BMI Calculator
Calculate Body Mass Index with WHO classification chart and ideal weight range.
WHO BMI Classification
| Category | BMI Range | Risk |
|---|---|---|
| Underweight | < 18.5 | Low (but increased risk of other problems) |
| Normal | 18.5 – 24.9 | Average |
| Overweight | 25 – 29.9 | Increased |
| Obese Class I | 30 – 34.9 | High |
| Obese Class II | 35 – 39.9 | Very High |
| Obese Class III | ≥ 40 | Extremely High |
What is BMI?
Body Mass Index, or BMI, is a single number derived from a person's height and weight. It is widely used as a quick screening tool to place an adult in a broad weight category and to flag possible weight-related health concerns.
The metric formula divides weight in kilograms by height in meters squared. The calculator accepts height in centimetres or in feet and inches, and weight in kilograms or pounds, converting everything to metric first. It uses 1 foot equals 30.48 centimetres, 1 inch equals 2.54 centimetres, and 1 pound equals 0.453592 kilograms.
The result is matched against the World Health Organization categories: underweight below 18.5, normal from 18.5 to 24.9, overweight from 25 to 29.9, and three classes of obesity from 30 upwards. A colour-coded bar places the value on a scale from 0 to 50 at a glance.
Because it only uses height and weight, BMI is a rough guide rather than a diagnosis. Very muscular people, pregnant women, children, and older adults can all fall outside the usual interpretation, so the tool also shows a healthy weight range for the entered height using a BMI of 18.5 to 24.9.
Key Concepts
- Body Mass Index — weight relative to height, used as a screening measure.
- Metric formula — BMI equals weight in kilograms divided by height in meters squared.
- Unit conversion — feet and inches are converted to centimetres, and pounds to kilograms.
- WHO categories — underweight, normal, overweight, and obesity classes one to three.
- Healthy weight range — the weight band for the entered height that maps to a BMI of 18.5 to 24.9.
- Screening tool — a fast indicator, not a substitute for medical assessment.
How to Use
- Choose the height unit — select centimetres or switch to feet and inches.
- Enter your height — type the value in centimetres, or the feet and inches separately.
- Choose the weight unit — select kilograms or pounds.
- Enter your weight — type the value in the selected unit.
- Read the result — the BMI value, WHO category, colour bar, and healthy weight range appear instantly.
BMI Classification Reference
The World Health Organization groups adult BMI values into the categories below. The healthy weight range shown by the calculator corresponds to the normal band, from 18.5 up to 24.9.
| Category | BMI Range | Typical Risk |
|---|---|---|
| Underweight | below 18.5 | Low, but increased risk of other problems |
| Normal | 18.5 – 24.9 | Average |
| Overweight | 25 – 29.9 | Increased |
| Obese Class I | 30 – 34.9 | High |
| Obese Class II | 35 – 39.9 | Very high |
| Obese Class III | 40 and above | Extremely high |
Conversions used by the calculator: 1 foot equals 30.48 cm, 1 inch equals 2.54 cm, and 1 pound equals 0.453592 kg. The healthy weight range is found by multiplying the squared height in metres by 18.5 for the lower bound and by 24.9 for the upper bound.
Worked Examples
- 170 cm, 70 kg — 70 / 1.70² = 70 / 2.89 = 24.2, in the normal range. Healthy weight is about 53.5 to 72.0 kg.
- 180 cm, 90 kg — 90 / 1.80² = 90 / 3.24 = 27.8, which is overweight.
- 160 cm, 45 kg — 45 / 1.60² = 45 / 2.56 = 17.6, which is underweight.
- 175 cm, 95 kg — 95 / 1.75² = 95 / 3.0625 = 31.0, which is Obese Class I.
- 5 ft 7 in, 70 kg — 5 ft 7 in is 170.18 cm, giving 70 / 1.7018² = 24.2, in the normal range.
- 170 cm, 160 lbs — 160 lbs is 72.57 kg, giving 72.57 / 2.89 = 25.1, which is overweight.
Tips and Common Mistakes
- A screening tool, not a diagnosis — BMI flags a possible concern but should not replace a medical assessment.
- Muscle weighs more than fat — athletes and very muscular people can have a high BMI while carrying little fat.
- Some groups need care — pregnancy, childhood, older age, and certain medical conditions make BMI less reliable.
- Squared height — the formula divides by height in metres squared, not by height itself.
- Convert before calculating — feet and inches, and pounds, are converted to metric first, so mixing units in one field causes errors.
- Healthy range depends on height — the same BMI band yields a different weight range for every height.
Common Use Cases
People use BMI as a first look at where their weight sits relative to their height. These are the most common situations where a quick BMI check is helpful.
- General health screening — get a fast baseline category before a medical appointment.
- Weight management goals — compare current weight with the healthy range for your height.
- Fitness tracking — log BMI over time alongside workouts and diet changes.
- Pre-appointment checks — complete a BMI field on a form or questionnaire.
- Comparing heights — see how the same weight lands in different categories at different heights.
- Education and study — illustrate weight categories and population statistics.
Frequently Asked Questions
How is BMI calculated?
BMI is weight in kilograms divided by height in meters squared (kg/m²). For example, 70 kg at 1.70 m gives 70 / 2.89 = 24.2. Height entered in feet and inches, or weight entered in pounds, is converted to metric first.
What units does the calculator support?
Height can be entered in centimetres or in feet and inches, and weight in kilograms or pounds. It uses 1 foot equals 30.48 cm, 1 inch equals 2.54 cm, and 1 pound equals 0.453592 kg.
What are the WHO BMI categories?
Underweight is below 18.5, normal is 18.5 to 24.9, overweight is 25 to 29.9, Obese Class I is 30 to 34.9, Obese Class II is 35 to 39.9, and Obese Class III is 40 and above.
What is a healthy BMI?
According to the WHO, a BMI between 18.5 and 24.9 is considered the normal, healthy range for most adults. Below 18.5 is underweight and 25 or above is overweight.
Is BMI accurate for everyone?
BMI is a general screening tool and may be misleading for athletes with high muscle mass, pregnant women, the elderly, and children. It is a starting point, not a complete assessment, so consult a healthcare professional for advice.
How is the healthy weight range found?
The calculator squares the height in metres and multiplies it by 18.5 for the lower bound and 24.9 for the upper bound. For a height of 1.70 m that gives roughly 53.5 to 72.0 kg.
Does BMI differ for men and women?
The standard BMI formula and the WHO categories are the same for adult men and women. Body composition differs between the sexes, but the calculation itself does not change.
Is this tool free and private?
Yes. All calculations happen in your browser and no height or weight data is sent to a server, so the tool is free and private.