BMI Calculator UK: What's a Healthy BMI for Adults?

Your BMI number takes about two seconds to calculate. Understanding what it actually means for your health takes a little longer, and it's worth doing properly, since BMI is a useful starting point but not the full picture on its own.

This guide covers how BMI is worked out, the NHS categories for UK adults, healthy weight ranges by height, and where BMI falls short as a single measure on its own. It sits in our Health category, alongside our other NHS-aligned calculators.

What is BMI and how is it calculated?

Body Mass Index (BMI) is a simple ratio of your weight to your height, designed to give a quick, population-level indicator of whether your weight falls within a healthy range. The formula is:

BMI = weight (kg) ÷ height (m)²

Someone who is 175cm tall and weighs 75kg has a BMI of 75 ÷ (1.75 × 1.75), which works out to 24.5, within the healthy range. If you'd rather work in feet, inches, stone and pounds, our BMI calculator converts automatically and gives the same result.

NHS BMI categories for UK adults

These categories are used across the NHS as a first, quick screening tool, typically alongside other measurements rather than in isolation. A GP or practice nurse considering your weight will usually look at your BMI alongside things like waist measurement, activity levels and overall health, not the number by itself.

These bands originate from World Health Organization classifications, adopted by the NHS as a consistent, simple starting point for assessing weight-related health risk across the whole adult population. Their strength is exactly that simplicity, and, as covered further down, that's also where their limits lie.

Healthy weight range for your height

Rather than calculating your own BMI category by hand, it can be more useful to see the actual healthy weight range for your height directly:

For example, someone 170cm tall has a healthy weight range of roughly 53.5kg to 72.0kg. Use the BMI calculator for your exact height rather than rounding to the nearest row above.

BMI categories differ by ethnic background

The 18.5 to 24.9 healthy range works reasonably well as a general guide, but it was originally based on data drawn largely from White European populations, and NHS guidance recognises that it doesn't fit everyone equally well. NHS guidance uses lower BMI thresholds for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds. For these groups, increased health risk is considered to start from a BMI of 23 rather than 25, and high risk from 27.5 rather than 30.

This adjustment exists because research shows conditions like type 2 diabetes and cardiovascular disease can appear at a lower BMI in these populations than the standard thresholds would suggest. If you fall into one of these groups, it's worth discussing the adjusted ranges with your GP rather than relying on the standard categories alone.

Why BMI isn't the whole story

BMI can't distinguish between weight from muscle and weight from fat. A very muscular person, such as an athlete or regular weightlifter, can register as overweight or even obese on BMI despite having low body fat, simply because muscle is denser than fat.

Where fat is carried on the body matters for health risk too, which BMI alone doesn't capture. That's why it's often used alongside waist circumference: general NHS guidance flags increased risk above roughly 94cm for men and 80cm for women, and high risk above roughly 102cm for men and 88cm for women. Waist-to-hip ratio and body fat percentage are other measures sometimes used alongside BMI for a fuller picture.

BMI for children and teenagers

Children and teenagers aren't assessed using the adult categories above. Instead, BMI is plotted against age and sex-specific percentile charts, since healthy body composition changes considerably during growth. If you have concerns about a child's weight, your GP, health visitor or school nurse is the right place to start rather than applying adult BMI ranges.

Pregnancy is another time standard BMI categories aren't applied in the usual way, since healthy weight gain is expected and tracked differently by your midwife. If you're expecting, our pregnancy due date calculator covers key NHS antenatal milestones instead.

What to do if your BMI is outside the healthy range

A BMI outside 18.5 to 24.9 isn't a diagnosis, and it doesn't mean anything is necessarily wrong. It can be a useful prompt to have a conversation with your GP or practice nurse, who can look at the fuller picture alongside your general health, activity levels and any existing conditions.

If change feels right for you, gradual and sustainable adjustments generally work better than drastic ones, and focusing on habits, such as regular movement and consistent sleep, tends to be more sustainable than fixating on a single number. Your mental health matters just as much as the physical side of this, and the NHS offers free weight management support for anyone who wants it, at whatever starting point.

The same applies if your BMI is below 18.5. Being underweight can carry its own health considerations, and a GP conversation is just as worthwhile here as it is at the higher end of the range. Whichever direction your number points, the most useful next step is usually the same one: a conversation with someone who can see your full health picture, not a search for a quick fix.

Frequently asked questions