I remember when I started tracking protein… and a friend asked me the simplest question: “But what is your BMI?” I had no idea. I knew my weight, I knew my lifts, but I had never placed myself on the most basic health scale there is. When I finally calculated it, I was 26.4 — technically overweight, mostly from the muscle I had built, but it was still a useful reality check about where I stood.
In this article
The BMI Calculator above gives you that reality check in seconds. Enter your weight in kilos and your height in centimetres. It returns your BMI to one decimal, the NHS weight band it falls in, and the healthy weight range for your height. It also tells you the honest limitation: BMI is a screening tool, not a diagnosis.
What BMI is and is not
BMI — Body Mass Index — is weight relative to height. It was designed for populations, not individuals, and it cannot tell muscle from fat. A rugby player and a sedentary office worker can share a BMI of 28 with completely different health profiles. I use it as a first glance, then look at waist measurement, fitness and habits for the real picture.
| NHS band | BMI range | What I take from it |
|---|---|---|
| Underweight | Below 18.5 | Worth a GP chat, especially if unintentional |
| Healthy weight | 18.5–24.9 | The zone to maintain |
| Overweight | 25–29.9 | Check waist and habits before worrying |
| Obese | 30 and above | A real health signal — act on it kindly |
The bands above are the adult NHS cut-offs used across the UK. The British Nutrition Foundation has useful context on what sits behind the categories.
The BMI formula
Weight in kilos divided by height in metres squared:
| Variable | What it is | Example |
|---|---|---|
| W | Weight in kilos | 70 kg |
| H | Height in metres (cm ÷ 100) | 1.70 m |
| BMI | The index, to one decimal | 24.2 |
The healthy weight range for your height is simply the band edges run backwards: 18.5 × H\u00B2 to 24.9 × H\u00B2. The NHS Eat Well guide pairs this with the lifestyle advice that actually moves the number.
A worked example: 70 kilos at 170 cm
Let me walk you through my exact setup — a straightforward case.
| Step | Calculation | Result |
|---|---|---|
| Height in metres | 170 ÷ 100 | 1.70 m |
| Height squared | 1.70 × 1.70 | 2.89 |
| BMI | 70 ÷ 2.89 | 24.2 |
| NHS band | 18.5–24.9 | Healthy weight |
| Healthy range | 18.5 × 2.89 to 24.9 × 2.89 | 53.5–72.0 kg |
A BMI of 24.2 sits near the top of the healthy band — fine, but with less headroom than 21. That framing helped me: the band is wide, and where you sit inside it is information, not a verdict.
Using the BMI Calculator step by step
Enter your two numbers at the top of this page, then press Calculate BMI.
| Input field | What to enter | Sample figure |
|---|---|---|
| Weight (kg) | Morning weight, lightly dressed | 70 |
| Height (cm) | Without shoes | 170 |
You get your BMI, your NHS weight band and the healthy weight range for your height. Everything runs in your browser. If your BMI says overweight and you want to act on it, the TDEE Calorie Calculator sets a sensible calorie target, the Protein Intake Calculator protects your muscle while you lose, and the Lean Body Mass Protein Calculator is the fairer tool if you carry a lot of muscle.
When BMI misleads — and what to check instead
If you lift weights. Muscle is dense. I have been “overweight” by BMI while visibly lean. In that case, waist measurement and progress photos tell the truth.
If you are very short or tall. The formula slightly favours the middle of the height range. Treat edge cases with extra scepticism.
Waist first. For health risk, waist circumference beats BMI. The NHS flags concern above 94 cm for men and 80 cm for women. I measure mine monthly — it moves before the scale does.
Questions I get asked about BMI
What is a healthy BMI in the UK?
The NHS defines healthy weight as a BMI of 18.5 to 24.9 for adults. I treat the middle of that band — around 21 to 23 — as a comfortable long-term home for most people. Near the edges is still healthy, but I would want to know why I was sitting there before deciding to stay.
Is BMI accurate for muscular people?
Not really — that is its best-known flaw. Muscle weighs more than fat per litre, so lifters routinely land in “overweight” while lean. If you train seriously and your waist is trim, I would largely ignore the BMI label. It was never designed for athletes, and punishing yourself with it is misusing the tool.
Can I use BMI for children?
Not the adult bands — children use age-and-sex-specific percentile charts, because healthy BMI changes as they grow. This calculator uses adult cut-offs only. For anyone under 18, the GP or school nurse has the right charts, and I would not apply this page’s bands to them.
My BMI is 27 but I feel fine. Should I worry?
I would investigate, not panic. Check your waist measurement, blood pressure and how you actually feel day to day — then decide. A BMI of 27 with a 90 cm waist and good fitness is a different story from 27 with a 104 cm waist. The number is a prompt for curiosity, not a diagnosis, and your GP can put it in context.
