BMI Calculator
Check your body mass index.
lb to kg converter
BMI formula
Metric
BMI = weight (kg) ÷ height (m)2
70.0 ÷ (1.75 × 1.75) = 22.9
Imperial
BMI = 703 × weight (lb) ÷ height (in)2
703 × 154.3 ÷ (68.9 × 68.9) = 22.9
The example uses 70 kg and 175 cm. For children and teens (2–19 years) the same BMI is converted to a percentile with the CDC BMI-for-age LMS method:
z = ((BMI ÷ M)L − 1) ÷ (L × S)
BMI categories for adults (20 and over)
| Classification | BMI (kg/m²) | Risk of health problems |
|---|---|---|
| Severe thinness | Below 16.00 | Increased (nutritional deficiency) |
| Moderate thinness | 16.00 – 16.99 | Increased (nutritional deficiency) |
| Mild thinness | 17.00 – 18.49 | Slightly increased |
| Normal weight | 18.50 – 24.99 | Average |
| Overweight (pre-obese) | 25.00 – 29.99 | Increased |
| Obese class I | 30.00 – 34.99 | Moderate |
| Obese class II | 35.00 – 39.99 | Severe |
| Obese class III | 40.00 and above | Very severe |
Source: World Health Organization adult BMI classification. The same fixed ranges apply to men and women aged 20 and over.
Your BMI chart
- UnderweightBMI below 18.5
- Normal weightBMI 18.5 to 24.9
- OverweightBMI 25 to 29.9
- ObesityBMI 30 and above
Categories follow the WHO and CDC adult BMI cut-offs. BMI is a screening tool, not a diagnosis. Muscle mass, age, ethnicity and body shape affect what is healthy for you, so talk to a doctor about your results.
Waist size matters too
Extra fat around the waist raises health risk even when BMI looks normal. Risk increases above 94 cm (37 in) for men and 80 cm (31.5 in) for women, and rises substantially above 102 cm (40 in) and 88 cm (35 in).
Asian adults
Health risk starts at a lower BMI. WHO advises that 23.0 to 27.4 is increased risk and 27.5 or above is high risk for people of Asian descent.
Sources: World Health Organization BMI classification and waist circumference guidance; WHO Expert Consultation (Lancet, 2004). BMI is a screening tool, not a diagnosis. Please consult a doctor for personal advice.
About this free BMI calculator for adults, teens and children
What this tool does
This calculator works out your Body Mass Index (BMI) from your height and weight and tells you which weight category you fall into. It also shows the healthy weight range for your height, how far you are from that range, and a chart that places you on the scale. You can enter metric (cm and kg) or imperial (feet, inches and pounds) values, and a quick converter sits right under each field so you never have to switch tabs to change units.
Two methods, one tool
Adults aged 20 and over are compared with the fixed World Health Organization categories. For anyone aged 2 to 19, a BMI number on its own means very little, because children are still growing. So the calculator compares a child or teen with others of the same age and sex using the CDC BMI-for-age percentile charts, and shows the result as a percentile instead of a single cut-off.
How to get the most accurate result
- Measure your height without shoes, standing straight against a wall.
- Weigh yourself in the morning, after using the toilet and before eating, in light clothing.
- Use the same scale each time if you are tracking change over weeks or months.
- For a child, enter the exact age in years and months, since the percentile shifts with every month.
Who it is useful for
It suits anyone who wants a quick, private health check: people starting a fitness or nutrition plan, parents following a child's growth, students working on health projects, and patients preparing questions for a doctor's visit. Nothing you type is stored, and you can download a one-page PDF report of the result to keep or share with your healthcare provider.
What BMI can and cannot tell you
BMI is a simple screening number, not a measure of health on its own. It does not separate muscle from fat, it ignores where fat is stored, and it does not account for age-related changes, pregnancy or ethnic differences in body composition. Use it as a starting point, and look at waist size, activity, diet and medical history alongside it.
Frequently asked questions
What is BMI?
Body Mass Index is a number calculated from your weight and height (kg/m²). It is a quick screening measure used to group people into weight categories. It does not measure body fat directly or diagnose any condition.
How is BMI calculated?
Divide your weight in kilograms by your height in metres squared. With pounds and inches, multiply weight in pounds by 703 and divide by height in inches squared. Both methods give the same result.
What is a healthy BMI for adults?
The World Health Organization defines a normal BMI for adults as 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.
Is BMI different for men and women?
The adult categories are the same for men and women. On average, women carry more body fat than men at the same BMI, so BMI alone does not describe body composition. For children and teens, results are compared with sex-specific growth charts.
How is BMI read for children and teens?
From 2 to 19 years, BMI is compared with other children of the same age and sex using CDC BMI-for-age percentiles. Below the 5th percentile is underweight, 5th to below 85th is healthy, 85th to below 95th is overweight, and 95th or above is obesity. For children under 2, doctors use WHO weight-for-length charts instead.
Is BMI accurate for athletes and muscular people?
Not always. BMI cannot tell muscle from fat, so very muscular people can have a high BMI with low body fat. Waist circumference and body fat percentage give a fuller picture.
Does BMI apply during pregnancy or in older adults?
BMI is not designed for use during pregnancy. In older adults, loss of muscle and height can make BMI understate body fat, so interpret the result together with a healthcare professional.
What is waist-to-height ratio?
Waist-to-height ratio is your waist measurement divided by your height. A ratio of 0.40 to 0.49 is considered healthy, 0.50 to 0.59 suggests increased fat around the middle, and 0.60 or above suggests high central fat. It is a useful second check alongside BMI, especially for muscular people.
What should I do if my BMI is outside the healthy range?
Treat it as a prompt to look closer rather than a diagnosis. A doctor can consider waist size, blood pressure, blood tests, diet, activity and family history before suggesting any changes.
Medical sources and references
The adult categories, cut-offs and growth charts used here are taken from the World Health Organization and the US Centers for Disease Control and Prevention. The references below are listed so you can read the original guidance yourself.
World Health Organization (WHO)
- WHO: Obesity and overweight fact sheet (opens in a new tab) Global definitions, causes and health consequences.
- WHO Global Health Observatory: Body mass index (opens in a new tab) Country-level BMI data and indicators.
- WHO growth reference data for ages 5 to 19 (opens in a new tab) BMI-for-age reference used for school-age children and teens.
Peer-reviewed research
- WHO Expert Consultation. Appropriate body-mass index for Asian populations. The Lancet, 2004. (opens in a new tab) Basis for the lower BMI action points used for Asian adults.
- Flegal KM et al. All-cause mortality with overweight and obesity using standard BMI categories. JAMA, 2013. (opens in a new tab) Systematic review and meta-analysis of the standard BMI categories.
- Prospective Studies Collaboration. BMI and cause-specific mortality in 900,000 adults. The Lancet, 2009. (opens in a new tab) Analysis of 57 prospective studies on BMI and death rates.
Clinical guidance (CDC and NIH)
- CDC: About adult BMI (opens in a new tab) How adult BMI is measured and interpreted.
- CDC: BMI for children and teens (opens in a new tab) How BMI-for-age percentiles work.
- NIDDK: Overweight and obesity in adults (opens in a new tab) National Institutes of Health guidance on risks and treatment.
Medical disclaimer
This calculator is for general information and education only. It is not a medical device and does not give a diagnosis, treatment or personal medical advice. BMI is a screening tool and can be misleading for athletes, older adults, pregnant women and some ethnic groups. Always talk to a doctor or qualified healthcare professional about your weight, your child's growth or any health concern, and never ignore professional advice because of a result you saw here. If you think you have a medical emergency, contact your local emergency services immediately.