BMI (body mass index)
Body mass index (BMI) is your weight in kilograms divided by your height in metres squared, a quick way to check whether your weight is in a healthy range for your height.
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Work out your BMI
The formula: BMI = weight (kg) ÷ height (m)². Someone who weighs 75 kg and is 1.75 m tall has a BMI of 75 ÷ (1.75 × 1.75) = 24.5.
What your BMI means
| BMI | Range |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 39.9 | Obese |
| 40 or above | Severely obese |
These ranges are for adults aged 18 and over. Children and teenagers use BMI centiles, which allow for age and sex, and BMI isn't used during pregnancy1.
What BMI can't tell you
- Muscle from fat. BMI only knows your weight. A 90 kg lifter who's 1.80 m tall has a BMI of 27.8, "overweight", even when lean.
- Where you carry fat. Fat around your middle is more closely linked to health risks than fat elsewhere.
- Changes with age. Older adults can have a "healthy" BMI while carrying more fat and less muscle than they used to.
BMI works reasonably well for spotting trends across large groups of people, but a review of the evidence found it's a poor guide to how much body fat any one person carries5. Treat it as a screening tool, not a diagnosis.
Waist-to-height ratio: a better second check
Measure your waist halfway between your lowest rib and the top of your hip bone, breathing out naturally, then divide it by your height (using the same units).
| Waist-to-height ratio | What it means |
|---|---|
| 0.4 to 0.49 | Healthy |
| 0.5 to 0.59 | Increased health risk |
| 0.6 or more | High health risk |
The simple rule: keep your waist to less than half your height. NICE advises using it alongside BMI for adults with a BMI under 353.
Research spotlight
Across 31 studies and more than 300,000 adults, waist-to-height ratio picked out people with higher risks of diabetes, high blood pressure and heart disease better than BMI did.
BMI for lifters
If you've built a lot of muscle, BMI will overestimate how much fat you carry. Better ways to track your progress:
- Waist-to-height ratio, or your waist measurement on its own, taken the same way each time.
- Your weight trend (not single weigh-ins) alongside your strength in the gym.
- Progress photos in the same light every 2–4 weeks.
Is a BMI of 26 bad if I lift weights?
Not necessarily. If your waist is less than half your height and you're getting stronger, a BMI slightly over 25 from muscle isn't a concern.
Should I use BMI to pick a goal weight?
Use it as a rough guide only. A goal based on your waist, how you feel and how you perform is usually more meaningful.
Track this in NutriLog
Track your weight and BMI over time
Log weigh-ins in kg, stone or lb and see your trend, your BMI and your progress towards a goal on one chart.
Free to use. No card needed.
References
- NHS. Calculate your body mass index (BMI) for adults. nhs.uk.
- World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO consultation (Technical Report Series 894). WHO, Geneva, 2000.
- National Institute for Health and Care Excellence. Overweight and obesity management (NG246). NICE, 2025.
- National Institute for Health and Care Excellence. BMI: preventing ill health and premature death in black, Asian and other minority ethnic groups (PH46). NICE, 2013.
- Nuttall FQ. Body mass index: obesity, BMI, and health: a critical review. Nutr Today. 2015;50(3):117-128. doi:10.1097/NT.0000000000000092
- Ashwell M, Gunn P, Gibson S. Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis. Obes Rev. 2012;13(3):275-286. doi:10.1111/j.1467-789X.2011.00952.x