BMI Calculator: Check Your Body Mass Index and Healthy Weight Range

Your BMI (body mass index) is your weight in kilograms divided by your height in metres squared. For adults, a BMI of 18.5 to under 25 falls in the healthy weight range, 25 to under 30 is overweight, and 30 or above is obesity. Enter your height and weight in the calculator above to see your number and category instantly.

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Created by Dr. Emily Carter

Last updated: October 9, 2026

This free BMI calculator works out your body mass index from your height and weight and shows your adult BMI category. BMI is the quickest screening measure doctors and public-health bodies use to relate your weight to your height, and it is the same measure used by the World Health Organization, the US Centers for Disease Control and Prevention, and the NHS.

BMI matters because weight that sits well above or below the healthy range is linked to long-term conditions such as type 2 diabetes, heart disease and high blood pressure, while being underweight can signal other health problems. It is also useful over time: tracking your BMI helps you notice weight changes when managing daily intake with a calorie calculator or planning energy balance with a TDEE calculator.

This page shows you how to use the calculator, the exact formula behind it (with the source for every number), a worked example you can follow by hand, and a plain guide to reading your result, including where the standard thresholds change for different populations, children, athletes and older adults. BMI is a screening tool, not a diagnosis, so treat your number as one useful data point rather than a verdict.

What this calculator does:

•Calculate BMI from metric or imperial height and weight
•Show your BMI category using WHO and CDC adult thresholds
•Return the healthy weight range for your height

How to use the BMI calculator

1
Choose your units.Select metric (centimetres and kilograms) or imperial (feet, inches and pounds).
2
Enter your height.For metric, type it in centimetres or metres; for imperial, enter feet and inches.
3
Enter your weight.Use kilograms for metric, pounds for imperial.
4
Add your ethnic background if the tool asks.This lets it apply the correct thresholds, because some groups use lower cut-offs.
5
Read your BMI number.It appears to one decimal place, for example 22.9.
6
Check your category.The calculator labels your result as underweight, healthy weight, overweight or obesity.
7
Compare against your healthy range.The tool shows the weight range that puts you between BMI 18.5 and 24.9 for your height, or you can find your exact target with our ideal weight calculator.
8
For anyone under 20, switch calculators.Children and teens need an age- and sex-specific child and teen BMI calculator, not the adult one.

The formula behind the BMI calculator

BMI uses one formula in every unit system; only the conversion changes.

Metric (kilograms and metres):

\text{BMI} = \frac{\text{weight (kg)}}{[\text{height (m)}]^2}

Metric by hand (kilograms and centimetres):

Multiply by 10,000 instead of converting centimetres to metres first:

\text{BMI} = \frac{\text{weight (kg)}}{[\text{height (cm)}]^2} \times 10{,}000

Both are the WHO and CDC definitions of body mass index.

Imperial (pounds and inches):

\text{BMI} = 703 \times \frac{\text{weight (lbs)}}{[\text{height (in)}]^2}

The 703 is a unit-conversion constant, not a separate rule: 1 pound = 0.45359237 kg and 1 inch = 0.0254 m, so $\frac{0.45359237}{(0.0254)^2} \approx 703.06957$ , rounded to 703. This is the same figure used in clinical BMI tools.

The metric formula is the definition used by WHO, the CDC and the NHS; the imperial version exists only to avoid converting units by hand.

Worked example

Take an adult who is 1.75 m tall and weighs 70 kg.

Square the height: $1.75 \times 1.75 = 3.0625\text{ m}^2$
Divide weight by the squared height: $70 / 3.0625 = 22.857...$
Round to one decimal place: BMI 22.9

A BMI of 22.9 sits between 18.5 and 25, so this person's weight is in the healthy weight category.

Now the same body in imperial units: 5 ft 9 in (69 in) and 160 lb.

Square the height in inches: $69 \times 69 = 4{,}761\text{ in}^2$
Multiply weight by 703: $160 \times 703 = 112{,}480$
Divide: $112{,}480 / 4{,}761 = 23.625...$ → BMI 23.6

The two methods give the same answer once rounding is applied (22.9 in metric, 23.6 in imperial for the equivalent height and weight), both in the healthy weight range. For reference, the CDC's own adult example for someone 5 ft 9 in shows underweight at 124 lb or less, healthy weight at 125–168 lb, overweight at 169–202 lb and obesity at 203 lb or more.

BMI Calculation & Classification Flowchart

Enter height and weight

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BMI = weight (kg) / height (m)²

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Check BMI Value

less than 18.5Underweight

18.5 to under 25Healthy weight

25 to under 30Overweight

30 or aboveObesity

How to read your BMI result

For adults aged 20 and over, the standard categories are the same for men and women and do not change with age or race.

BMI range (kg/m²)	Category
Less than 18.5	Underweight
18.5 to under 25	Healthy weight
25 to under 30	Overweight
30 to under 35	Obesity, Class 1
35 to under 40	Obesity, Class 2
40 or above	Obesity, Class 3 (severe obesity)

Where the thresholds are lower. The WHO kept 18.5/25/30 as its international classification but added public-health action points of 23, 27.5, 32.5 and 37.5 kg/m² for Asian populations. The UK's NICE applies lower thresholds to people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds: overweight from BMI 23 and obesity from BMI 27.5, with obesity classes 2 and 3 reduced by 2.5 kg/m². If you are in one of these groups, a “normal” result on the standard scale may still deserve a conversation with a clinician.

BMI is not a measure of body fat. BMI treats muscle, fat and bone as the same weight, and it does not show where fat is carried. A muscular athlete can read as “overweight” with very little body fat, and an older adult who has lost muscle can read as “healthy” while carrying excess fat; measuring lean body mass offers a clearer picture.

Add a waist measurement. NICE recommends a waist-to-height ratio of 0.5 as a practical estimate of central (abdominal) fat, used alongside BMI. A quick rule: keep your waist measurement to less than half your height. This matters because central fat carries extra risk even when BMI looks normal.

Children and teens are different. For ages 2 to 19, BMI is interpreted with age- and sex-specific percentiles, not fixed adult numbers: underweight below the 5th percentile, healthy weight 5th to under 85th, overweight 85th to under 95th, and obesity at or above the 95th. Severe obesity is at or above 120% of the 95th percentile, or a BMI of 35 or more. Never apply the adult chart to a child.

Older adults. NICE advises interpreting BMI with caution in people aged 65 and over, because a slightly higher BMI can be protective and other conditions affect the picture.

Methodology & sources

This page's thresholds and formulas come from primary guidance, not secondary summaries. The base formula is the WHO and CDC definition of BMI (weight in kilograms divided by height in metres squared). Adult categories follow the CDC's adult BMI classification. Ethnicity-adjusted thresholds follow the WHO 2004 expert consultation and NICE guidance. The child and teen percentiles follow CDC growth-chart guidance. The waist-to-height ratio comes from NICE CG189. All clinical guidelines and thresholds are detailed in the Sources list below.

Frequently Asked Questions

What is a healthy BMI for adults?

For most adults, a healthy BMI is 18.5 to under 25. A BMI below 18.5 is underweight, 25 to under 30 is overweight, and 30 or above is obesity. These thresholds come from the World Health Organization and are used by the CDC and NHS.

How do I calculate my BMI?

Divide your weight in kilograms by your height in metres squared. For example, 70 kg divided by 1.75 m × 1.75 m equals 22.9. In imperial units, multiply your weight in pounds by 703, then divide by your height in inches squared.

What BMI is considered obese?

For adults, a BMI between 25 and 29.9 is categorized as overweight (which you can evaluate on an overweight calculator), while obesity starts at a BMI of 30 or above. It is divided into Class 1 (30 to under 35), Class 2 (35 to under 40) and Class 3 or severe obesity (40 or above). These categories follow the CDC's adult BMI classification.

Is BMI accurate for athletes and muscular people?

No, BMI can overestimate body fat in athletes and muscular people. BMI measures weight relative to height and cannot tell the difference between muscle, fat and bone. A muscular person may have a BMI in the overweight range while carrying very little body fat.

Are BMI thresholds different for different ethnicities?

Yes. For people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, the NHS and NICE use lower thresholds: overweight from a BMI of 23 and obesity from 27.5. The WHO also supports lower action points for Asian populations.

Does BMI work the same way for children?

No. For children and teens aged 2 to 19, BMI is interpreted using age- and sex-specific percentiles rather than fixed adult numbers. A BMI at or above the 95th percentile is classed as obesity. Always use a child and teen BMI calculator for anyone under 20.

What is a healthy waist size alongside BMI?

NICE recommends a waist-to-height ratio of 0.5 or less as a practical measure of central fat. Your waist should measure less than half your height. Waist measurement adds useful information because BMI does not show where fat is stored on the body.

What is the difference between BMI and body fat?

BMI is a screening measure based on height and weight, not a direct measure of body fat. It does not indicate body composition or where fat is carried. Body fat is measured directly with tools such as DEXA scans, which are more accurate but less practical.

Is BMI different for men and women?

For adults aged 20 and over, the BMI formula and categories are the same for men and women and do not change with age or sex. However, the same BMI can correspond to different amounts of body fat depending on factors such as muscle mass and body composition.

Sources

World Health Organization. Obesity and overweight (fact sheet). https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight.
WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet 2004;363:157–163. https://pubmed.ncbi.nlm.nih.gov/14726171/.
US Centers for Disease Control and Prevention. Adult BMI Categories (updated 26 June 2024). https://www.cdc.gov/bmi/adult-calculator/bmi-categories.html.
US Centers for Disease Control and Prevention. About Body Mass Index (BMI). https://www.cdc.gov/bmi/about/index.html.
US Centers for Disease Control and Prevention. Adult BMI Calculator. https://www.cdc.gov/bmi/adult-calculator/index.html.
US Centers for Disease Control and Prevention. Child and Teen BMI Categories. https://www.cdc.gov/bmi/child-teen-calculator/bmi-categories.html.
US Centers for Disease Control and Prevention. Plotting and Interpreting BMI-for-Age (Growth Chart Training). https://www.cdc.gov/growth-chart-training/hcp/using-bmi/plotting-interpreting-bmi.html.
National Institute for Health and Care Excellence (NICE). Obesity: identification, assessment and management (CG189), Recommendations. https://www.nice.org.uk/guidance/cg189/chapter/Recommendations.
National Institute for Health and Care Excellence (NICE). Overweight and obesity management (NG246), Identifying and assessing overweight, obesity and central adiposity. https://www.nice.org.uk/guidance/ng246/chapter/Identifying-and-assessing-overweight-obesity-and-central-adiposity.
NHS. Calculate your body mass index (BMI) for adults. https://www.nhs.uk/health-assessment-tools/calculate-your-body-mass-index/calculate-bmi-for-adults.
NHS Digital. Overweight and obesity (Health Survey for England: ethnicity and health). https://digital.nhs.uk/data-and-information/publications/statistical/health-survey-england-additional-analyses/ethnicity-and-health-2011-2019-experimental-statistics/overweight-and-obesity.