Body Mass Index (BMI) is one of the most widely used health screening numbers in the world. Doctors, insurers, and fitness apps all use it, yet many people are unsure how it is calculated or what their result actually says about them. This guide walks through the formula step by step, shows real worked examples, explains each category, and is honest about where BMI falls short.
What BMI Measures
BMI is a ratio of your weight to your height. It does not measure body fat directly. Instead, it gives a quick estimate of whether your weight is in a range that is statistically associated with lower or higher health risk for adults. Because it only needs two measurements, it is cheap, fast, and easy to compare across large groups of people, which is exactly why public health organisations use it.
The BMI Formula
Metric (kilograms and metres)
BMI = weight (kg) ÷ height (m)²
Square your height in metres first, then divide your weight by that number.
Imperial (pounds and inches)
BMI = 703 × weight (lb) ÷ height (in)²
The factor 703 converts the imperial units so the result matches the metric scale.
Worked Examples
Example 1 (metric): A person who weighs 70 kg and is 1.75 m tall.
- Height squared: 1.75 × 1.75 = 3.0625
- BMI: 70 ÷ 3.0625 = 22.9
A BMI of 22.9 falls in the normal weight range.
Example 2 (imperial): A person who weighs 150 lb and is 5 feet 7 inches tall.
- Height in inches: (5 × 12) + 7 = 67 inches
- Height squared: 67 × 67 = 4,489
- BMI: 703 × 150 ÷ 4,489 = 23.5
This is also in the normal range. You can check both examples with the ToolGhost BMI Calculator, which accepts either metric (cm and kg) or imperial (feet, inches, and pounds) and does the conversion for you.
BMI Categories for Adults
The World Health Organization uses the following categories for adults aged 20 and over:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
Some health bodies recommend lower action points for people of South Asian, Chinese, and other Asian backgrounds, because health risks such as type 2 diabetes tend to rise at a lower BMI in these groups. If this applies to you, ask your doctor which thresholds they use.
Where BMI Can Mislead You
BMI is a screening tool, not a diagnosis. It treats all weight the same, whether it is muscle, bone, water, or fat. That creates predictable blind spots:
- Muscular people: A rugby player or bodybuilder can have a BMI over 30 while carrying very little body fat.
- Older adults: Muscle mass naturally declines with age, so an older person can have a "normal" BMI while carrying more fat than is healthy.
- Children and teenagers: BMI changes as children grow and differs between boys and girls, so under-20s are assessed with BMI-for-age percentile charts rather than the adult table.
- Pregnancy: Weight gain during pregnancy is expected, so standard BMI categories do not apply.
- Fat distribution: BMI says nothing about where fat is stored. Fat around the abdomen carries more health risk than fat on the hips or thighs.
Useful Measurements to Pair With BMI
Because of those limits, BMI is most useful alongside other simple measurements:
- Waist circumference: Measure around your middle, just above the hip bones. Health guidelines commonly flag a waist above about 102 cm (40 in) for men and 88 cm (35 in) for women as a sign of higher risk.
- Waist-to-height ratio: Divide your waist by your height (same units). Keeping it below 0.5 is a widely used rule of thumb.
- Blood pressure, blood sugar, and cholesterol: These tell you far more about your actual health than weight alone.
Getting an Accurate Reading
- Weigh yourself at the same time of day, ideally in the morning before eating.
- Measure height without shoes, standing straight against a wall.
- Use the same scale each time; home scales can differ by a kilogram or more.
- Track the trend over weeks rather than reacting to a single number, since daily weight can swing by 1–2 kg from water alone.
What to Do With Your Result
If your BMI is in the normal range and you feel well, it is a reasonable sign that your weight is not a major health concern. If it falls outside that range, treat it as a prompt for a conversation rather than a verdict. A doctor can look at your waist size, blood tests, family history, and activity level to give you a complete picture. Sudden or unexplained weight change, in either direction, is always worth checking with a professional.
Frequently Asked Questions
Is BMI different for men and women?
For adults, the same formula and categories are used for everyone. Men and women with the same BMI can have different body fat levels, however, which is another reason BMI is only a starting point.
Why does my BMI say overweight when I am fit?
Muscle is dense, so muscular people often score higher. If you exercise regularly and your waist measurement is in a healthy range, a slightly high BMI is usually less of a concern.
Can children use this calculator?
The adult categories should not be used for anyone under 20. Children are assessed with growth charts that compare their BMI with other children of the same age and sex.
How often should I check my BMI?
Once a month is plenty for most people. Frequent checks mostly capture normal day-to-day fluctuations.
Reliable Sources
This article is for general information and is not medical advice. Speak to a qualified health professional about your individual situation.
Ready to check your own number? Use the free BMI Calculator in metric or imperial units. It runs in your browser and nothing you enter is stored.