BMI Calculator
Calculate your Body Mass Index instantly. Supports both metric (kg/cm) and imperial (lbs/ft) units. Understand what your BMI means and what a healthy range looks like for your age.
BMI Categories
BMI is a screening tool, not a diagnostic measure. Athletes and elderly individuals may receive misleading BMI readings. Always consult a healthcare professional for a full assessment.
What is BMI?
Body Mass Index (BMI) is a numerical measure of a person's body weight relative to their height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has since become the world's most widely used screening tool for categorising weight status in adults. Despite its simplicity, BMI remains a useful first indicator for identifying potential weight-related health risks in large populations.
BMI is calculated by dividing a person's weight in kilograms by the square of their height in metres. The resulting number falls into one of several categories ranging from underweight to severely obese, each associated with different health risk profiles.
The BMI Formula
There are two versions of the formula depending on the unit system you use:
| Unit System | Formula | Example |
|---|---|---|
| Metric (SI) | BMI = weight (kg) ÷ height (m)² | 70 kg ÷ (1.75 m)² = 22.9 |
| Imperial (US) | BMI = 703 × weight (lbs) ÷ height (in)² | 703 × 154 lbs ÷ (69 in)² = 22.7 |
BMI Categories for Adults
The World Health Organization (WHO) defines BMI categories for adults aged 18 and over as follows:
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Malnutrition, osteoporosis, anaemia risk |
| 18.5 – 24.9 | Normal (Healthy) weight | Lowest risk in most populations |
| 25.0 – 29.9 | Overweight | Increased risk of cardiovascular disease |
| 30.0 – 34.9 | Obese (Class I) | High risk of type 2 diabetes, hypertension |
| 35.0 – 39.9 | Obese (Class II) | Very high risk of metabolic disorders |
| 40.0 and above | Obese (Class III / Severe) | Extremely high risk, may require medical intervention |
Research has shown that people of Asian descent may face elevated health risks at lower BMI values. Many health authorities recommend adjusted thresholds: overweight at BMI ≥ 23 and obese at BMI ≥ 27.5 for Asian populations. Consult your local health guidelines for more information.
How to Use This BMI Calculator
- Select your unit system — toggle between Metric (kg/cm) and Imperial (lbs/ft). The calculator remembers your preference.
- Enter your weight — in kilograms if using metric, or pounds if using imperial.
- Enter your height — in centimetres if using metric, or feet and inches if using imperial.
- Click Calculate — your BMI, category, and healthy weight range appear instantly.
Limitations of BMI
While BMI is widely used, it has several well-documented limitations you should be aware of before interpreting your result:
- It does not distinguish fat from muscle. Athletes and bodybuilders often have high BMIs despite low body fat percentages because muscle is denser than fat.
- It does not account for fat distribution. Visceral fat (around the abdomen) carries higher health risks than subcutaneous fat, but BMI cannot differentiate between them.
- It may be inaccurate for older adults. Muscle mass naturally decreases with age, so older adults can have a "normal" BMI while having a high body fat percentage.
- It is not suitable for children or pregnant women. For children, use BMI-for-age percentiles specific to sex. Pregnant women should consult their midwife or obstetrician.
Frequently Asked Questions
What is a healthy BMI for adults?
The World Health Organization defines a healthy BMI as between 18.5 and 24.9 for adults aged 18 and over. Within this range, health risks related to weight are considered minimal. For people of Asian descent, some health authorities recommend a lower threshold — overweight at BMI ≥ 23 and obese at ≥ 27.5.
Is a BMI of 25 considered overweight?
Yes, according to the WHO classification, a BMI of exactly 25.0 falls at the border between normal weight and overweight. A BMI from 25.0 to 29.9 is classified as overweight. However, a single number should not be cause for alarm — factors like muscle mass, age, sex, and ethnicity all influence how BMI maps to actual health risk.
How accurate is BMI as a health measure?
BMI is a useful population-level screening tool but is less accurate at the individual level. Studies show it has about 50% sensitivity for identifying individuals with high body fat percentage — meaning roughly half of people with unhealthy levels of body fat will fall in the "normal" BMI range. More comprehensive assessments include waist circumference, body fat percentage measurement (DEXA scan, skinfold callipers), and blood markers.
Can a normal BMI mean someone is unhealthy?
Yes — this is sometimes called "normal weight obesity" or "metabolically obese normal weight." A person can have a BMI within the healthy range but carry excess visceral fat around their abdominal organs. This is associated with elevated risks of type 2 diabetes, cardiovascular disease, and metabolic syndrome. Waist circumference is a better predictor of visceral fat than BMI alone.
What BMI is considered morbidly obese?
A BMI of 40 or above is classified as Class III obesity, commonly referred to as morbid obesity or severe obesity. At this level, the associated health risks are extremely elevated and medical intervention — including dietary, pharmacological, or surgical options — may be recommended. Some classifications also include a BMI of 35 or above combined with obesity-related comorbidities in this highest-risk category.
Should I use BMI or waist-to-hip ratio?
Both measures have value and they assess different things. BMI reflects overall weight relative to height. Waist-to-hip ratio (WHR) reflects fat distribution — specifically how much fat is stored around your abdomen versus your hips, which is a strong predictor of cardiovascular risk. The ideal approach is to track both. A healthy WHR is below 0.90 for men and below 0.85 for women (WHO guidelines). Using both measurements together gives a more complete picture than either alone.