What Is BMI? How to Calculate It and What It Means
Updated · 4 min read · By Taohidul Islam
Body mass index (BMI) is your weight divided by your height squared: weight in kilograms divided by height in meters squared. For adults, a BMI of 18.5 to 24.9 is classed as a healthy weight, 25 to 29.9 as overweight and 30 or more as obesity. It is a quick screening number, not a measure of body fat or a diagnosis.
How to calculate BMI in metric units
The formula is:
BMI = weight (kg) / height (m)²
- Convert your height to meters. For example, 175 cm is 1.75 m.
- Multiply your height by itself: 1.75 x 1.75 = 3.0625.
- Divide your weight by that number: 70 kg / 3.0625 = 22.9.
A person who is 175 cm tall and weighs 70 kg has a BMI of about 22.9.
How to calculate BMI in pounds and inches
With imperial units you need one extra step, multiplying by 703 to convert the result to the same scale:
BMI = weight (lb) / height (in)² x 703
- Convert your height to inches. For example, 5 ft 7 in is 67 inches.
- Multiply your height by itself: 67 x 67 = 4,489.
- Divide your weight by that number: 160 lb / 4,489 = 0.0356.
- Multiply by 703: 0.0356 x 703 = 25.1.
A person who is 5 ft 7 in tall and weighs 160 lb has a BMI of about 25.1. If you would rather not do the maths, the NHLBI and CDC both offer free online BMI calculators (linked below).
What do the BMI categories mean?
The World Health Organization defines overweight as a BMI of 25 or more and obesity as 30 or more. The CDC uses the same adult cut-offs and splits obesity into three classes:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity, class 1 |
| 35.0 to 39.9 | Obesity, class 2 |
| 40.0 or higher | Obesity, class 3 (severe obesity) |
These categories describe groups of people, and higher categories are linked with a higher average risk of conditions such as heart disease and type 2 diabetes. A very low BMI carries risks too, including poor nutrition and weaker bones. Your own risk depends on much more than this one number.
Do BMI ranges differ by ethnicity?
Yes, in some guidance. The NHS uses lower cut-offs for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, because health risks tend to rise at a lower BMI in these groups. For these groups the NHS treats a BMI of 23 to 27.4 as overweight and 27.5 or more as obesity.
What are the limitations of BMI?
BMI is popular because it is free, fast and only needs a tape measure and scales. But the CDC is clear that it does not directly measure body fat. Its main blind spots are:
- Muscle versus fat. BMI cannot tell fat, muscle and bone apart. A muscular athlete can be classed as overweight while carrying little fat.
- Where fat sits. Fat around the waist is linked to higher health risk than fat on the hips or thighs, and BMI cannot see the difference.
- Age. Older adults tend to lose muscle, so two people with the same BMI can have very different body composition.
- Pregnancy. Adult categories do not apply during pregnancy.
- Children and teens. Under 20 (in the US) or under 18 (in the UK), BMI is compared with growth charts for age and sex instead of the adult table.
Key takeaway: BMI is a starting point for a conversation, not a verdict. The CDC advises that for any individual it should be considered alongside other factors such as medical history, habits, a physical exam and lab results.
Simple measures to use alongside BMI
- Waist circumference. According to the NHLBI, a waist of more than 35 inches (89 cm) for women or more than 40 inches (102 cm) for men increases the risk of heart disease and type 2 diabetes.
- Waist-to-height ratio. The NHS suggests keeping your waist to less than half your height.
- Fitness and daily habits. How active you are, how well you sleep and how fit your heart is all affect health in ways that weight alone does not show. Our guide to why exercise matters covers the benefits that go beyond the scales.
To measure your waist, find the point midway between your lowest rib and the top of your hip bone, breathe out normally and wrap the tape around without pulling it tight.
When to talk to a doctor about your weight
- Your BMI is below 18.5 or 30 or above.
- You have lost or gained weight without trying.
- Your waist measurement is above the thresholds above.
- You want to change your weight and have a health condition, take regular medicine, or are pregnant.
- Food, eating or your body shape is causing you distress.
Seeing your BMI in FitRest
FitRest shows your BMI in Personal Metrics, using the height and weight saved in Apple Health, alongside your BMR and a weight trend over time. If you weigh yourself with a scale that saves to Apple Health, or log your weight in the Health app, the numbers stay up to date without extra typing. It sits next to your daily scores, so you can view weight in the context of sleep and activity rather than on its own. Your daily calorie burn and target are on the Energy page.
Sources
FitRest is a wellness app, not a medical device. It does not diagnose, treat or prevent any condition. Talk to a qualified health professional about any health concern.