What is a healthy BMI?

According to the WHO, a healthy BMI for adults is between 18.5 and 24.9. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese. These thresholds apply to most adults but may not be suitable for athletes, the elderly, or certain ethnic groups.

How is BMI calculated?

BMI is calculated by dividing your weight in kilograms by the square of your height in metres: BMI = kg / m². In imperial units, use: BMI = (lbs / in²) × 703. For example, a person weighing 70kg at 1.75m has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

What are the WHO BMI categories?

The WHO classifies BMI as follows: Under 18.5 = Underweight; 18.5–24.9 = Normal weight; 25.0–29.9 = Overweight; 30.0–34.9 = Obese Class I; 35.0–39.9 = Obese Class II; 40 or above = Obese Class III (severely obese). Our BMI calculator shows which category you fall into instantly.

Is BMI accurate for everyone?

BMI has limitations. It does not distinguish between muscle and fat, so athletes may be classified as overweight despite being healthy. It can also underestimate health risk in some ethnic groups and is less reliable for the elderly. For a fuller picture, consider waist circumference and body fat percentage alongside BMI.

BMI Prime is your BMI divided by 25 (the upper limit of the healthy range). A BMI Prime of 1.0 means you are exactly at the upper boundary of a healthy weight. Below 0.74 is underweight; 0.74–1.0 is normal; above 1.0 is overweight or obese. It makes it easy to see how close you are to the healthy range.

⚕️ BMI Calculator

Free, accurate, GDPR compliant. All calculations run in your browser — no data is ever stored or shared.

Unit System

Weight (kg)

Height (cm)

⚠️ Not medical advice. BMI is a screening tool only and does not diagnose health conditions. Consult a qualified healthcare professional for personal health guidance. WHO classification used.

About the BMI Calculator

The BMI Calculator is free to use and all calculations run entirely in your browser. Body Mass Index (BMI) is a widely used screening tool that estimates whether a person has a healthy body weight relative to their height. It is calculated by dividing body weight in kilograms by the square of height in metres (kg/m²). Our calculator supports both metric (kg/cm) and imperial (lbs/ft/in) inputs.

BMI Categories (WHO Classification)

Below 18.5 — Underweight
18.5 – 24.9 — Healthy weight
25.0 – 29.9 — Overweight
30.0 – 34.9 — Obese (Class I)
35.0 and above — Obese (Class II / III)

What Is BMI Prime?

BMI Prime is a dimensionless number that represents the ratio of your actual BMI to the upper boundary of the healthy weight range (25). A BMI Prime of 1.0 means you are exactly at the top of the healthy range. Values below 1.0 indicate underweight or healthy weight, while values above 1.0 indicate overweight or obesity.

Limitations of BMI

While BMI is a useful population-level screening tool, it does not directly measure body fat and does not account for age, sex, ethnicity, or muscle mass. A person with high muscle mass may have a high BMI without excess body fat, while an older person may have a normal BMI but carry an unhealthy proportion of body fat. Always consider BMI alongside other health indicators.

Healthy Weight Range

After calculating your BMI, this tool also displays the weight range that corresponds to a healthy BMI (18.5–24.9) at your specific height. This gives you a practical target range rather than a single number, which is more useful for setting realistic health goals in consultation with your healthcare provider.

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Important Note: This tool is intended to provide estimates and should not be used as a substitute for professional advice. Information generated by these calculators may be incomplete and does not account for all individual circumstances. Always seek the counsel of a certified expert (such as a financial advisor, healthcare provider, or licensed engineer) before taking action based on these results.

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