BMI, Waist Size and What the Numbers Really Mean
BMI is a useful screening number and a poor verdict on your health. Here's how to read it, and what to look at alongside it.
The short version
- BMI is a quick screening tool based on height and weight. It doesn't measure body fat directly.
- It can mislead for muscular people, older adults and some ethnic groups.
- Waist size adds useful information, because belly fat is more strongly tied to health risk.
- Blood pressure, blood sugar and cholesterol tell you far more about your actual health.
What BMI is
Body mass index is your weight in kilograms divided by your height in meters squared. It's cheap, quick and useful for spotting patterns across large groups. Calculate yours here.
| BMI | Adult category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class 1 |
| 35 to 39.9 | Obesity class 2 |
| 40 or more | Obesity class 3 |
These categories apply to adults. Children and teens are assessed using growth charts.
Where BMI goes wrong
BMI can't tell muscle from fat or see where fat is stored, so it can mislead:
- Muscular people can land in "overweight" or "obesity" without excess fat.
- Older adults may have a "normal" BMI while having lost muscle and gained fat.
- People of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean descent tend to develop weight-related problems such as diabetes at lower BMIs. UK guidelines use lower cutoffs for these groups: overweight from 23, and obesity from 27.5.
- Any individual can have good metabolic health at a higher BMI, or poor metabolic health at a lower one.
Waist size
Fat around your organs is the kind most linked with type 2 diabetes and heart disease, and waist size is a simple way to estimate it.
How to measure: Stand up and find the bottom of your ribs and the top of your hip bones. Wrap a tape measure around your bare stomach, halfway between the two, keeping it level and snug but not squeezing. Read it just after you breathe out normally, and use the same spot each time.
Waist-to-height ratio: Divide your waist by your height, in the same units. The UK's national health guideline body suggests keeping your waist to less than half your height.
| Waist-to-height | What it suggests |
|---|---|
| 0.40 to 0.49 | Healthy range for central fat |
| 0.50 to 0.59 | Increased risk |
| 0.60 or more | High risk |
A long-used US guide also flags a waist above 35 inches for women or 40 inches for men as higher risk, with lower cutoffs for some ethnic groups.
Numbers that often matter more
Ask your clinician about these, and keep track of them over time:
- Blood pressure: see high blood pressure.
- A1C or fasting blood sugar: see prediabetes and type 2 diabetes.
- Cholesterol and triglycerides: see cholesterol.
- Liver enzymes: see fatty liver.
How to use all this
Use BMI and your waist measurement together as a first look, then let your bloodwork and how you feel fill in the picture. These numbers should inform your decisions. They shouldn't define you, and an "ideal" number on a chart isn't the goal if a higher weight is where you're healthy, active and sustainable.
Sources
This guide is general information, not medical advice. Please check with a clinician before making big changes, especially if you have a health condition, take medication, or are pregnant. Read the full disclaimer.
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