BMI, BMR and TDEE Explained: Formulas, Limits and a Worked Example

BMI: what it measures and how it is calculated

Body mass index is weight divided by height squared. In metric units, BMI equals weight in kilograms divided by height in meters, squared. In US units, BMI equals 703 times weight in pounds, divided by height in inches, squared. The factor 703 converts the US units so both versions produce the same number.

For adults aged 20 and over, the US Centers for Disease Control and Prevention and the World Health Organization use the same basic categories. A BMI below 18.5 is classed as underweight, 18.5 to 24.9 as healthy weight, 25 to 29.9 as overweight, and 30 or above as obesity, which is further divided into classes starting at 30, 35 and 40. Children and teenagers are assessed differently, using BMI percentiles for their age and sex rather than these fixed cutoffs.

BMI became popular because it needs only a scale and a tape measure, which makes it practical for comparing large populations and for flagging who might benefit from a closer look.

The limits of BMI

BMI does not measure body fat directly, and it cannot tell muscle from fat or show where fat is carried. A muscular athlete can easily register as overweight while being very lean, and an older adult who has lost muscle can fall in the healthy range while carrying more body fat than is ideal. Fat stored around the abdomen is associated with higher health risk than fat stored elsewhere, and BMI says nothing about it.

The relationship between BMI and health risk also varies between groups. Some health bodies recommend lower action thresholds for people of South Asian, Chinese and other Asian backgrounds, because risk of type 2 diabetes and heart disease tends to rise at lower BMI values. BMI is not intended for pregnancy. For these reasons clinicians treat BMI as a starting point and combine it with waist measurement, blood pressure, blood tests, and personal and family history.

BMR and the Mifflin-St Jeor equation

Basal metabolic rate is the energy your body uses to stay alive at rest: breathing, circulating blood, maintaining body temperature and keeping organs working. For most people it is the largest share of daily calorie use. Strictly speaking, BMR is measured in a lab after an overnight fast at complete rest, and the closely related resting metabolic rate is measured under slightly looser conditions. Online calculators estimate rather than measure, and the two terms are often used interchangeably.

The most widely recommended estimate for adults today is the Mifflin-St Jeor equation, published in 1990. Using weight in kilograms, height in centimeters and age in years, it is 10 times weight, plus 6.25 times height, minus 5 times age, plus 5 for men. For women the final term is minus 161 instead of plus 5. Studies comparing equations against measured values have generally found Mifflin-St Jeor to be among the more accurate for healthy adults, though individual errors of 10 percent or more are still common.

An older alternative is the Harris-Benedict equation, first published in 1918 and revised in 1984. The revised version for men is 88.362 plus 13.397 times weight, plus 4.799 times height, minus 5.677 times age. It tends to give somewhat higher results than Mifflin-St Jeor, so seeing both side by side is a useful reminder that every equation is an estimate.

TDEE and activity multipliers

Total daily energy expenditure is the estimated number of calories you burn in a full day. It includes BMR, the energy used to digest food, which is roughly 10 percent of intake, and all physical activity, from structured exercise to walking, fidgeting and standing. The simplest way to estimate TDEE is to multiply BMR by an activity factor.

The commonly used multipliers are 1.2 for sedentary people with desk jobs and little exercise, 1.375 for light activity of about one to three days a week, 1.55 for moderate activity of about three to five days a week, 1.725 for very active people training six or seven days a week, and 1.9 for very hard physical work or intense training. These factors are rough categories, and the biggest source of error in TDEE estimates is that people tend to overestimate how active they are. If you are unsure between two levels, the lower one is usually the safer starting point.

Worked example

Take a 35 year old man who is 178 cm tall and weighs 80 kg, which is about 5 feet 10 inches and 176 pounds. His BMI is 80 divided by 1.78 squared. 1.78 squared is 3.1684, and 80 divided by 3.1684 is about 25.2. Using US units gives the same answer: 703 times 176 is 123,728, and 70 inches squared is 4,900, so the result is about 25.3, the tiny difference coming from rounding the conversions. A BMI of 25.2 sits just inside the overweight category. For his height, the healthy range of 18.5 to 24.9 corresponds to about 58.6 to 78.9 kg.

His Mifflin-St Jeor BMR is 10 times 80, which is 800, plus 6.25 times 178, which is 1,112.5, minus 5 times 35, which is 175, plus 5. That gives 800 plus 1,112.5 minus 175 plus 5, or 1,742.5, about 1,743 calories per day. The revised Harris-Benedict equation gives about 1,816 calories for the same person, roughly 4 percent higher.

He works at a desk but trains four days a week, so he picks the moderate multiplier of 1.55. His estimated TDEE is 1,742.5 times 1.55, or about 2,701 calories per day. If he chose the sedentary factor instead, the estimate would be about 2,091, a difference of 610 calories, which shows how much the activity choice matters. A common general approach to gradual weight loss is a moderate deficit of around 500 calories below TDEE, which here would be about 2,200 calories per day, but the right target depends on the person and is best set with a professional.

Using the numbers sensibly

Treat every result as a starting estimate to test, not a fixed truth. If you eat close to your estimated TDEE for two or three weeks and your weight trend stays flat, the estimate is about right for you. If it drifts steadily up or down, adjust by 100 to 200 calories and observe again. Daily weight fluctuates with water, salt and digestion, so weekly averages are far more informative than single readings.

Body composition changes the picture too. Two people with the same weight and height can have different BMRs if one carries much more muscle. Very low calorie targets can make it hard to meet protein and micronutrient needs. Anyone who is pregnant or breastfeeding, under 18, managing a medical condition such as diabetes, kidney disease or an eating disorder, or taking medication that affects appetite or metabolism should get personalized advice from a doctor or registered dietitian before changing how they eat.

How ToolWorld calculators can help

The ToolWorld BMI Calculator accepts metric or US measurements, shows the result with the adult CDC category, and gives a reference weight range for your height, so you can see figures like the 58.6 to 78.9 kg range above without doing the arithmetic. The BMR Calculator compares adult resting energy estimates from Mifflin-St Jeor and the revised Harris-Benedict equation side by side, without activity multipliers, which makes the gap between equations easy to see.

When you want the full daily picture, the Calorie and TDEE Calculator estimates resting energy with Mifflin-St Jeor, applies an activity level from sedentary through intense physical work, and lets you customize the macro split. Each tool runs in the browser with no signup required. Use them to understand the numbers and to have a clearer conversation with a healthcare professional, rather than as a substitute for one.