BMI Explained: How to Calculate It and What It Really Means
Body Mass Index (BMI) is a quick screening number that relates your weight to your height. Doctors and health organisations use it as a rough first check, and it is easy to work out yourself. It is genuinely useful — but only when you also understand what it cannot tell you.
The BMI formula
BMI is your weight in kilograms divided by your height in metres squared: BMI = weight (kg) ÷ height (m)². For example, someone who is 1.70 m tall and weighs 65 kg has a BMI of 65 ÷ (1.70 × 1.70) = 65 ÷ 2.89 ≈ 22.5. If you prefer pounds and inches, the formula is weight (lb) ÷ height (in)² × 703.
What the categories mean
- Below 18.5: underweight
- 18.5 to 24.9: normal weight
- 25.0 to 29.9: overweight
- 30.0 and above: obese
These ranges come from population studies linking BMI to health risks. A BMI in the normal range is associated, on average, with lower risk of certain conditions — but average is the key word.
The important limitations
BMI is a blunt instrument because it only knows your height and weight. It cannot tell muscle from fat. A very muscular athlete can have a high BMI while carrying little fat, and an older person can have a normal BMI while having lost muscle. It also does not account for where fat is stored, which matters for health, and the standard cut-offs may not fit every ethnic group equally well — some health bodies use lower thresholds for South Asian populations, for instance.
How to use BMI sensibly
- Treat it as a starting signal, not a diagnosis.
- Combine it with other measures like waist circumference and how you feel and function.
- Track trends over time rather than fixating on a single reading.
- Talk to a healthcare professional about what a healthy weight means for your body specifically.
Waist size tells you something BMI can't
Here's what BMI quietly misses: where you carry weight matters more than how much you weigh. Two people can share an identical BMI while one carries fat around the hips and the other around the middle — and it's the belly fat, the kind wrapped around your organs, that's most strongly tied to heart disease and diabetes. That's why a tape measure makes a useful sidekick. As a rough guide, a waist over about 90 cm for men or 80 cm for women is worth raising with your doctor, whatever the BMI happens to say.
BMI for children, athletes and older adults
The standard adult bands don't fit everyone. For children and teens, BMI is read against age-and-sex percentile charts rather than the fixed adult cut-offs. Athletes carrying real muscle often land in the 'overweight' range while holding very little fat. And older adults can sit comfortably in the 'normal' band while having quietly lost the muscle that keeps them steady on their feet. If you're in one of those groups, take the number with a pinch of salt.
Frequently asked questions
Is a 'normal' BMI the same as being healthy?
Not really. BMI knows nothing about your fitness, diet, blood pressure or blood sugar. It's a quick screen, not a clean bill of health — plenty of normal-BMI people aren't especially healthy, and some people in the overweight band are genuinely fit.
Why do some guidelines use lower cut-offs for South Asians?
Studies show people of South Asian descent tend to develop weight-related health risks at lower BMIs than the original thresholds assumed. Several health bodies therefore use a lower overweight cut-off — around 23 rather than 25 — for these populations.
How often should I check it?
No need to obsess. Glancing at it every few months to watch the trend beats weighing in daily — a single reading drifts with water, food and the time of day, so the direction matters far more than any one number.
The bottom line
BMI is a fast, free way to see roughly where your weight sits relative to your height, and it is worth knowing. Just remember it is one data point among many — useful as a nudge, not a verdict. Calculate yours in seconds with our tool, then use it as one input into a fuller picture of your health.