What Is BMI?

BMI stands for Body Mass Index. It is a number we get by using a person’s height and weight. This number helps us understand if someone’s weight is healthy for their height or not.

Doctors and health workers use BMI as a simple check to see if a person may be too thin, at a healthy weight, overweight, or obese. You only need two things to find BMI — your weight and your height — which makes it very easy to use.

How to Calculate BMI

You can find your BMI using kilograms and meters (metric) or using pounds and inches (US units). Both ways give you the same answer in the end.

Metric Formula (kg and meters)

US Standard Formula (pounds and inches)

BMI Categories for Adults

For grown-ups aged 20 and older, BMI is read using a fixed range of numbers. These ranges are the same for both men and women. They help us know which weight group a person falls into. The table below shows the main categories:

BMI for Children and Teenagers (Ages 2–20)

BMI is worked out the same way for children and teens as it is for adults. But the result means something different for kids. This is because children’s bodies keep changing as they grow, and boys and girls grow differently from each other.

So instead of using fixed numbers like adults, a child’s BMI is compared to other children of the same age and sex. This comparison is shown as a percentile. Here is how to read it:

Health Risks of Being Overweight

When a person’s BMI is in the overweight or obese range, the body is carrying more weight than it should. Over time, this extra weight puts pressure on the heart, bones, and other organs. Some health problems that can happen because of high BMI are:

• High blood pressure — the heart has to work harder than normal
• Bad cholesterol levels — too much bad fat and not enough good fat in the blood
• Type 2 diabetes — the body stops using sugar properly
• Heart disease and risk of heart attack
• Stroke — when blood flow to the brain is blocked
• Gallbladder problems and gallstones
• Joint pain, especially in the knees and hips
• Difficulty breathing during sleep (sleep apnea)
• Higher chance of some cancers
• Feeling tired, low mood, and poor quality of life
• Greater chance of serious illness compared to people with a normal BMI

The good news is that even losing a small amount of weight can help. Blood pressure, blood sugar, and energy levels can all improve. If you are worried about your weight, speak to a doctor.

Health Risks of Being Underweight

Being too thin — a BMI below 18.5 — is also a health concern. When the body does not have enough weight, it may not be getting the food and nutrients it needs. Some problems that can come from being underweight are:

• Not getting enough vitamins and nutrients — this can cause tiredness and weak blood (anemia)
• Weak bones that break more easily
• A weaker immune system — getting sick more often
• Slow growth in children and teenagers
• Hormonal problems in women that can affect their periods
• Higher risk during surgery or illness
• Greater chance of serious illness compared to people with a normal BMI

BMI Prime

BMI Prime is a simple version of BMI that tells you how close your BMI is to the top of the normal range. The top of the normal range is 25. So we just divide your BMI by 25 to get BMI Prime.

If your BMI Prime is below 1.0, you are in the normal or underweight range. If it is above 1.0, you are overweight or obese. It is an easy way to see at a glance how far your BMI is from the healthy limit.

Ponderal Index

The Ponderal Index (PI) is similar to BMI — it also uses height and weight to check if a person has a healthy body size. The difference is in the formula. BMI uses height squared, but PI uses height cubed (multiplied three times). This makes PI a better tool for people who are very tall or very short, where BMI can sometimes give a misleading result.

Limitations of BMI

BMI is a helpful tool but it is not perfect. It only uses weight and height — it cannot tell the difference between fat and muscle. So two people with the same BMI can have very different bodies.

Limitations in Adults

Here are some cases where BMI may not give the full picture:

• A sportsperson or bodybuilder may have a high BMI because of muscle, not fat — but they are still very healthy
• Older people often have more body fat than younger people at the same BMI, because muscles get smaller with age
• Women generally carry more body fat than men even at the same BMI
• Fat stored around the belly is more dangerous than fat on the arms or legs — but BMI cannot tell where fat is stored
• BMI may not work as well for people from different ethnic backgrounds, because the link between BMI and body fat can differ

Limitations in Children and Teenagers

The same issues apply to children too. Also, because kids grow at different speeds and go through puberty at different times, their BMI can change a lot in a short time. This is why we use percentiles for children instead of fixed numbers.

Other Measurements Used Alongside BMI

These are some other ways to check body health that can be used together with BMI:

Worked Examples