BMI calculator: your height-to-weight ratio
Enter your height and weight to see your body mass index, what category it falls into and the healthy weight range for your height.
BMI categories for adults
| BMI | Category (WHO) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value of 1.00 sits exactly on that limit; 0.74–0.99 falls within the healthy range.
Some health organisations use lower thresholds (for example, 23 and 27.5) for people of South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds, because health risks can rise at a lower BMI.
What is BMI?
Body mass index (BMI) is a simple ratio of your weight to your height. It was designed for studying populations, and today it’s used as a quick screening number by doctors and public-health bodies around the world.
The formula is your weight in kilograms divided by the square of your height in metres:
Imperial: BMI = 703 × weight (lb) ÷ height (in)²
For example, someone who is 1.78 m tall and weighs 76 kg has a BMI of 76 ÷ (1.78 × 1.78) = 24.0.
What BMI doesn’t tell you
BMI is quick and free, which is why it’s so widely used — but it’s a blunt instrument. It doesn’t distinguish muscle from fat, it says nothing about where fat is stored, and it doesn’t account for age, sex or ethnicity on its own.
- Muscular builds: a strength athlete can be “overweight” by BMI while carrying very little body fat.
- Older adults: people can lose muscle with age while their BMI stays the same.
- Fat distribution: fat around the waist is more closely linked to health risk than fat elsewhere.
That’s why it’s worth pairing BMI with a second quick check: your waist-to-height ratio. Measure your waist around the midpoint between your lowest rib and the top of your hip bone, then divide it by your height. Keeping your waist to less than half your height is a widely used guideline.
What to do with your number
Whatever your result, the habits that improve health are the same: regular movement, strength training, mostly whole foods, enough protein and good sleep. If you want to change your weight, our guides on sustainable fat loss and protein intake are good places to start, and our free programs give you a structured plan to follow.
If your BMI is under 18.5 or 30 and over, or if you have any concerns about your weight, it’s worth speaking to your doctor.
Frequently asked questions
How is BMI calculated?
BMI is 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.
What is a healthy BMI?
For most adults, the World Health Organization classifies 18.5–24.9 as a healthy weight range, 25–29.9 as overweight and 30 or above as obesity. Some health bodies use lower thresholds for people of certain ethnic backgrounds.
Is BMI accurate for muscular people?
Not always. BMI doesn’t distinguish muscle from fat, so very muscular people can have a high BMI while carrying little body fat. Waist measurement and waist-to-height ratio add useful context.
Can children use this BMI calculator?
No. This calculator is for adults aged 18 and over. Children and teenagers are assessed with age- and sex-specific BMI percentile charts.