BMI calculator
Body mass index from height and weight, with the WHO ranges.
How it works
BMI is weight in kg divided by height in metres squared. WHO ranges: under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 and over obese. It does not see muscle.
How to read your result
BMI is your weight in kilograms divided by your height in metres squared. For adults, the World Health Organization (WHO) groups the result like this: under 18.5 is underweight, 18.5 to 24.9 is healthy weight (WHO's own label is normal weight), 25 and over is overweight, and 30 and over is obesity. The same cut-offs apply to women and men and to every adult age.
BMI is a rough screen, not a diagnosis. It does not tell fat from muscle and bone, and it does not show where fat sits on the body. WHO also notes that the same BMI may not mean the same degree of fatness in different populations, partly because body proportions differ. A muscular person can therefore land in the overweight range.
Use the number as one clue next to other measures, such as waist size, and talk to a doctor if the result worries you. In Asian populations, a WHO expert group found that a large share of people already have a high risk of type 2 diabetes and heart disease below a BMI of 25. It kept the standard cut-offs and added extra action points at 23, 27.5, 32.5 and 37.5.
A worked example
Take a person who weighs 70 kg and is 175 cm tall. Step 1: turn the height into metres: 175 ÷ 100 = 1.75 m. Step 2: square it: 1.75 × 1.75 = 3.0625. Step 3: divide the weight by that: 70 ÷ 3.0625 = 22.857, shown as 22.9. That sits inside the WHO range of 18.5 to 24.9. In imperial units this person is about 154 lb and 69 in (5 ft 9 in). The page converts them to kg and cm first, and because those figures are rounded it shows 22.7.
You can also work backwards. At 175 cm, a BMI of 18.5 is 18.5 × 3.0625 = 56.7 kg and a BMI of 24.9 is 24.9 × 3.0625 = 76.3 kg. So the normal range for that height is about 56.7 to 76.3 kg (125 to 168 lb). At 85 kg and 180 cm, 1.80 × 1.80 = 3.24 and 85 ÷ 3.24 = 26.2, which is in the overweight range. BMI cannot tell you whether that weight is muscle or fat.
What is a healthy BMI?
For adults, WHO puts the range at 18.5 to 24.9 (its label is normal weight). It is a range, not a target. WHO says the health risks rise steadily as BMI goes up.
Is BMI accurate for athletes or people with a lot of muscle?
Only partly. BMI does not tell fat from muscle and bone, so a muscular person can read as overweight. Look at other measures too, such as waist size, and ask a doctor if you are unsure.
Is BMI the same for men, women and every age?
For adults, yes. WHO says the values do not change with age and are the same for both genders. For children and teens these cut-offs do not apply, because WHO takes age into account for them.
Does Zefro use BMI?
Zefro uses your height, weight and other details to work out your calorie targets. If you choose to lose weight and your BMI is under 18.5, the app shows a notice during setup: check your details, and talk to a doctor before starting a deficit.