BMI Calculator

Body Mass Index calculator for quick weight category checks

This BMI calculator estimates your Body Mass Index from your weight and height and shows which weight category the result falls into. It supports both metric (kilograms and centimetres) and imperial (pounds, feet and inches) inputs, so you can use whichever system you are most comfortable with. The result includes your BMI value, the WHO weight category, and a suggested healthy weight range for your specific height.

To use it, select your measurement system, enter your current weight and height, and click Calculate BMI. The result appears immediately. If you switch between metric and imperial, the input fields update accordingly — enter fresh values in the new unit system rather than trying to convert the numbers you already typed.

BMI is calculated as weight in kilograms divided by height in metres squared (kg/m²). For imperial inputs, the calculator converts your measurements to metric internally before applying the formula. The WHO classification used here is: underweight (BMI below 18.5), normal weight (18.5 to 24.9), overweight (25.0 to 29.9), and obese (30.0 and above). These categories were established as population-level screening benchmarks and have been used in clinical settings worldwide for several decades.

BMI is a widely used tool because it requires only two measurements that are easy to obtain, and it provides a quick sense of whether a person's weight is broadly appropriate for their height at a population level. It is used by doctors, public health agencies, and researchers as a screening tool for weight-related health risks. However, it is a simplified metric and has well-documented limitations when applied to individuals. A BMI number on its own does not distinguish between fat mass and lean mass, does not account for where fat is distributed on the body, and does not factor in age, sex, ethnicity, or fitness level — all of which affect what a given BMI value means for an individual's health.

These limitations are most significant for people who are very muscular, older adults who may have lost muscle mass, pregnant women, and people from populations where the standard WHO thresholds are less predictive. Some health authorities use adjusted thresholds for certain ethnic groups because the relationship between BMI and health risk differs across populations. If your BMI is near a category boundary or you have specific health concerns, a conversation with a doctor or registered dietitian will give you a much more complete picture than a number alone.

Assumptions and how to use this calculator

• Intended for adults aged 18 and over. BMI interpretation for children and teenagers requires age- and sex-specific growth charts.
• BMI formula: weight (kg) ÷ height (m)². Imperial inputs are converted to metric internally.
• Weight categories follow WHO standard ranges: underweight <18.5, normal weight 18.5–24.9, overweight 25.0–29.9, obese ≥30.0.
• Healthy weight range is calculated as the weight range that corresponds to a BMI of 18.5–24.9 for your entered height.
• Results may be less accurate for very muscular individuals, older adults, pregnant women, or certain ethnic groups.
• This is a screening tool only. Use the result as a starting point for discussion with a qualified health professional.

Common questions

What is BMI and how is it calculated?

Body Mass Index is your weight in kilograms divided by your height in metres squared. For example, a person who weighs 75 kg and is 1.75 m tall has a BMI of 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 ≈ 24.5. The result places them in the normal weight category. BMI is used as a quick population-level indicator of whether weight may be contributing to health risk.

What BMI range is considered healthy?

For most adults, a BMI between 18.5 and 24.9 falls within the WHO normal weight range. This calculator also shows the weight range that corresponds to that BMI bracket for your specific height, so you can see what maintaining a normal BMI looks like in terms of actual kilograms or pounds.

Is BMI accurate for athletes or very muscular people?

No, not reliably. BMI cannot distinguish between muscle and fat — both contribute equally to weight. A person with high muscle mass may show a BMI in the overweight or obese range even if their body fat percentage is low. In those cases, measurements like body fat percentage, waist circumference, or waist-to-height ratio give a more accurate picture of health risk than BMI does.

Can I use this calculator for children or teenagers?

No. For people under 18, BMI must be interpreted using age- and sex-specific growth charts because body composition changes significantly during development. A BMI that would be normal for an adult may be high or low for a child of the same height. Speak with a paediatrician or GP for the correct approach for a young person.

What should I do if my BMI is outside the healthy range?

A BMI outside the 18.5–24.9 range does not automatically mean there is a health problem — as noted above, it is a simplified screening tool with real limitations. However, if your result suggests you may be significantly underweight or overweight, it is worth discussing with your doctor or a dietitian who can assess your full health picture, consider factors BMI does not capture, and suggest evidence-based steps if they are needed.