BMI Calculator

The BMI formula (and a worked example)

BMI (Body Mass Index) is a simple ratio of your weight to the square of your height. This tool computes it exactly as the standard formula defines:

BMI = weight (kg) ÷ (height (m))²

Worked example: a person who is 175 cm (1.75 m) and weighs 70 kg gets 70 ÷ (1.75 × 1.75) = 22.9, which falls in the "normal weight" band. For imperial input the tool first converts inches to metres (× 0.0254) and pounds to kilograms (× 0.453592), then applies the same formula, so metric and imperial results always agree.

The categories used

The four bands shown in the gauge follow the adult classification published by the World Health Organization and used by the US CDC and NIH:

• Underweight: BMI below 18.5
• Normal weight: BMI 18.5 – 24.9
• Overweight: BMI 25 – 29.9
• Obesity: BMI 30 or above

The "healthy weight range" figure is the weight span that keeps your BMI between 18.5 and 24.9 for your height, rearranged from the same formula.

What BMI cannot tell you

BMI is a screening indicator, not a diagnosis. It measures total mass, so it cannot distinguish muscle from fat, and it does not show where fat is stored. It tends to under-state risk for very muscular people (athletes may read "overweight" while carrying little fat) and over-state leanness in older adults who have lost muscle. It is also less reliable for children, teenagers, pregnancy, and some ethnic groups, for whom different cut-off points apply. Waist circumference, body-fat percentage and a clinician's assessment give a fuller picture.

Sources & review

Categories and interpretation follow public guidance from the World Health Organization (WHO), the US Centers for Disease Control and Prevention (CDC), and the National Institutes of Health (NIH). Methodology last reviewed: October 2026. Calculations run entirely in your browser; no height or weight data is uploaded or stored.

Medical disclaimer: This tool is for general information only and is not medical advice. Consult a qualified healthcare professional before making any health decisions.

Full adult BMI classification

The gauge above uses four broad bands. The World Health Organization splits them further - including obesity classes - like this:

These are adult cut-offs (age 20+). They are a population-level guide to statistical risk, not a verdict on any one person’s health.

Healthy weight for your height

Because a healthy BMI is 18.5–24.9, the healthy weight window for a given height is fixed by the same formula. Common heights:

Enter your own height in the calculator to get your exact range - it recalculates live.

Beyond BMI: waist and waist-to-height

BMI cannot see where you carry fat. Central (abdominal) fat is a stronger predictor of heart disease and type-2 diabetes than BMI alone, so two simple tape-measure checks add real information.

Waist circumference - one widely used set of action levels (NHS / International Diabetes Federation) is:

Different bodies publish slightly different thresholds (US guidelines often use > 102 cm for men and > 88 cm for women as the high-risk line), so treat these as guidance, not absolutes.

When BMI can mislead

• Athletes / high muscle: BMI can read "overweight" or even "obese" for lean, muscular people.
• Older adults & small-framed: BMI can look "normal" while muscle is low and fat is high.
• Ethnicity: for people of South Asian, Chinese and other Asian descent, cardiometabolic risk rises at a lower BMI; some authorities use overweight around 23 and obesity around 27.5 instead of 25 / 30.
• Not for: children, pregnancy, or eating-disorder assessment - use clinical tools instead.

BMI in children and teens (2–20 years)

Adult cut-offs do not apply to growing children. The CDC judges a child’s BMI against age- and sex-specific percentiles on growth charts:

A calculator can give a number, but a child’s healthy weight should be assessed by a pediatrician using the official growth charts, not the adult bands above.

If your BMI is outside the healthy range

A single BMI number is a starting prompt, not a plan. General, evidence-based direction:

• Above range: small, sustainable changes tend to beat crash diets - a modest calorie reduction, more movement, protein and fibre-forward meals, and better sleep. A commonly advised safe pace is gradual loss of roughly 0.25–0.5 kg (0.5–1 lb) per week.
• Below range: a healthcare professional or dietitian can rule out underlying causes and guide nutrient-dense, adequate-calorie eating plus strength training.
• Always: BMI is one signal. Waist measurement, blood pressure, blood sugar and lipids, fitness, and how you feel matter just as much.

Talk to a qualified professional before starting any weight-loss, medication or intensive exercise plan - especially with existing conditions, pregnancy, or for children.