BMI Calculator
Calculate your Body Mass Index and see which weight category you fall into.
Try an example
24.2
Healthy weightAbout this calculator
What Is BMI?
Body Mass Index (BMI) is a simple number that relates your weight to your height. It is widely used as a first screening tool for whether a person's weight falls in a healthy range, and it sorts adults into underweight, normal weight, overweight and obese categories. Enter your height and weight in this BMI calculator to see your result and where it lands on the scale.
How BMI Is Calculated
BMI = weight (kg) ÷ height² (m²). In US units the formula is BMI = 703 × weight (lb) ÷ height² (in²). For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which is in the normal range.
Worked Example
At 170 cm and 70 kg, BMI = 70 ÷ (1.70 × 1.70) = 24.2 — inside the healthy weight range.
BMI Categories for Adults
The World Health Organization uses these ranges for adults aged 20 and over, for both men and women:
- Underweight: below 18.5 (further split into mild, moderate and severe thinness)
- Normal weight: 18.5 – 24.9
- Overweight: 25 – 29.9
- Obesity class I: 30 – 34.9
- Obesity class II: 35 – 39.9
- Obesity class III: 40 and above
For children and teens aged 2–19, doctors do not use these fixed cut-offs. They compare BMI with percentiles for the child's age and sex, because body composition changes as children grow.
Health Risks at Either End of the Scale
A BMI above the healthy range is associated with higher risk of high blood pressure, type 2 diabetes, heart disease, stroke, sleep apnea, joint problems and some cancers. A BMI that is too low can point to malnutrition, weakened bones, a weaker immune system and fertility problems. Neither is a diagnosis on its own — they are reasons to talk with a doctor.
Limitations of BMI
BMI cannot tell fat from muscle, so a muscular athlete can land in the "overweight" band while carrying little body fat. It also ignores where fat is stored, which matters for health, and it can be less accurate for older adults, pregnant women, and people of different ethnic backgrounds. Use it as one data point alongside waist size, body fat percentage, fitness, and advice from a healthcare professional. If you want a weight target for your height, the Ideal Weight Calculator is a good next step.
Worked example
At 170 cm and 70 kg, BMI = 70 ÷ (1.70 × 1.70) = 24.2 — inside the healthy weight range (18.5–24.9).
Frequently asked questions
What do the BMI categories mean?
Under 18.5 is underweight, 18.5–24.9 is a healthy weight, 25–29.9 is overweight, and 30+ is considered obesity — these are the standard adult thresholds used by most health authorities.
Is BMI accurate for everyone?
No. It doesn't distinguish muscle from fat, so athletes and very muscular people often score higher without excess body fat. It also doesn't account for age, sex, or where fat is distributed.
What's the formula in imperial units?
BMI = 703 × weight (lb) ÷ height (in)². The 703 factor puts the result on the same scale as the metric kg/m² formula.
Should I use BMI alone to judge my health?
Treat it as a quick screening tool, not a diagnosis — waist circumference, body fat percentage, and a conversation with a doctor give a fuller picture.
