What Is BMI? Formula, Adult Ranges, and Limitations
What Is BMI? Formula, Adult Ranges, and Limitations
BMI stands for body mass index. It is calculated by dividing weight in kilograms by height in meters squared. BMI is a quick screening measure for whether an adult’s weight is relatively high or low for their height; it does not directly measure body fat, muscle mass, fitness, or where fat is stored.
Calculating BMI is simple. Understanding what the result can—and cannot—tell you is the more important part.
How do you calculate BMI?
The metric formula is:
BMI = weight (kg) ÷ height² (m²)
For someone who is 170 cm tall and weighs 65 kg:
65 ÷ 1.70² = 22.5
Their BMI is approximately 22.5.
Using pounds and inches, the formula is:
BMI = weight (lb) ÷ height² (in²) × 703
For someone who is 5 feet 7 inches tall (67 inches) and weighs 143 pounds:
143 ÷ 67² × 703 ≈ 22.4
What are the adult BMI ranges?
The U.S. Centers for Disease Control and Prevention uses these categories for adults age 20 and older:
| BMI | Adult screening category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0 or higher | Obesity |
Some countries and clinical guidelines use different cutoffs for particular populations because health risk may appear at different BMI levels. Always check which standard a calculator or clinician is using.
These labels are screening categories, not complete assessments of a person. A BMI inside the “healthy weight” range does not guarantee good health, and a result outside it is not a diagnosis by itself.
Why is BMI used so often?
BMI has three practical strengths:
- It requires only height and weight.
- It is inexpensive and consistent enough for population research and initial screening.
- At a population level, higher BMI is associated with increased risk of several health conditions.
Public-health agencies and clinicians use BMI because it is accessible, not because it can see everything happening inside a body.
What does BMI not tell you?
It does not separate fat from muscle
Seventy-five kilograms can include very different proportions of fat, muscle, bone, and water. A muscular athlete may have a BMI in the overweight category without having the same body composition as an inactive person at the same height and weight.
It does not show fat distribution
Two people with the same BMI may store fat in different places. More fat around the abdomen can carry a different health context from more fat around the hips and thighs. BMI knows the total weight but not where it sits.
It does not capture age-related body change
Muscle and fat can change substantially with age, training, illness, or lifestyle while BMI remains exactly the same.
It does not measure health behaviors or function
Blood pressure, blood sugar, blood lipids, sleep, strength, mobility, diet, menstrual health, and medical history cannot be inferred from BMI alone.
BMI vs. body-fat percentage vs. waist size
| Measure | What it uses | What it describes | Main limitation |
|---|---|---|---|
| BMI | Height and weight | Weight relative to height | Cannot separate fat from muscle |
| Body-fat percentage | An estimate of fat and total mass | Proportion of body weight estimated as fat | Home methods can have substantial error |
| Waist circumference | A measuring tape | Abdominal size and a clue to central fat | Sensitive to landmark and short-term conditions |
| Waist-to-height ratio | Waist and height | Waist size relative to stature | Still does not directly measure visceral fat |
These measures complement one another. BMI can be a first screen, waist metrics add distribution, and body-fat estimates attempt to add composition.
Can your BMI be normal while your waist is high?
Yes. BMI only reflects weight relative to height. Someone can have a BMI within the healthy-weight category while carrying more fat around the abdomen and having relatively less muscle.
That does not mean a waist measurement can diagnose a condition either. It means BMI should sometimes be interpreted alongside waist size, blood pressure, lab results, personal history, and a clinician’s assessment.
The U.K. National Institute for Health and Care Excellence recommends considering waist-to-height ratio for some adults with a BMI below 35 to help assess central adiposity. Learn more in waist measurement vs. body weight.
Who should not use standard adult BMI categories?
Adult BMI categories do not apply in the same way to:
- Children and teenagers, who need age- and sex-specific growth charts
- People who are pregnant
- Highly muscular athletes or strength-trained people
- People with edema, limb differences, or a health condition that substantially affects body weight or composition
Older adults may also need muscle mass, nutritional risk, and physical function considered alongside BMI. Aiming for a particular category without that context can miss the real issue.
Frequently asked questions
What is a healthy BMI?
For most U.S. adults, 18.5 to 24.9 is labeled the healthy-weight range. Cutoffs may differ in other countries or for certain populations.
Is a BMI of 25 overweight?
Under standard U.S. adult categories, 25.0 begins the overweight range. It is a screening result, so waist size, body composition, health markers, and individual circumstances still matter.
Is a lower BMI always better?
No. A low BMI can be associated with low muscle mass, undernutrition, illness, or other risks. Health is not a contest to produce the smallest possible BMI.
Can BMI calculate body-fat percentage?
No. Formulas that combine BMI with age and sex can estimate body fat at a population level, but they can be inaccurate for individuals.
Should you lose weight if your BMI is normal?
BMI alone cannot answer that. Consider waist size, body composition, medical results, habits, physical capacity, and what you actually want to improve. A clinician can help when health risk is the concern.
If you are using BMI to work backward to a target weight, read how to find a healthy weight range.
Use BMI as a starting point, not a verdict
BMI works best when it prompts the next useful question rather than placing a label on a body.
Contura calculates BMI while keeping it in context with waist size, waist-to-height ratio, body-fat estimates, and other measurements. Instead of asking whether 22.5 is simply “good” or “bad,” you can ask whether it fits your body shape and long-term trend.
Explore Contura on the App Store