BMI calculator

Body mass index — usually shortened to BMI — is a number that compares your weight to your height. It's been used for decades as a quick way to estimate whether someone's weight may be linked to certain health risks.

Healthcare providers use it as one of many markers to look at — alongside things like blood pressure, cholesterol, blood sugar, waist circumference, family history, and how you feel day-to-day. On its own, BMI can flag a question. It can't answer it.

BMI is your weight in kilograms divided by your height in metres, squared.

Someone who weighs 75kg and is 1.68m tall would calculate it as 75 ÷ (1.68 × 1.68) = 26.6. The calculator above does the maths — it's the explanation that matters more than the arithmetic.

The categories below follow World Health Organization (WHO) and CDC guidelines. They apply to adults aged 20 and over. They do not apply to children, teenagers, athletes with high muscle mass, or anyone who is pregnant.

• Underweight: below 18.5
• Healthy range: 18.5 to 24.9
• Overweight: 25 to 29.9
• Class I obesity: 30 to 34.9
• Class II obesity: 35 to 39.9
• Class III obesity: 40 and above

Falling in the healthy range generally suggests your weight is unlikely to be a major contributor to weight-related health risks. Being in the overweight or obesity range can be associated with higher risk of conditions like type 2 diabetes, high blood pressure, heart disease, and sleep apnoea — particularly when paired with other markers.

BMI is useful because it's simple, but a number based only on height and weight has obvious blind spots.

• Muscle versus fat. BMI doesn't distinguish between the two. A muscular person can register as overweight while having very low body fat.
• Where fat is stored. Fat carried around the abdomen carries more health risk than fat stored elsewhere.
• Metabolic markers. BMI says nothing about your blood pressure, cholesterol, blood sugar, or inflammation.
• Fitness. An active person with a higher BMI is often healthier than a sedentary person with a lower one.
• Age, sex, and ethnicity. The original BMI formula was developed using almost exclusively white, European populations.
• Habits and context. Sleep, stress, your food environment, medication, hormones, and life circumstances all shape weight. None of those are inputs to BMI.

Treat it as a starting point for a conversation, not a conclusion. If your BMI is in the healthy range, that's useful information — but it doesn't mean other health markers are fine. If it's in the overweight or obesity range, that doesn't make you unhealthy by definition — but it may be a signal to take a closer look at the wider context, ideally with a clinician who can see all of it.

Weight is medical, not moral. If your weight has been a long, exhausting conversation in your life, BMI isn't the answer to that conversation. A proper clinical assessment is.