you within a few hundred calories; your own results get you the rest of the way.
Sources
BMI categories are the CDC’s adult definitions. The Mifflin-St Jeor equation is from Mifflin et al., American Journal of Clinical Nutrition, 1990, and the activity multipliers are the standard Harris-Benedict factors used with it. This guide is general information, not medical advice.