What BMI is and what it tells you
Body mass index is your weight in kilograms divided by your height in metres squared. A person who weighs 68 kg and is 1.70 m tall has a BMI of 68 divided by 2.89, which is 23.5. Doctors use BMI as a quick screening number because it is cheap, fast and works for large groups of people.
BMI is a screen and not a diagnosis. It does not separate muscle from fat, it ignores where fat sits on the body, and it does not account for age, sex or ethnicity. Use it as one signal next to waist size, blood pressure, blood sugar, fitness and sleep.
BMI categories for adults
The World Health Organization sets the global adult cutoffs. Many Asian health bodies use lower cutoffs because health risks such as type 2 diabetes rise at a lower BMI in South and East Asian populations. The calculator lets you pick either scale.
| Category | WHO standard | Asian cutoffs |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Healthy weight | 18.5 to 24.9 | 18.5 to 22.9 |
| Overweight | 25 to 29.9 | 23 to 27.4 |
| Obesity range | 30 and above | 27.5 and above |
Some countries use 25 as the overweight line even for Asian adults, so check which scale your own doctor follows.
How sleep and BMI are connected
Short sleep and weight gain travel together. In a large 2004 study of more than 1,000 adults, people who slept less had lower leptin, the hormone that signals fullness, and higher ghrelin, the hormone that raises hunger. A 2008 review of 36 studies found that short sleepers had a clearly higher chance of obesity in both children and adults.
The practical lesson is simple. If you are trying to manage weight, protect seven to nine hours of sleep first. Use the bedtime calculator to pick a bedtime that matches your alarm, then check your habits with the sleep hours calculator.
Healthy weight by height
The table shows the weight range that gives a BMI of 18.5 to 24.9 on the WHO scale. For the Asian scale, the upper end is about 8 percent lower.
| Height | Lowest healthy weight | Highest healthy weight |
|---|---|---|
| 150 cm (4 ft 11 in) | 41.6 kg (92 lb) | 56.0 kg (123 lb) |
| 160 cm (5 ft 3 in) | 47.4 kg (104 lb) | 63.7 kg (140 lb) |
| 170 cm (5 ft 7 in) | 53.5 kg (118 lb) | 72.0 kg (159 lb) |
| 180 cm (5 ft 11 in) | 59.9 kg (132 lb) | 80.7 kg (178 lb) |
| 190 cm (6 ft 3 in) | 66.8 kg (147 lb) | 89.9 kg (198 lb) |
What to do with your result
- If your BMI is in the healthy range, keep your habits and keep a regular sleep schedule.
- If it is above range, aim for slow change. Losing 5 to 10 percent of body weight over several months already improves blood pressure and blood sugar.
- If it is below range, see a doctor to rule out illness before you try to gain weight.
- Pair BMI with the body fat calculator and calorie calculator for a fuller picture.