OnlineCalculator.si

BMI Calculator

Body mass index for adults, children and teens, with WHO and Asian cut-offs, CDC and WHO percentiles, your healthy weight range and waist measures.

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Calculate your BMI

Enter your details. Results update as you type and stay on your device.

Units

Sex at birth

Used for child percentiles, waist and body-fat estimates.

Height
cm

Weight
kg

Advanced options (optional)
Measure midway between your lowest rib and the top of your hip bone, after breathing out.
cm

Measure around the widest part of your buttocks.
cm

Some groups have higher health risks at a lower BMI. This only changes which cut-offs are used.

Your result

Enter your age, height and weight to see your BMI.

Your history (optional)

You can keep a record of your results on this device only. Nothing is sent to us.

BMI categories for adults

CategoryBMI (kg/m²)
Severe thinnessBelow 16.0
Moderate thinness16.0 – 16.9
Mild thinness17.0 – 18.4
Healthy weight (normal range)18.5 – 24.9
Overweight (pre-obesity)25.0 – 29.9
Obesity class I30.0 – 34.9
Obesity class II35.0 – 39.9
Obesity class III40.0 or more
WHO adult BMI categories (20 years and over)

Source: World Health Organization[1][2]. Thinness grades apply below 18.5. See the full BMI chart by height and weight.

Reading the categories

The categories describe how health risk tends to change with BMI across large groups of people. They are not a judgement about any person, and they do not diagnose a condition.

We use person-first, weight-neutral language. For example, we say “a person with obesity”, not labels that define someone by their body.

What BMI means

BMI compares your weight with your height. Because taller people are usually heavier, dividing weight by height squared gives a number that can be compared between people of different heights.

At a population level, a BMI that is well below or well above the healthy range is linked with higher risks of some health problems. Examples include type 2 diabetes, high blood pressure, heart disease, some cancers and joint problems at higher BMIs, and undernutrition and bone loss at lower BMIs[1][4].

Your own risk depends on much more than BMI: where fat is stored, fitness, sleep, diet quality, smoking, family history and conditions you may already have.

How we calculate

  • BMI = weight (kg) ÷ height (m)². US and UK units are converted with exact factors (1 lb = 0.45359237 kg, 1 in = 2.54 cm). This is equivalent to the 703 × lb ÷ in² shortcut.
  • Adults (20+): WHO categories[1][2], or optional Asian cut-offs[3][5]. Categories use the full-precision BMI. The display is rounded to one decimal place.
  • Children and teens (2–19): BMI-for-age z-score and percentile with the LMS method[12]: Z = ((BMI/M)L − 1) / (L × S). L, M and S come from the CDC 2000 growth charts[6] (default) or the WHO 2007 reference for ages 5–19[10]. Categories follow the CDC[7] or WHO definitions. Severe obesity is 120% or more of the 95th percentile[8][9].
  • Healthy weight range = the weights at BMI 18.5 and 24.9 (or 18.5 and 22.9 with Asian cut-offs) for your height.
  • Waist measures: waist circumference (WHO / NHLBI thresholds[13][4]; IDF thresholds for Asian populations[14]), waist-to-height ratio (0.5 and 0.6[5][15]) and waist-to-hip ratio (0.90 men, 0.85 women[13]).
  • Estimated body fat (adults only) uses the Deurenberg formula[16]. It is a rough estimate with an error of about ±4 percentage points.

Every threshold is stored in one configuration file with its source, so the clinical team can review it and update it without changing the code.

Limitations of BMI

  • It does not measure fat. Muscle, bone and water all add to weight. Athletes may be placed in a higher category than their health risk suggests.
  • It does not show where fat is stored. Fat around the waist carries more risk. Measure your waist as well.
  • Ethnic background matters. Many Asian populations have higher risk at lower BMIs[3]. The relationship also differs in other groups.
  • Age matters. In older adults, a slightly higher BMI may be protective[17], and muscle loss can hide behind a “normal” BMI.
  • Children are still growing. Their BMI must be read against age- and sex-specific growth charts.
  • Pregnancy. BMI categories do not apply. Pre-pregnancy BMI is used instead[18].
  • Some conditions, such as fluid retention, amputation or reduced muscle mass, make BMI less meaningful.

BMI for children and teens

For ages 2 to 19, the same BMI number means different things at different ages, and boys and girls grow differently. BMI is therefore interpreted by age and sex, as a percentile on a growth chart.

A child at the 60th percentile has a BMI higher than 60% of children of the same age and sex in the reference population. One measurement is a snapshot, and growth over time tells a clinician much more.

Please do not put a child on a weight-loss diet without professional advice. For babies under 2, growth is checked with weight-for-length charts[11].

More about BMI for children and teens

CategoryBMI-for-age percentile
UnderweightBelow the 5th percentile
Healthy weight5th to below the 85th percentile
Overweight85th to below the 95th percentile
Obesity95th percentile or above
Severe obesity120% of the 95th percentile or above
CDC BMI-for-age categories (2–19 years)
CategoryBMI-for-age z-score
Severe thinnessBelow −3 SD
Thinness−3 SD to below −2 SD
Healthy weight−2 SD to +1 SD
OverweightAbove +1 SD (about BMI 25 at age 19)
ObesityAbove +2 SD (about BMI 30 at age 19)
WHO 2007 BMI-for-age categories (5–19 years)

BMI for Asian populations

In 2004, a WHO expert consultation reviewed evidence that many Asian populations have a higher percentage of body fat, and a higher risk of type 2 diabetes and cardiovascular disease, at a given BMI than European populations[3]. It kept the international categories but added public-health action points at 23 (increased risk) and 27.5 (high risk).

NICE recommends using 23 and 27.5 for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background[5]. A 2009 consensus for Asian Indians proposed overweight from 23 and obesity from 25[19]. This option is available to clinicians but is switched off by default.

In the calculator, open Advanced options and choose your background, or tick Use Asian BMI cut-offs.

CategoryBMI (kg/m²)
Severe thinnessBelow 16.0
Moderate thinness16.0 – 16.9
Mild thinness17.0 – 18.4
Healthy weight (lower risk)18.5 – 22.9
Overweight (increased risk)23.0 – 27.4
Obesity (high risk)27.5 – 32.4
Obesity (high risk)32.5 – 37.4
Obesity (very high risk)37.5 or more
Asian population cut-offs (WHO Expert Consultation 2004)

Supporting a healthy weight: general, evidence-based tips

These are general suggestions for most adults. They are not a personal plan, diet or calorie prescription.

  • Aim for regular physical activity that you enjoy. Many guidelines suggest at least 150 minutes of moderate activity a week, plus strength activities twice a week.
  • Build meals around vegetables, fruit, whole grains, pulses and other fibre-rich foods. Choose water instead of sugary drinks.
  • Get enough sleep and look for ways to manage stress, because both affect appetite and energy.
  • Focus on habits you can keep. Even modest, sustained changes can improve blood pressure, blood sugar and wellbeing.
  • Ask a health professional before making big changes, especially if you have a health condition, take medicines, are pregnant, or are supporting a child.

Frequently asked questions

What is BMI?

Body mass index (BMI) is your weight in kilograms divided by your height in metres squared (kg/m²). It is a quick, low-cost way to screen for weight ranges that are linked with health risks across large groups of people. It does not measure body fat or health directly.

How is BMI calculated?

BMI = weight (kg) ÷ height (m)². For example, 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) = 22.9. In US units the shortcut is 703 × weight (lb) ÷ height (in)². This calculator converts pounds and inches to metric with exact factors, then uses the metric formula.

What is a healthy BMI for adults?

For most adults aged 20 and over, the World Health Organization’s healthy (normal) range is 18.5 to 24.9. For many people of Asian background, health risks start to rise at a lower BMI, so a range of 18.5 to 22.9 is often used instead.

How accurate is BMI?

BMI works well as a screening tool for populations, but it can be wrong for individuals. It cannot tell muscle from fat or show where fat is stored, and the link between BMI and health risk varies with age, sex and ethnic background. Waist measurement and a clinical assessment add important information.

Why are the cut-offs lower for Asian populations?

Studies summarised by a WHO expert consultation in 2004 found that many Asian populations have a higher risk of type 2 diabetes and heart disease at a lower BMI than European populations. WHO suggested extra action points of 23 (increased risk) and 27.5 (high risk). NICE in the UK recommends similar lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background.

How is BMI interpreted for children and teens?

For ages 2 to 19, BMI is compared with children of the same age and sex using growth charts. The result is a percentile. With the CDC charts, below the 5th percentile is underweight, 5th to below 85th is a healthy weight, 85th to below 95th is overweight, and 95th or above is obesity. Adult categories should not be used for children.

Why does the calculator not give a BMI for babies under 2?

BMI-for-age charts start at age 2. For babies and toddlers, growth is assessed with weight-for-length charts (WHO Child Growth Standards), ideally by a health professional who can look at growth over time.

Can I use BMI during pregnancy?

No. Weight gain is expected in pregnancy, so BMI categories do not apply. Midwives and doctors use your BMI from before pregnancy to guide advice on healthy weight gain. The calculator does not classify BMI if you tick “I am pregnant”.

I am very muscular. Does BMI apply to me?

BMI may place athletes and people with a lot of muscle in a higher category than their actual health risk, because muscle is denser than fat. A waist measurement, waist-to-height ratio or a professional body-composition assessment is more informative.

Is BMI different for older adults?

The same formula is used, but research suggests the lowest health risk for adults over 65 may be at a slightly higher BMI. In older age, muscle strength, frailty and unintended weight loss matter more than BMI alone.

Is BMI different for men and women?

Adult BMI categories are the same for men and women. Women usually have more body fat than men at the same BMI, and waist-size thresholds differ by sex. For children, BMI percentiles are always interpreted by sex because growth patterns differ.

What is a better measure than BMI?

No single measure is perfect. Waist circumference and waist-to-height ratio show fat stored around the middle, which is closely linked with health risk. NICE suggests keeping your waist to less than half your height (a ratio below 0.5). Blood pressure, blood sugar, cholesterol, fitness and family history also matter.

What are BMI Prime and the Ponderal index?

BMI Prime is your BMI divided by 25, the upper limit of the WHO healthy range, so 1.0 means you are at that limit. The Ponderal index (weight ÷ height³) is less affected by height, which can be useful for very tall or very short people. Both are shown under “More measures”.

Is my data stored or shared?

No. All calculations happen in your browser, and nothing you enter is sent to us. If you choose to switch on “Save my results”, your history is stored only in this browser, and you can delete it at any time with “Clear all data”.

How often should I check my BMI?

For most adults, checking now and then (for example once or twice a year, or at routine health checks) is enough. Day-to-day weight changes are normal. For children, growth is best followed at routine appointments over time.

Does this calculator give medical advice?

No. It provides general information to help you understand your result. It does not diagnose any condition or recommend treatments, diets or calorie targets. Please talk to a qualified health professional about your individual situation.

Glossary

  • BMI (body mass index): Weight in kilograms divided by height in metres squared (kg/m²).
  • BMI-for-age percentile: How a child’s BMI compares with children of the same age and sex in a reference population.
  • z-score (SD score): How many standard deviations a measurement is above or below the reference median. A z-score of 0 is the 50th percentile.
  • LMS method: A statistical method that describes a growth chart with three values for each age: L (skewness), M (median) and S (spread).
  • Healthy weight range: The range of weights that gives a BMI within the healthy category for a given height.
  • Waist-to-height ratio: Waist circumference divided by height. Below 0.5 is generally associated with lower health risk.
  • Waist-to-hip ratio: Waist circumference divided by hip circumference. A marker of fat stored around the middle.
  • BMI Prime: BMI divided by 25, the upper limit of the WHO healthy range.
  • Ponderal index: Weight divided by height cubed (kg/m³).
  • Screening tool: A quick test that identifies people who may benefit from further assessment. It is not a diagnosis.

References

  1. World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO Consultation. WHO Technical Report Series 894. Geneva: WHO; 2000. iris.who.int/handle/10665/42330
  2. World Health Organization. Physical status: the use and interpretation of anthropometry. WHO Technical Report Series 854. Geneva: WHO; 1995. iris.who.int/handle/10665/37003
  3. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157–163. doi.org/10.1016/S0140-6736(03)15268-3
  4. National Heart, Lung, and Blood Institute (NIH). Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report. NIH Publication No. 98-4083; 1998. www.ncbi.nlm.nih.gov/books/NBK2003/
  5. National Institute for Health and Care Excellence (NICE). Overweight and obesity management. NICE guideline NG246 (2025), which replaced CG189 including its 2022 update on lower BMI thresholds and waist-to-height ratio. www.nice.org.uk/guidance/ng246
  6. Kuczmarski RJ, Ogden CL, Guo SS, et al. 2000 CDC Growth Charts for the United States: methods and development. Vital Health Stat 11. 2002;(246):1–190. www.cdc.gov/growthcharts/cdc-data-files.htm
  7. Centers for Disease Control and Prevention. Child and Teen BMI Categories. www.cdc.gov/bmi/child-teen-calculator/bmi-categories.html
  8. Kelly AS, Barlow SE, Rao G, et al. Severe obesity in children and adolescents: identification, associated health risks, and treatment approaches. A scientific statement from the American Heart Association. Circulation. 2013;128(15):1689–1712. doi.org/10.1161/CIR.0b013e3182a5cfb3
  9. National Center for Health Statistics (CDC). Extended BMI-for-age growth charts for children and teens with very high BMI. 2022. www.cdc.gov/growthcharts/extended-bmi.htm
  10. de Onis M, Onyango AW, Borghi E, Siyam A, Nishida C, Siekmann J. Development of a WHO growth reference for school-aged children and adolescents. Bull World Health Organ. 2007;85(9):660–667. www.who.int/tools/growth-reference-data-for-5to19-years
  11. WHO Multicentre Growth Reference Study Group. WHO Child Growth Standards (birth to 5 years). Geneva: WHO; 2006. www.who.int/tools/child-growth-standards
  12. Cole TJ, Green PJ. Smoothing reference centile curves: the LMS method and penalized likelihood. Stat Med. 1992;11(10):1305–1319. doi.org/10.1002/sim.4780111005
  13. World Health Organization. Waist circumference and waist–hip ratio: report of a WHO expert consultation, Geneva, 8–11 December 2008. Geneva: WHO; 2011. www.who.int/publications/i/item/9789241501491
  14. International Diabetes Federation. The IDF consensus worldwide definition of the metabolic syndrome. Brussels: IDF; 2006. idf.org/
  15. Ashwell M, Gibson S. Waist-to-height ratio as an indicator of ‘early health risk’: simpler and more predictive than using a ‘matrix’ based on BMI and waist circumference. BMJ Open. 2016;6(3):e010159. doi.org/10.1136/bmjopen-2015-010159
  16. Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. Br J Nutr. 1991;65(2):105–114. doi.org/10.1079/BJN19910073
  17. Winter JE, MacInnis RJ, Wattanapenpaiboon N, Nowson CA. BMI and all-cause mortality in older adults: a meta-analysis. Am J Clin Nutr. 2014;99(4):875–890. doi.org/10.3945/ajcn.113.068122
  18. Institute of Medicine and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. Washington, DC: National Academies Press; 2009. doi.org/10.17226/12584
  19. Misra A, Chowbey P, Makkar BM, et al. Consensus statement for diagnosis of obesity, abdominal obesity and the metabolic syndrome for Asian Indians and recommendations for physical activity, medical and surgical management. J Assoc Physicians India. 2009;57:163–170.

Cite this page

APA 7

admin,  (2026, October 5). BMI Calculator. OnlineCalculator.si. Retrieved October 5, 2026, from https://onlinecalculator.si/health/bmi-calculator/

Vancouver

admin . BMI Calculator [Internet]. OnlineCalculator.si; 2026 [updated 2026 Oct 5; cited 2026 Oct 5]. Available from: https://onlinecalculator.si/health/bmi-calculator/

Harvard

admin,  (2026) BMI Calculator. OnlineCalculator.si. Available at: https://onlinecalculator.si/health/bmi-calculator/ (Accessed: October 5, 2026).

BibTeX

@misc{admin2026,
  author       = {admin, },
  title        = {BMI Calculator},
  year         = {2026},
  organization = {OnlineCalculator.si},
  howpublished = {\url{https://onlinecalculator.si/health/bmi-calculator/}},
  note         = {Accessed: 2026-10-05}
}