What is BMI?
BMI stands for Body Mass Index. It is a number calculated from your height and weight that gives a rough indication of whether you have a healthy amount of body fat for your size. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s — which is worth keeping in mind, because some of its limitations trace back to that era.
Despite being over 150 years old, BMI is still widely used by the NHS, the World Health Organization, and doctors worldwide as a quick, cost-free screening tool. It takes about ten seconds to calculate, requires no special equipment, and produces a single number that is easy to track over time. That’s precisely why it has stuck around.
How is BMI calculated?
The formula is simple. You divide your weight in kilograms by the square of your height in metres:
If you prefer to think in stones and feet, you don’t need to do the conversion yourself — our BMI calculator handles imperial measurements automatically and gives you your result instantly.
BMI in pounds and inches
If you’re working in imperial units, the formula is slightly different:
The result is the same number regardless of which unit system you use — BMI is dimensionless.
BMI categories explained
Once you have your BMI number, it falls into one of several categories. The UK follows the WHO classification system for adults aged 18 and over:
| BMI range | Category | What it means |
|---|---|---|
| Below 18.5 | Underweight | May indicate insufficient nutrition or an underlying health condition |
| 18.5 – 24.9 | Healthy weight | Associated with lower risk of weight-related health problems |
| 25 – 29.9 | Overweight | Moderately increased risk of conditions such as type 2 diabetes and heart disease |
| 30 – 34.9 | Obese (Class I) | Significantly increased health risk; lifestyle changes strongly recommended |
| 35 – 39.9 | Obese (Class II) | High health risk; medical review recommended |
| 40 and above | Obese (Class III) | Very high health risk; also called ‘severe’ or ‘morbid’ obesity |
These thresholds apply to adults. Children and teenagers use different, age- and sex-adjusted BMI charts because their bodies are still developing. The NHS has a separate BMI calculator for children.
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. Within this range, research consistently shows lower rates of cardiovascular disease, type 2 diabetes, certain cancers, and early mortality compared to BMI values outside the range.
That said, the “healthy” label does not mean that a BMI of 18.6 is definitively better than one of 25.1. The real-world health risks associated with BMI form a curve rather than a cliff — someone at BMI 25 is not suddenly at drastically higher risk than someone at BMI 24.9.
Adjusted thresholds for people of South Asian heritage
The NHS and WHO both recognise that the standard BMI thresholds may overestimate healthy weight in people of South Asian, Chinese, Middle Eastern, and Black African descent. For these groups, some guidance suggests:
- Overweight: BMI 23 and above
- Obese: BMI 27.5 and above
This is because at the same BMI, people from these ethnic groups can carry a higher proportion of body fat and face elevated risk of conditions like type 2 diabetes at lower BMI levels. Your GP can advise on the most appropriate threshold for you.
BMI’s limitations
BMI has been criticised for decades — and many of those criticisms are valid. Here are the most important limitations to understand:
Should you use BMI?
Yes — with appropriate expectations. BMI is a reasonable starting point for most adults to get a rough sense of whether their weight is in a healthy range. It is free, instant, and gives a number you can track over time to see whether you’re moving in the right direction.
Where BMI becomes less reliable:
- If you’re very muscular — gym-goers, athletes, and people with physically demanding jobs may find BMI overestimates their body fat
- If you’re older — in your 60s and above, waist circumference becomes a more reliable indicator
- If you’re of South Asian, East Asian, or Middle Eastern heritage — lower BMI thresholds may apply to you
- If you’re pregnant — BMI is not used as a weight measure during pregnancy
For everyone else, BMI is a perfectly reasonable number to know — as long as you use it as one input among several, not as the final word on your health.
Beyond BMI: other measures to know
If you want a fuller picture of your weight and body composition, these complementary measures are worth looking at alongside BMI:
Waist circumference
This is one of the most clinically useful measurements you can take at home. Fat carried around the middle — the abdomen — is more metabolically active and associated with higher risk of heart disease and diabetes than fat carried on the hips and thighs. The NHS recommends:
- Men: below 94 cm (37 inches)
- Women: below 80 cm (31.5 inches)
Waist-to-height ratio
A simple rule of thumb: your waist circumference should be less than half your height. So if you’re 5’10” (177 cm), aim for a waist below 88 cm (35 inches). Some research suggests this measure outperforms BMI as a predictor of cardiometabolic risk.
Body fat percentage
Measured by DEXA scan, underwater weighing, or consumer smart scales (with varying accuracy), body fat percentage directly measures what BMI can only estimate. Generally considered healthy ranges are 10–20% for men and 18–28% for women, depending on age.