BMI limitations: what your BMI does and does not tell you
BMI is a screening number worked out from height and weight. It cannot see body fat, muscle, bone or where fat is stored, so two people with the same BMI can have very different health. Treat it as a starting point, then add waist size, body fat and a conversation with your doctor.
Calculators for this guide: BMI Calculator · Body Fat Percentage Calculator · Ideal Weight Calculator
What does BMI actually measure?
Body mass index (BMI) is your weight in kilograms divided by your height in metres squared. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. The CDC describes BMI as a screening tool: it can point to possible weight-related health risks, but it is not a diagnosis of body fatness or of health. Our BMI calculator does the sum for you and shows a healthy weight range for your height.
| Category | BMI (kg/m²) | Weight at 175 cm |
|---|---|---|
| Underweight | Below 18.5 | Below 56.7 kg |
| Healthy weight | 18.5 to 24.9 | 56.7 to 76.3 kg |
| Overweight | 25.0 to 29.9 | 76.6 to 91.6 kg |
| Obesity, class 1 | 30.0 to 34.9 | 91.9 to 106.9 kg |
| Obesity, class 2 | 35.0 to 39.9 | 107.2 to 122.2 kg |
| Obesity, class 3 | 40.0 and above | 122.5 kg and above |
The adult categories are the same for men and women.
Try it: a quick BMI check
Metric only here. The full BMI calculator also takes pounds and feet, and shows your healthy weight range. For adults aged 18 and over.
How wide is the "healthy" BMI range?
The healthy band (18.5 to 24.9) covers a wide span of weights at any height. At 170 cm it runs from about 53 kg to 72 kg, a gap of about 19 kg. Inside that span, build, bone structure and muscle mean the weight where you feel and function best can be almost anywhere. That is why our ideal weight calculator shows a range, alongside four classic formulas, rather than one "perfect" number.
Calculated from the BMI formula, rounded to the nearest kilogram.
View the chart data as a table
| Height | Lowest (kg) | Highest (kg) |
|---|---|---|
| 150 cm | 42 | 56 |
| 155 cm | 44 | 60 |
| 160 cm | 47 | 64 |
| 165 cm | 50 | 68 |
| 170 cm | 53 | 72 |
| 175 cm | 57 | 76 |
| 180 cm | 60 | 81 |
| 185 cm | 63 | 85 |
| 190 cm | 67 | 90 |
| 195 cm | 70 | 95 |
Why can the same BMI hide very different bodies?
BMI uses only two inputs, so it cannot separate fat from muscle, bone or water. Muscle is denser than fat, so a muscular person can have a high BMI with little body fat. The reverse also happens: someone with a "healthy" BMI can carry less muscle and more body fat than the number suggests, which becomes more common as people age and lose muscle. A body fat calculator gives a second view of the same body.
Illustrative figures chosen to show the idea, not measurements of real people.
View the chart data as a table
| Example | Lean mass (kg) | Fat mass (kg) |
|---|---|---|
| Person A: 175 cm, 80 kg, 12% body fat | 70.4 | 9.6 |
| Person B: 175 cm, 80 kg, 32% body fat | 54.4 | 25.6 |
BMI: myth or fact?
Myth BMI directly measures body fat.
Fact BMI is calculated from height and weight only. On average it tracks body fatness, but for any one person it can be well off.
Myth A BMI in the healthy range means you are healthy.
Fact It says nothing about where fat is stored, fitness, blood pressure, blood sugar or cholesterol. Those need separate checks.
Myth BMI is useless, so ignore it.
Fact It is a cheap, quick screening tool, and it is useful as a first step. The mistake is treating it as the whole story.
Myth The same BMI cut-offs suit everyone.
Fact Asian adults have WHO action points at lower BMI values, and children and teens are judged against percentiles for age and sex.
Who does BMI fit less well?
- Athletes and very muscular adults. Muscle weight can push BMI into the overweight or obesity range even when body fat is low.
- Older adults. Losing muscle with age can leave a healthy-looking BMI that hides extra body fat.
- Asian adults. Health risks rise at lower BMI values. A WHO expert consultation in 2004 identified action points at 23 and 27.5, and our BMI calculator has an option for them.
- Children and teenagers. The adult categories do not apply. BMI is read as a percentile for age and sex on growth charts, so ask your child's doctor.
- Pregnancy. The adult categories are not designed for use during pregnancy. Your midwife or doctor will advise on healthy weight gain.
The healthy band ends at 24.9 on the standard scale and at 22.9 on the Asian scale.
View the chart data as a table
| Cut-offs | Band | From | To |
|---|---|---|---|
| WHO / CDC adult cut-offs | Underweight | 0 | 18.5 |
| WHO / CDC adult cut-offs | Healthy | 18.5 | 25 |
| WHO / CDC adult cut-offs | Overweight | 25 | 30 |
| WHO / CDC adult cut-offs | Obesity 1 | 30 | 35 |
| WHO / CDC adult cut-offs | Obesity 2 | 35 | 40 |
| WHO cut-offs for Asian adults | Underweight | 0 | 18.5 |
| WHO cut-offs for Asian adults | Healthy | 18.5 | 23 |
| WHO cut-offs for Asian adults | Overweight | 23 | 27.5 |
| WHO cut-offs for Asian adults | Obesity | 27.5 | and above |
What should you check alongside BMI?
No single number captures health. Useful companions to BMI include your waist measurement (where fat is stored matters), an estimate of body fat percentage, routine checks of blood pressure, blood sugar and cholesterol arranged by your doctor, and everyday signs such as energy, sleep, strength and stamina. Use this checklist to build a fuller picture.
Beyond BMI: a checklist
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Test what you have learned
BMI quiz
Five questions. Pick an answer for instant feedback.
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Show the answer
Answer: Weight in kg divided by height in metres squared. BMI = kg ÷ m². For 70 kg and 1.75 m, that is 70 ÷ 3.0625 = 22.9.
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Show the answer
Answer: Only height and weight, combined into one number. BMI uses height and weight only. Body fat, muscle and waist size need other measurements.
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Show the answer
Answer: Muscle weight is raising the BMI. BMI counts all weight the same, so muscle can push BMI up even when body fat is low.
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Show the answer
Answer: 23. The WHO expert consultation identified 23 as the first action point for Asian populations, lower than the standard 25.
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Show the answer
Answer: As a percentile for age and sex. The CDC compares a child's BMI with growth charts for the same age and sex, giving a percentile.
Frequently asked questions
Is BMI accurate?
BMI is an exact calculation, but only an indirect guide to body fatness. It works reasonably well as a first screen and for groups of people. It works less well for individuals who are very muscular, older, or of Asian descent, so use it alongside other measures.
Why is my BMI high when I am fit?
BMI counts all weight the same, including muscle. A very muscular person can land in the overweight or obesity range with low body fat. A body fat estimate and a waist measurement give a fuller picture.
Is BMI different for men and women?
The formula and the adult categories are the same for men and women. On average women carry more body fat than men at the same BMI, which is one reason BMI is only a guide.
What is a better measure than BMI?
No single measure is best. Waist circumference shows where fat is stored, body fat percentage estimates fat directly, and blood pressure and blood tests show health markers. A doctor can combine them with your history.
Does BMI apply to children?
Not the adult categories. For children and teenagers, BMI is compared with percentiles for age and sex on growth charts, and their doctor can interpret the result.
When should I talk to a doctor about my weight?
If your BMI is below 18.5 or well above 30, if your weight has changed a lot without trying, or if you have symptoms or conditions you are concerned about. A doctor can look at the whole picture.
Calculators that go with this guide
More guides
Sources
- About Body Mass Index (BMI) (opens in a new tab) — Centers for Disease Control and Prevention (CDC)
- Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies (opens in a new tab) — WHO Expert Consultation, The Lancet, 2004;363:157–163
- Child and Teen BMI Calculator — Centers for Disease Control and Prevention (CDC)
Disclaimer and editorial note
For information only, not medical advice. This calculator gives general estimates from the numbers you enter. It cannot diagnose, treat or prevent any condition and does not replace advice from a doctor, midwife, registered dietitian or other qualified professional who knows your history.
Written and maintained by the HealthFitnessHQ editorial team. We name the source of every figure and formula, and we review each guide when guidance changes. Read how we work in our editorial policy and methodology.