Measure height and weight
Finding your BMI takes two measurements and one calculation. Stand without shoes to measure height in metres (or inches), then weigh yourself in kilograms (or pounds) at roughly the same time of day, ideally unclothed or in light clothing, for consistency.
The formula is weight divided by height squared:
BMI = weight (kg) ÷ height (m)²
For readers working in pounds and inches, the equivalent calculation multiplies weight by 703 before dividing by height in inches squared, which converts the result to the same metric scale. So someone weighing 70 kg at 1.70 m calculates 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89, giving a BMI close to 24.2.
An online or chart-based BMI calculator does this same arithmetic for you — you enter height and weight, it applies the formula, and it returns the number and sometimes the category directly, which is useful for checking the maths or for repeating the calculation after a change in weight without redoing the division by hand each time. The result is identical either way; a calculator just saves the manual step.
Once you have the number, compare it against the standard adult categories used by most health systems:
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0 and above | Obesity |
These cut-offs are the ones referenced by the World Health Organization's body mass index data, and they apply to adults generally rather than to children, whose BMI is instead plotted against age-and-sex growth percentiles rather than fixed numeric bands. A reader's single number, placed against this table, tells them which of the four bands they currently sit in — nothing more.
It's worth noting the WHO data also shows how much these figures vary by population: global adult BMI patterns differ substantially between regions, which is one reason some national health bodies apply adjusted thresholds for certain ethnic groups rather than treating the four-band table as universal.
BMI falls in the overweight or obesity band
A BMI in the overweight or obesity range is a flag for further review, not a diagnosis on its own. Research summarized by Cigna and other health organizations links higher BMI with increased population-level risk of conditions including type 2 diabetes, high blood pressure, and heart disease, which is why the number is used so widely as a first-pass screening tool.
But association at the population level doesn't mean every individual with a raised BMI carries the same risk. A healthcare provider reviewing a high BMI result typically looks at it alongside other information: blood pressure, blood glucose, cholesterol, family history, and where on the body fat is carried. As the Heart Foundation of New Zealand explains, BMI works best as a starting point for a conversation rather than an endpoint.
What typically follows a high BMI result, depending on that fuller picture:
- A review of diet, physical activity levels, and sleep as a first step
- Additional measurements — waist circumference, blood pressure, blood tests
- A discussion of realistic, gradual changes rather than an immediate prescription
- Referral for further clinical assessment if other risk markers are also present
Because height cannot change, BMI only moves through a change in weight — an obvious point that gets lost in general "eat better, move more" advice. Someone whose BMI sits above the healthy band and wants to shift it has exactly one lever to pull: weight, achieved through sustained changes to energy intake and activity rather than any adjustment to the formula's other input.
BMI uses only height and weight
BMI is calculated from exactly two inputs, and that simplicity is both its strength and its central limitation. It cannot tell the difference between muscle and fat, and it says nothing about where on the body fat is stored — a distinction that matters because fat carried around the abdomen is linked to higher metabolic risk than fat carried elsewhere, even at the same BMI value.
This is why specific groups are routinely misclassified by the number alone:
- Athletes and muscular individuals, whose weight from lean tissue can push BMI into the "overweight" band despite low body fat
- Older adults, who often lose muscle mass over time in a way that can mask a high fat percentage behind a normal-looking BMI
- People of certain ethnic backgrounds, for whom research has suggested the standard cut-offs don't map onto the same disease risk as they do in the populations the scale was originally built around
Clinicians at MD Anderson and researchers at the University of Rochester Medical Center have both pointed out that BMI was developed as a population-screening measure, useful for tracking trends across large groups, rather than as a precise individual diagnostic tool. Treating a single person's BMI as a definitive verdict on their health overstates what the number was built to do.
The practical fix is to pair BMI with a second measurement rather than reading it alone. Waist circumference adds information about where fat is concentrated, and a provider can factor in muscle mass, age, and ethnicity in a way the formula itself cannot. A reader whose BMI seems to conflict with how they look or feel — a strength-trained person flagged as overweight, for instance — has good reason to treat the figure as a prompt for a broader conversation rather than a settled result.
Body mass index (BMI)
At its core, BMI is a single number derived from a person's weight divided by the square of their height, designed to screen populations for weight-related risk rather than to measure an individual's body composition directly. It's quick to calculate, requires no special equipment, and is used consistently enough across health systems — from WHO's global monitoring to routine checks described by Duly Health and Care — that it remains the default first step in a weight conversation.
Use it that way: as the number that tells you which band you're in and whether it's worth asking a provider a follow-up question, not as the final word on your health.