Your Body Mass Index (BMI) checks whether your weight is in a healthy range for your height. This calculator uses the categories NICE recommends — including the lower thresholds that apply to a large share of the UK population, which most tools ignore. Add your waist and it also works out your waist-to-height ratio.
What you’ll need
Your height, your weight, and your age. Pick the option that describes your background, because the healthy thresholds are different for some groups (more on that below). If you can, measure your waist too — it takes the reading from good to genuinely useful.
You don’t need to enter your sex: neither BMI nor the NICE waist-to-height ratio uses it.
How BMI is worked out
BMI is your weight in kilograms divided by your height in metres, squared:
BMI = weight (kg) ÷ height (m)²
So someone who is 1.78 m tall and weighs 92 kg has a BMI of 92 ÷ (1.78 × 1.78) = 29.0. The calculator does the sum in metric or feet-and-stones — knowing the formula helps explain why very muscular people sometimes get a misleading number.
What your result means
BMI on its own is just a number; the category is what matters. For most adults, NICE uses these bands:
| Category | BMI |
|---|---|
| Underweight | below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity class 1 | 30.0 – 34.9 |
| Obesity class 2 | 35.0 – 39.9 |
| Obesity class 3 | 40.0 or more |
If your result sits in the overweight or obese range, it doesn’t mean anything is wrong today — it’s a flag that losing some weight could lower your future risk of conditions like type 2 diabetes, high blood pressure and heart disease. If you’re in the healthy range, it means your weight is broadly in proportion to your height.
Why your background can change the thresholds
Here’s the part generic calculators get wrong. NICE recommends lower BMI thresholds for people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background. The reason is physiological: people in these groups tend to carry more fat around the middle, and face weight-related health risks at a lower BMI than the thresholds originally developed for White European populations.
| Category | Standard | Adjusted |
|---|---|---|
| Healthy weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25.0 – 29.9 | 23.0 – 27.4 |
| Obesity | 30.0+ | 27.5+ |
The difference is not small. A 165 cm adult weighing 68 kg has a BMI of 25.0 — “healthy” on a calculator that uses one set of numbers for everyone, but overweight under the thresholds NICE recommends for these groups. This calculator applies the right set based on the option you choose. It’s the single biggest reason to use it over a generic one.
Waist-to-height ratio: the measure BMI misses
BMI can’t tell where your weight sits, and fat around the middle is the kind that matters most for your heart and your risk of diabetes. That’s why NICE now recommends measuring your waist-to-height ratio alongside BMI (for anyone with a BMI under 35). The rule of thumb is simple enough to remember without a calculator:
Keep your waist to less than half your height.
To measure your waist properly: find the bottom of your ribs and the top of your hips, wrap a tape measure around the midpoint (roughly just above your belly button), and breathe out naturally before you read it. Divide that by your height in the same units. NICE reads the result like this:
| Waist-to-height ratio | What it means |
|---|---|
| 0.40 – 0.49 | Healthy — no increased risk |
| 0.50 – 0.59 | Increased risk |
| 0.60 or more | High risk |
Because it works the same way for everyone regardless of sex, ethnicity or how muscular you are, waist-to-height ratio is often a fairer read than BMI — which is exactly why it’s worth entering your waist.
Your healthy weight range — and what to aim for
Rather than just labelling you, the calculator shows the weight range that would put you in the healthy band for your height, in both kilograms and stones. If you’re above it, it tells you how many kilos you’d need to lose to reach the top of that range; if you’re below it, how much to gain to reach the bottom. Change the weight figure and recalculate to see how a target would land. It’s a range, not a single “ideal weight” — there’s no such thing — and it’s arithmetic from the NICE bands, not a prescription.
When BMI can mislead you
BMI is a useful screen, not a diagnosis, and there are a few people it doesn’t serve well:
- Very muscular people. Muscle is dense, so serious strength athletes can land in the “overweight” band with very little fat. If that’s you, trust your waist-to-height ratio over your BMI.
- Adults over 65. A slightly higher BMI can be protective in later life, so the categories should be read with more caution.
- Pregnancy. BMI isn’t valid during pregnancy — speak to your midwife or GP instead.
- Children and teenagers. Under-18s are assessed on age- and sex-adjusted growth charts, not adult categories. Use the NHS BMI healthy weight calculator, which handles this properly.
What to do next
If your result nudges you into thinking about your weight, the NHS has free tools and support, and your GP can talk through your waist-to-height ratio and any other risk factors. This calculator is a starting point for a conversation, not a substitute for one. CalcLab doesn’t sell diet plans — just a neutral UK reading against NICE guidance.
Frequently asked questions
Is this the same as the NHS BMI calculator? It uses the same underlying formula and the same NICE categories, and adds the ethnicity-adjusted thresholds, your waist-to-height ratio and your target weight range in one place.
What’s a healthy BMI? For most adults, 18.5 to 24.9. For people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, NICE recommends a healthy range of 18.5 to 22.9.
Why doesn’t it ask for my sex? Because neither BMI nor the NICE waist-to-height ratio classification uses it. We don’t collect information the calculation doesn’t need.
Is a BMI of 25 bad? It’s the bottom of the “overweight” band on the standard thresholds — a flag rather than a problem in itself. On the adjusted thresholds it would already be overweight. Your waist-to-height ratio adds context.
Can I be healthy and still have a high BMI? Yes — particularly if you’re very muscular. That’s exactly the situation waist-to-height ratio is better at reading.
Sources & how this is kept current
The formula, categories, ethnicity-adjusted thresholds and waist-to-height bands come from NICE guideline NG246, Overweight and obesity management (published 2025, last updated 2026). See the review date at the top. This is information to help you understand your numbers, not medical advice — for anything specific to you, speak to a healthcare professional.