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What your BMI actually means
Body Mass Index is a single number that compares your weight to your height. It's a screening tool — a quick way for you and your doctor to flag whether your weight could be raising your risk of weight-related health problems.
A BMI in the healthy range (18.5–24.9) is associated, on average, with the lowest risk of conditions linked to excess or insufficient body weight, including type 2 diabetes, high blood pressure, and heart disease. A number outside that band isn't a diagnosis — it's a prompt to look closer. Two people can share the same BMI and have very different health profiles, which is why the result on its own is a starting point rather than a verdict.
BMI categories and what each band signals
For adults aged 18 and over, the World Health Organization uses these standard cut-offs:
| BMI (kg/m²) | Category | What it suggests |
|---|---|---|
| Below 18.5 | Underweight | Possible undernutrition; worth checking with a clinician |
| 18.5 – 24.9 | Healthy weight | Lowest average weight-related risk |
| 25.0 – 29.9 | Overweight | Modestly raised risk; lifestyle changes help |
| 30.0 – 34.9 | Obesity (Class 1) | Clearly raised risk |
| 35.0 – 39.9 | Obesity (Class 2) | High risk |
| 40.0 and above | Obesity (Class 3) | Very high risk; medical support recommended |
How BMI is calculated
The maths is simple, which is part of why BMI has stayed popular for over 150 years. In metric units:
BMI = weight (kg) ÷ height (m)²
So someone who is 1.80 m tall and weighs 80 kg has a BMI of 80 ÷ (1.80 × 1.80) = 24.7. In imperial units the formula is 703 × weight (lb) ÷ height (in)², where the 703 factor converts pounds and inches into the metric result. This calculator handles the conversion for you whichever units you choose.
Where BMI gets it wrong
BMI is a brilliantly simple tool, and simplicity has a cost. Because it only knows your height and weight, it can't see what that weight is made of. Keep these blind spots in mind:
- Muscle reads as "overweight." Muscle is denser than fat, so athletes and regular lifters often post a high BMI while carrying very little fat. If that's you, check your body fat percentage instead.
- It ignores where fat sits. Fat around the waist (visceral fat) is riskier than fat on the hips and thighs. Two people at the same BMI can have very different risk profiles.
- Age and ethnicity matter. Older adults tend to carry more fat at a given BMI, and some populations face raised risk at lower BMI thresholds.
- It doesn't apply to children or pregnancy. Under-18s use age- and sex-specific percentile charts, not these fixed cut-offs.
What to do with your number
If your BMI sits in the healthy range, the goal is simply to maintain it — find your maintenance calories and keep an eye on the trend over months, not days. If it's higher or lower than you'd like, change happens through a sustained, modest calorie shift rather than crash dieting:
- Find your daily burn with the BMR calculator and calorie calculator.
- Aim for a gentle deficit or surplus — around 250–500 calories a day is sustainable for most people.
- Protect muscle by hitting a protein target; the macro calculator turns your calories into grams.
- Re-check your BMI every few weeks to confirm you're trending the right way.
None of this replaces personalised advice. If your BMI is well outside the healthy range, or you have existing health conditions, speak with a doctor or registered dietitian before making big changes.
Helpful tools to track your progress
Tracks weight and estimated body fat over time.
View options →For waist and body-fat measurements at home.
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Frequently asked questions
What is a healthy BMI?
For most adults a BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is classified as obesity.
How is BMI calculated?
BMI equals your weight in kilograms divided by your height in metres squared (kg/m²). In imperial units it is 703 × weight in pounds ÷ height in inches squared.
Is BMI accurate for athletes?
Not always. BMI can't tell muscle from fat, so very muscular people can show a high BMI while having low body fat. In those cases a body fat percentage estimate is more useful.
Does BMI work for children?
Adult BMI categories don't apply to people under 18. Children are assessed with age- and sex-specific BMI percentile charts instead of fixed cut-offs.