Free BMI calculator for adults with CDC categories, plus what BMI cannot tell you, why waist size matters and how BMI works for children and teens.
Key takeaways
- For adults 20 and older, the CDC classes a BMI of 18.5 to under 25 as healthy weight, 25 to under 30 as overweight and 30 or more as obesity.[1]
- BMI does not separate fat, muscle and bone, and does not show where fat is carried, so it is a screening number, not a diagnosis.[2]
- A waist of more than 35 inches (women) or 40 inches (men) raises health risk, whatever the BMI.[5]
- NICE uses lower BMI thresholds (23 for overweight, 27.5 for obesity) for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background.[4]
- For ages 2 to 19, BMI is read as a percentile for age and sex, not with the adult categories.[3]
In short: BMI (body mass index) compares your weight with your height. For adults 20 and older, a BMI from 18.5 to under 25 is in the CDC’s healthy-weight range, 25 to under 30 is overweight and 30 or more is obesity. It is a quick screening number, not a diagnosis: it cannot see muscle, fat or where fat sits, so read it next to your waist size and your overall health.
BMI calculator
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Educational estimate, not medical advice. Talk with a healthcare professional before changing how you eat, move or take medicines.
How this works
BMI is your weight in kilograms divided by your height in meters squared (in pounds and inches: 703 × lb ÷ in²).
The result is placed in the CDC adult categories: under 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, 30 or more obesity (class 1: 30 to 34.9, class 2: 35 to 39.9, class 3: 40 or more).
Limits
- BMI is a screening number, not a diagnosis. It does not measure body fat, muscle or where fat sits.
- Muscular people can have a high BMI with little body fat; older adults can have a "normal" BMI with low muscle.
- Health risk at a given BMI differs between ethnic groups; some guidelines use lower cut-offs for people of Asian descent.
- For ages 2 to 19, BMI is read with BMI-for-age growth charts, not the adult categories.
Sources
What to do with your number
A BMI result is a starting point for a conversation, not a verdict. The CDC calls BMI one possible health indicator that should be considered together with other factors, such as your medical history, habits, a physical exam and lab results. Two people with the same BMI can be in very different health.
- In the healthy-weight range: keep doing what supports your health: regular activity, enough sleep and mostly whole foods. If your waist is large for your height, that is still worth a look (see below).
- In the overweight range: small, steady changes are where most benefits start. The CDC notes that losing 5% of your weight, 10 pounds for someone who weighs 200, can lower the risk of some chronic diseases. Our guide on how to lose weight lays out the basics.
- In the obesity range: you have more options than diet and exercise alone, and asking for help is not a failure. A clinician can check blood pressure, blood sugar and cholesterol and talk through structured programs, medicines or surgery.
- Under 18.5: a low BMI can also signal a health problem, especially if you lost weight without trying. Mention it to a doctor.
Why the same BMI can mean different things
BMI is popular because it is free, fast and works the same way for everyone, which makes it useful for tracking large groups. For one person it is less precise. The CDC points out that BMI does not separate fat, muscle and bone, and does not show what type of fat you have or where you carry it. A strong, muscular person can land in the overweight range with little body fat, while an older adult who has lost muscle can have a “healthy” BMI and still carry a lot of fat.
Background matters too. The UK’s NICE guideline notes that people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background tend to carry more fat around the middle and develop heart and metabolic risks at a lower BMI. For these groups NICE uses lower thresholds: overweight from a BMI of 23 and obesity from 27.5. NICE also advises reading BMI with caution in people with a lot of muscle and in adults 65 and older.
| Item | CDC, adults 20+ | NICE, some ethnic backgrounds |
|---|---|---|
| Overweight from | 25 | 23 |
| Obesity from | 30 | 27.5 |
NICE uses the lower cut-offs for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background.
Source: CDC Adult BMI Categories and NICE guideline NG246 (checked on October 6, 2026)
Check your waist too
Where fat sits matters as much as how much there is. Fat deep in the belly, around the organs, is the type most closely linked with type 2 diabetes and heart disease; our guide to visceral fat explains why. Two simple checks add what BMI misses:
- Waist-to-height ratio. NICE suggests keeping your waist to less than half your height, a ratio under 0.5. Ratios from 0.5 to 0.59 mean increased health risk, and 0.6 or more a further increased risk. Try our waist-to-height calculator.
- Waist size. The NIH’s heart, lung and blood institute (NHLBI) says a waist of more than 35 inches for women or more than 40 inches for men raises risk. Measure standing, with the tape just above your hip bones, right after you breathe out.
Waist checks at a glance
- Under 0.5waist-to-height ratio NICE suggests aiming forNICE NG246
- 0.6+waist-to-height ratio with further increased health riskNICE NG246
- Over 35 inwaist size that raises health risk for womenNHLBI (NIH)
- Over 40 inwaist size that raises health risk for menNHLBI (NIH)
Checked on October 6, 2026
If you want a closer look at body composition, our body fat calculator estimates body fat from a few tape measurements.
Children and teens: BMI works differently
For ages 2 through 19, the adult categories do not apply. Children are still growing, and healthy amounts of body fat differ between boys and girls and change with age. The CDC compares a child’s BMI with others of the same age and sex and reports a percentile: under the 5th percentile is underweight, the 5th to under the 85th is a healthy weight, the 85th to under the 95th is overweight, and the 95th percentile or above is obesity. Use the CDC child and teen BMI calculator and talk with a pediatrician rather than setting weight goals for a young person.
| Item | Low | High |
|---|---|---|
| Underweight | 0 | 5 |
| Healthy weight | 5 | 85 |
| Overweight | 85 | 95 |
| Obesity | 95 | 100 |
Each band runs up to just under the next one: for example, healthy weight is the 5th to under the 85th percentile.
Source: CDC, BMI Categories for Children and Teens (checked on October 6, 2026)
Where BMI shows up in treatment decisions
Even with its limits, BMI is the number most guidelines use to decide which options to discuss. According to the NIH, prescription weight-management medicines are generally considered for adults with a BMI of 30 or more, or 27 or more with a weight-related condition such as high blood pressure or type 2 diabetes. The U.S. Preventive Services Task Force recommends that clinicians offer or refer adults with a BMI of 30 or more to intensive behavioral programs. These are general criteria, not an eligibility check: only you and your clinician can decide what fits. Our weight-loss medications hub and medical weight loss guide explain the options, and Find My Options sorts them by what matters to you.
Next steps
- Measure your waist and work out your waist-to-height ratio.
- If you want to change your weight, see a healthy range for your height in the ideal weight calculator and estimate your daily needs with the calorie calculator.
- Take your numbers to your next checkup and ask what they mean for you. Our other free tools can help you prepare.
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- Given my BMI and waist size, are there health risks we should check for?
- Which numbers besides BMI should we track, such as blood pressure, blood sugar or cholesterol?
- Is my BMI a fair picture of my health, given my muscle mass, age and background?
- Would a change in weight help my health, and what amount would be a reasonable first goal?
Frequently asked questions
What is a healthy BMI for adults?
The CDC classes a BMI from 18.5 to under 25 as the healthy-weight range for adults 20 and older. Some guidelines, such as NICE in the UK, use lower cut-offs for people from some ethnic backgrounds because risk rises at a lower BMI.
Is BMI accurate for muscular people?
Not very. BMI cannot tell muscle from fat, so people with a lot of muscle can have a high BMI and little body fat. A waist measurement or a body fat estimate adds useful context.
Is BMI different for women and men?
For adults, the same BMI categories are used for women and men. But a study of 1,626 adults found that body fat at a given BMI differs with sex, age and ethnic group, which is one reason BMI is best read alongside other measures.
Can I use this calculator for my child?
No. For ages 2 through 19, BMI is compared with children of the same age and sex and given as a percentile. Use the CDC child and teen BMI calculator and talk with a pediatrician.
Does my BMI mean I qualify for weight-loss medication?
BMI is one of the criteria guidelines use, but it never decides on its own. Whether any treatment fits you depends on your health history and is a decision for you and your clinician.
References
- Adult BMI Categories. Centers for Disease Control and Prevention. (accessed October 7, 2026) Government page
- About Body Mass Index (BMI). Centers for Disease Control and Prevention. (accessed October 7, 2026) Government page
- BMI Categories for Children and Teens. Centers for Disease Control and Prevention. (accessed October 6, 2026) Government page
- Overweight and obesity management (NG246): identifying and assessing overweight, obesity and central adiposity. National Institute for Health and Care Excellence (NICE), 2025. (accessed October 7, 2026) Guideline
- Assessing Your Weight and Health Risk. National Heart, Lung, and Blood Institute (NIH). (accessed October 7, 2026) Government page
- Steps for Losing Weight. Centers for Disease Control and Prevention. (accessed October 7, 2026) Government page
- Prescription Medications to Treat Overweight & Obesity. National Institute of Diabetes and Digestive and Kidney Diseases (NIH). (accessed October 7, 2026) Government page
- Gallagher D, Heymsfield SB, Heo M, Jebb SA, Murgatroyd PR, Sakamoto Y. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. American Journal of Clinical Nutrition, 2000. doi:10.1093/ajcn/72.3.694 · PMID 10966886 (accessed October 6, 2026) Other
- Weight Loss to Prevent Obesity-Related Morbidity and Mortality in Adults: Behavioral Interventions. U.S. Preventive Services Task Force, 2018. (accessed October 7, 2026) Guideline
Facts checked on October 6, 2026
Educational information, not medical advice. Talk with a qualified healthcare professional about your own situation. In an emergency call 911.