BMI Online Calculator
Your BMI – or Body Mass Index – is a single number that compares your weight to your height and tells you whether you fall within a healthy range. If you want to check yours right now, the calculator below gives you an instant answer.
How Does the BMI Formula Work?
BMI was introduced by the Belgian mathematician Adolphe Quetelet in the 1830s. The formula is straightforward:
BMI = weight (kg) ÷ height² (m²)
For a person who weighs 75 kg and is 1.78 m tall: 75 ÷ 1.78² = 23.7. In the imperial system the equivalent formula is (weight in pounds × 703) ÷ height in inches². Both produce the same index value.
Because BMI depends on only two measurements, it is cheap, fast, and reproducible – the reason public-health agencies worldwide still use it for population-level screening.
WHO BMI Categories for Adults
The World Health Organization classifies adult BMI into the following bands:
| Category | BMI Range |
|---|---|
| Underweight (severe thinness) | < 16.0 |
| Underweight (moderate thinness) | 16.0 – 16.9 |
| Underweight (mild thinness) | 17.0 – 18.4 |
| Normal weight | 18.5 – 24.9 |
| Overweight (pre-obese) | 25.0 – 29.9 |
| Obese class I | 30.0 – 34.9 |
| Obese class II | 35.0 – 39.9 |
| Obese class III | ≥ 40.0 |
Most adults with a BMI between 18.5 and 24.9 carry a statistically lower risk of cardiovascular disease, type 2 diabetes, and certain cancers compared with those outside this range.
How to Calculate BMI Online
To get your result from the calculator above you need just two inputs:
- Your height – in centimeters or feet/inches
- Your weight – in kilograms or pounds
The tool applies the Quetelet formula instantly and maps your result to the WHO classification table. If you switch between metric and imperial units, the output stays the same because the calculator handles conversion internally.
For the most reliable reading, measure yourself at the same time of day (morning is standard), without shoes, and in light clothing.
Limitations of BMI You Should Know
BMI is a screening tool, not a diagnosis. Here are the main reasons a doctor may look beyond it:
- Muscle mass. Athletes and strength-trained individuals often have a BMI above 25 despite low body fat. A 90 kg bodybuilder at 1.75 m registers a BMI of 29.4 – technically “overweight” – while carrying minimal visceral fat.
- Age. Older adults naturally lose muscle and bone density, so the same BMI may represent a higher fat percentage than in a younger person.
- Ethnicity. Studies published in The Lancet show that Asian populations experience elevated metabolic risk at lower BMI values. The WHO Western Pacific office recommends an overweight threshold of 23 for Asian adults instead of 25.
- Fat distribution. Two people with identical BMIs can have very different health profiles depending on whether fat accumulates around the waist (visceral) or hips (subcutaneous). Waist circumference is a useful complementary metric.
This tool is for informational purposes only and does not replace professional medical advice.
BMI for Children and Teenagers
For individuals aged 2 through 19, fixed BMI cut-offs do not apply. Instead, pediatricians use BMI-for-age percentiles based on growth reference data from organizations like the CDC.
- Underweight: below the 5th percentile
- Healthy weight: 5th to 84th percentile
- Overweight: 85th to 94th percentile
- Obese: 95th percentile or above
Because children’s bodies change rapidly with growth spurts, the percentile approach accounts for age and sex in a way that a single number cannot. If you are evaluating a child’s weight status, consult a pediatric growth chart or your family doctor.
Practical Tips for Moving Toward a Healthy BMI
If your BMI falls outside the normal range, small, consistent changes are more effective than crash diets:
- Aim for 0.5–1 kg per week. Losing more than 1% of body weight weekly increases the chance of muscle loss and rebound weight gain.
- Combine diet and activity. A daily deficit of 500 kcal (through food and exercise) produces roughly 0.45 kg of fat loss per week.
- Track waist circumference. Men should stay below 102 cm; women below 88 cm, according to the National Heart, Lung, and Blood Institute guidelines.
- Re-check BMI monthly rather than daily. Day-to-day weight fluctuates by 1–2 kg due to water retention, food volume, and hormonal cycles.