BMI Calculator
Calculate your Body Mass Index using the WHO standard formula. This free tool supports both metric (kg, cm) and imperial (lbs, ft + inches) units, automatically converting values when you switch. Results include your BMI number, weight category, a colour-coded scale bar, your ideal weight range using four established formulas, BMI Prime, and Ponderal Index — all updating in real time as you type.
BMI Calculator
Enter your height and weight above to see your BMI.
BMI Prime
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Ponderal Index
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BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, or fat distribution. Consult a healthcare professional for a full health assessment.
What is BMI Prime?
BMI Prime is your BMI divided by 25 — the upper boundary of the normal range. A value below 1.0 means you are in the normal or underweight range, exactly 1.0 is the upper limit of normal, and above 1.0 indicates you are above the normal range. It makes it easy to see at a glance how far you are from the healthy upper boundary without memorising numerical cut-offs.
What is the Ponderal Index?
The Ponderal Index (PI) divides weight by the cube of height rather than the square used in BMI, which makes it more accurate for people who are significantly taller or shorter than average. For very tall individuals, standard BMI tends to overestimate heaviness; for very short individuals, it tends to underestimate it. The Ponderal Index corrects for this. A healthy PI range is approximately 11–14 kg/m³.
Related Calculators
Ideal Weight Calculator
Based on your height (— cm) and gender
Ideal weight formulas are based on height and gender and do not account for muscle mass, frame size, or individual health conditions. Use these figures as a general guide, not a clinical target.
BMI Chart for Adults
WHO BMI Classification
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Malnutrition risk |
| 18.5 – 24.9 | Normal weight | Low (healthy range) |
| 25.0 – 29.9 | Overweight | Moderate |
| 30.0 – 34.9 | Obese Class I | High |
| 35.0 – 39.9 | Obese Class II | Very high |
| 40.0 and above | Obese Class III | Extremely high |
Healthy BMI Range by Age Group
| Age Group | Healthy BMI Range | Notes |
|---|---|---|
| 20 – 34 | 18.5 – 24.9 | Standard WHO range applies |
| 35 – 44 | 18.5 – 24.9 | Standard WHO range applies |
| 45 – 54 | 18.5 – 24.9 | Standard WHO range applies |
| 55 – 64 | 18.5 – 26.9 | Slight upward shift is acceptable |
| 65 and over | 22.0 – 27.9 | Higher range may be protective |
BMI Calculator for Women
The BMI formula is the same for men and women, but healthy body fat percentages differ between the sexes. Women naturally carry more body fat than men at the same BMI. This calculator reflects those differences in the personalised message shown with your result. A BMI of 18.5–24.9 is the healthy range for adult women of any age, though women over 65 may fall in a healthy range slightly higher than this.
BMI Calculator for Men
Men tend to have more muscle mass and less body fat than women at the same BMI, which can cause standard BMI to slightly overestimate adiposity in muscular men. Despite this limitation, the 18.5–24.9 healthy range applies to adult men according to WHO guidelines. Athletes and men with high muscle mass may benefit from supplementing BMI with body fat percentage or waist circumference measurements.
What is BMI?
Body Mass Index (BMI) is a numerical value derived from an individual's height and weight. It was developed by Belgian statistician Adolphe Quetelet in the 1830s and later adopted by the World Health Organization as a simple screening tool for weight status in populations. It is not a direct measure of body fat, but it correlates well with more accurate body composition measures in most people.
BMI is calculated by dividing weight in kilograms by the square of height in metres. The result is a single number that places individuals into one of six categories — from underweight to Obese Class III. It is best used alongside other health measurements rather than as a standalone diagnostic tool.
Am I Overweight?
According to WHO classification, a BMI between 25.0 and 29.9 is classified as overweight, and a BMI of 30 or above is classified as obese. If your BMI falls in either of these ranges, it does not necessarily mean you have a health problem — muscle mass, bone density, and other factors all play a role. A conversation with your doctor will give you a clearer picture of your personal health status.
What is a Healthy BMI?
The World Health Organization defines a healthy BMI as between 18.5 and 24.9 for adults aged 20 and over. A BMI below 18.5 is classified as underweight and above 24.9 as overweight. This range is the same for men and women, though the health implications at each BMI level can differ between the sexes due to differences in body composition.
Limitations of BMI
BMI does not directly measure body fat. It cannot distinguish between muscle mass and fat mass, which means a muscular athlete may be classified as overweight or obese despite having a healthy or low body fat percentage. Conversely, someone with low muscle mass and high body fat — sometimes called "skinny fat" — may have a normal BMI despite carrying metabolically harmful visceral fat.
BMI also does not account for age, sex, ethnicity, or fat distribution. Research has shown that for some Asian populations, health risks begin at lower BMI thresholds than the standard WHO cut-offs. For the elderly, muscle loss can mask higher body fat at a normal BMI. For these reasons, BMI should be used alongside waist circumference, body fat percentage, and clinical assessment rather than in isolation.
BMI Calculator for Seniors
For people aged 65 and over, the standard BMI range is often adjusted. Some research suggests a BMI of 22–27 may be more appropriate for older adults, as a slightly higher BMI may be associated with lower mortality risk and better protection against age-related muscle and bone loss. This calculator notes these considerations in the personalised message when age is entered.
BMI Calculator for Children and Teens
Adult BMI categories do not apply to children and teenagers. For those under 18, BMI is interpreted using age- and sex-specific percentile charts that account for the fact that body fat naturally changes with age and differs between boys and girls during development. A child's BMI must be compared against these percentile charts — not the adult categories shown here. Please consult your child's paediatrician for a proper assessment.
Frequently Asked Questions
How to use BMI Calculator
- 1Enter your height and weight
Type your height (in cm, or feet and inches) and your weight (in kg or lbs) into the input fields. Results appear instantly as you type — no button press needed.
- 2Choose metric or imperial units
Click the unit toggle (kg/cm | lbs/ft) to switch between metric and imperial. Your existing values are automatically converted so you never have to retype them.
- 3Read your BMI result and weight category
Your BMI number and category (Underweight, Normal weight, Overweight, or Obese Class I–III) appear with a colour-coded scale bar showing exactly where you fall. A personalised message based on your age and gender is shown below.
- 4Check your ideal weight range
Scroll to the Ideal Weight Calculator section to see your target weight range based on four established formulas — Devine, Robinson, Miller, and Hamwi — along with how much you would need to lose or gain to reach the midpoint.
Frequently Asked Questions
What is BMI?
- BMI (Body Mass Index) is a number calculated from your height and weight that provides a rough indication of whether you have a healthy body weight for your height. It is widely used as a screening tool by healthcare professionals around the world. A BMI between 18.5 and 24.9 is generally considered healthy for adults.
How is BMI calculated?
- In metric units, BMI is calculated by dividing your weight in kilograms by the square of your height in metres (kg/m²). In imperial units, the formula is 703 × weight(lbs) ÷ height(inches)². This calculator performs both conversions automatically based on whichever unit system you select.
What is a healthy BMI for women?
- For adult women, a BMI between 18.5 and 24.9 is considered healthy according to WHO guidelines. However, women naturally carry a higher proportion of body fat than men at the same BMI, which is factored into the personalised message shown by this calculator.
What is a healthy BMI for men?
- For adult men, the healthy BMI range is also 18.5 to 24.9 per WHO classification. Men tend to have more muscle mass and less body fat than women at the same BMI level, so the interpretation can differ slightly depending on body composition.
What is a normal BMI by age?
- For adults aged 20 and over, the standard healthy BMI range is 18.5 to 24.9 regardless of age. For people over 65, some research suggests a slightly higher BMI (up to 27) may be associated with better outcomes. Children and teenagers use age- and gender-specific percentile charts rather than these fixed ranges.
Is BMI accurate for muscular people?
- BMI is not always accurate for highly muscular individuals because muscle weighs more than fat. A bodybuilder or elite athlete may have a high BMI despite having very low body fat. In these cases, other measures such as waist circumference, body fat percentage, or the Ponderal Index may give a more accurate picture.
What BMI is considered obese?
- A BMI of 30 or above is classified as obese. This is further divided into Obese Class I (30–34.9), Obese Class II (35–39.9), and Obese Class III (40 and above, sometimes called severe or morbid obesity). Each class is associated with progressively higher health risks.
How can I lower my BMI?
- BMI can be lowered by reducing body fat through a combination of a modest calorie deficit and regular physical activity. Sustainable approaches include eating more whole foods, reducing ultra-processed foods, increasing daily movement, and building muscle through resistance training. Rapid or extreme dieting is not recommended — small consistent changes produce lasting results.
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