Guide · Health
BMI: What the Number Tells You (and Its Limitations)
Body Mass Index is a widely used screening tool, but it was designed for populations not individuals. Understanding both what it measures and where it falls short helps you put your number in context.
How BMI is calculated and the NHS categories
BMI is calculated as weight in kilograms divided by height in metres squared: BMI = weight(kg) ÷ height(m)². In imperial units: BMI = [weight(lb) ÷ height(in)²] × 703. A person weighing 70 kg at 1.75 m has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
The NHS and WHO use the following adult categories: Underweight (BMI below 18.5), Healthy weight (18.5–24.9), Overweight (25–29.9), Obese Class I (30–34.9), Obese Class II (35–39.9), Obese Class III (40 and above). These categories apply to adults aged 18 and over. Children use age- and sex-specific centile charts instead.
| Height | BMI 18.5 (underweight boundary) | BMI 22 (mid-healthy) | BMI 25 (overweight boundary) | BMI 30 (obese boundary) |
|---|---|---|---|---|
| 160 cm | 47.4 kg | 56.3 kg | 64.0 kg | 76.8 kg |
| 170 cm | 53.5 kg | 63.6 kg | 72.3 kg | 86.7 kg |
| 180 cm | 59.9 kg | 71.3 kg | 81.0 kg | 97.2 kg |
| 190 cm | 66.8 kg | 79.4 kg | 90.3 kg | 108.3 kg |
What BMI means for a person at 175 cm
For a person standing 175 cm tall, the key weight thresholds are:
The overweight boundary is the most commonly asked-about threshold. At 175 cm, that is 76.6 kg. A person at 80 kg has a BMI of 26.1—technically overweight, but close to the boundary and unlikely to represent meaningful health risk in isolation, particularly if their waist circumference is in a healthy range.
The well-documented limitations of BMI
It cannot distinguish muscle from fat. Because muscle is denser than fat, a person with high muscle mass can have a high BMI despite very low body fat. Many elite athletes and bodybuilders fall into the “overweight” or even “obese” category despite being in excellent health. Conversely, someone with a “healthy” BMI can have high body fat and low muscle mass—a condition sometimes called “skinny fat” or, clinically, normal-weight obesity.
It ignores fat distribution. Where you carry fat matters as much as how much you carry. Visceral fat—stored around the organs in the abdominal area—is strongly associated with metabolic disease, cardiovascular risk, and type 2 diabetes. Subcutaneous fat (under the skin at the hips and thighs) is much less metabolically active. BMI provides no information about fat distribution. Waist circumference and waist-to-height ratio are better proxies for visceral fat.
It does not account for age or sex. Women naturally carry more body fat than men at the same BMI, reflecting hormonal differences and reproductive function. As people age, they tend to lose muscle and gain fat, so older adults can have a “healthy” BMI while carrying clinically significant fat mass.
It is less accurate for some ethnic groups. People of South Asian, Chinese, and other East Asian descent have higher metabolic risk at lower BMI values than white European populations. The NHS recommends using lower cut-off points for these groups: overweight at BMI 23, obese at BMI 27.5.
Better alternatives include waist-to-height ratio (healthy: below 0.5), DEXA (dual-energy X-ray absorptiometry) body composition scanning, and hydrostatic weighing. These are more accurate but less accessible than a simple height and weight measurement. BMI remains useful as a quick, free, population-level screening tool—it is not a clinical diagnosis.
Frequently asked questions
Is a BMI of 25 unhealthy?
A BMI of 25 sits at the boundary between the 'normal' and 'overweight' categories. For most people it is not a cause for clinical concern on its own. BMI is a screening tool, not a diagnosis—a GP will consider waist circumference, blood pressure, cholesterol, family history, and other factors before drawing any health conclusions. Many people with a BMI of 25–27 have no elevated health risk, particularly if they have good cardiorespiratory fitness.
Why is BMI not accurate for muscular people?
BMI cannot distinguish between muscle mass and fat mass—it only measures total weight relative to height. Muscle is denser than fat, so a bodybuilder or rugby player with very low body fat percentage may have a BMI in the overweight or even obese range. This is why many sports scientists prefer direct body composition measurements (DEXA scan, hydrostatic weighing, or skinfold calipers) over BMI for athletes.
What is a healthy BMI range?
The WHO and NHS define a healthy BMI as 18.5 to 24.9 for adults. Below 18.5 is underweight; 25 to 29.9 is overweight; 30 and above is obese. These thresholds were established from population-level data correlating BMI with health outcomes. However, for people of South Asian, Chinese, or other East Asian background, the NHS recommends using lower thresholds (overweight at BMI 23+, obese at BMI 27.5+) because metabolic risk increases at lower BMIs in these groups.
What is the waist-to-height ratio and why does it matter?
Waist-to-height ratio (WHtR) is your waist circumference divided by your height, both in the same units. A healthy WHtR is generally considered to be below 0.5 for adults under 40, rising to 0.5–0.6 for those aged 40–50. It measures abdominal fat (visceral fat around the organs) more directly than BMI, which is a stronger predictor of cardiovascular disease, type 2 diabetes, and metabolic syndrome. You can measure it with a tape measure at home: waist circumference at the midpoint between the bottom of your lowest rib and the top of your hip bone.