Free BMI Calculator - Body Mass Index & Health Assessment Tool

What is BMI (Body Mass Index)?

BMI, or Body Mass Index, is a numerical value derived from a person's height and weight. It provides a simple, widely used screening tool to categorise individuals into weight status categories — underweight, normal weight, overweight, and obese — based on guidelines from the World Health Organization (WHO).

The BMI formula is straightforward: divide your weight in kilograms by your height in metres squared. Our free BMI calculator does this automatically in both metric and imperial units, giving you instant results with a detailed health assessment.

BMI Formula

For example: A person weighing 70 kg with a height of 1.75 m has a BMI of: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 — a healthy, normal weight.

BMI Categories (WHO Standards)

Health Risks Associated with BMI

Underweight (BMI < 18.5)

Being underweight can indicate nutritional deficiency, eating disorders, or underlying medical conditions. Health risks include weakened immune function, osteoporosis, anemia, and fertility issues.

Normal Weight (BMI 18.5–24.9)

This range is associated with the lowest health risks for most adults. Maintaining a normal BMI through balanced diet and regular exercise supports cardiovascular health, metabolic function, and longevity.

Overweight (BMI 25–29.9)

Excess weight increases the risk of type 2 diabetes, high blood pressure, heart disease, joint problems, and sleep apnea. Lifestyle changes including diet modification and increased physical activity can significantly reduce these risks.

Obese (BMI ≥ 30)

Obesity substantially elevates the risk of serious health conditions including cardiovascular disease, stroke, type 2 diabetes, certain cancers, and reduced life expectancy. Medical guidance is strongly recommended.

Limitations of BMI

BMI is a useful screening tool but has important limitations:

• Muscle mass — Athletes and muscular individuals may have a high BMI without excess body fat.
• Age — BMI categories are designed for adults (18–65). Different thresholds apply for children and the elderly.
• Ethnicity — Research suggests lower BMI thresholds for cardiovascular risk in some Asian populations.
• Body fat distribution — BMI does not account for where fat is stored (e.g., abdominal vs. peripheral fat).
• Pregnancy — BMI is not an appropriate measure during pregnancy.

Always consult a qualified healthcare professional for personalised medical advice about your weight and health.

Frequently Asked Questions

What is a healthy BMI for adults?

The WHO considers a BMI between 18.5 and 24.9 as "normal weight" for adults. However, optimal BMI can vary based on age, sex, ethnicity, and muscle mass.

Is BMI accurate for women?

BMI applies the same formula to men and women, but women naturally have higher body fat percentages than men at the same BMI. Some researchers suggest slightly lower thresholds for women when assessing health risks.

How do I lower my BMI?

BMI decreases when body weight decreases relative to height. Sustainable approaches include a balanced caloric deficit diet, regular aerobic and strength exercise, adequate sleep, and stress management. Always consult a doctor before starting a weight-loss programme.

Can children use a BMI calculator?

Children and teenagers use age- and sex-specific BMI percentile charts (not adult BMI categories). This calculator is designed for adults aged 18 and over.