How to use the BMI calculator
- Choose cm · kg or ft · in · lb.
- Enter your height and weight. Decimals are fine, and the result updates as you type.
- Read your BMI, the marker on the scale, and the category under each of the two scales.
- The last column shows the weight range that counts as healthy for your height under each scale — often the more useful number, because it answers "what should I weigh?" directly.
How BMI is calculated and classified
Body mass index is weight in kilograms divided by the square of height in metres: BMI = kg ÷ m². In imperial units the usual shortcut is 703 × pounds ÷ inches²; this calculator converts to metric with the exact factors instead (1 lb = 0.45359237 kg, 1 in = 2.54 cm), although the two differ by less than 0.01.
The WHO and CDC scale
The World Health Organization defines overweight in adults as a BMI of 25 or more and obesity as 30 or more. The US Centers for Disease Control and Prevention uses the same cut-offs and splits obesity into three classes:
| BMI | Category |
|---|---|
| below 18.5 | Underweight |
| 18.5 to less than 25 | Healthy weight |
| 25 to less than 30 | Overweight |
| 30 to less than 35 | Obesity, class 1 |
| 35 to less than 40 | Obesity, class 2 |
| 40 or more | Obesity, class 3 |
The Chinese standard
People of East Asian descent tend to carry more body fat, and face higher risk of diabetes and heart disease, at a given BMI. China's health standard WS/T 428-2013, *Criteria of weight for adults*, therefore sets lower cut-offs: 18.5 to less than 24 is normal, 24 to less than 28 is overweight and 28 or more is obesity. The same standard adds waist circumference: 85–89 cm for men and 80–84 cm for women indicates pre-central obesity, and 90 cm or more for men and 85 cm or more for women central obesity. The National Health Commission's 2024 dietary guideline for adult obesity reproduces these thresholds.
Why the result is truncated, not rounded
All the cut-offs are whole tenths, so this calculator shows BMI cut down to one decimal place rather than rounded. That keeps the number and the category consistent: a BMI of 24.97 is shown as 24.9 and classed as healthy under WHO, instead of being shown as 25.0 next to the word "healthy".
Examples
| Height and weight | BMI | WHO / CDC | China |
|---|---|---|---|
| 1.75 m, 70 kg | 22.8 | Healthy weight | Normal |
| 1.70 m, 72 kg | 24.9 | Healthy weight | Overweight |
| 1.72 m, 75 kg | 25.3 | Overweight | Overweight |
| 5 ft 9 in, 160 lb | 23.6 | Healthy weight | Normal |
The second row shows why the scale matters: the same person is at a healthy weight by the WHO scale and overweight by the Chinese one. For a height of 1.70 m, the healthy range is 53.5–72.2 kg on the WHO scale and 53.5–69.3 kg on the Chinese scale; for 1.60 m on the Chinese scale it is 47.4–61.4 kg.
Limits
- Adults only. Children and teenagers are assessed with BMI-for-age percentiles, which depend on age and sex; this calculator does not do that.
- BMI ignores body composition. It cannot tell muscle from fat, so athletes may be classed as overweight, and older adults who have lost muscle may look healthier than they are. It also says nothing about where fat is stored; waist circumference adds that.
- Not for pregnancy. Weight gain during pregnancy is judged differently.
- A screening tool, not a diagnosis. A doctor looks at BMI together with blood pressure, blood tests, family history and other factors. Use the number as a prompt for that conversation.
Frequently asked questions
What is a healthy BMI?
For adults, 18.5 to less than 25 on the WHO scale, or 18.5 to less than 24 on the Chinese scale.
Why is the Chinese overweight threshold 24 instead of 25?
Studies of Chinese adults found that health risks rise at lower BMI values than in the populations the WHO scale was based on, so the national standard uses 24 and 28.
How do I calculate BMI in pounds and inches?
Multiply your weight in pounds by 703 and divide by your height in inches squared. For 5 ft 9 in (69 in) and 160 lb: 160 × 703 ÷ 69² ≈ 23.6.
Is BMI accurate for athletes?
No. Muscle is dense, so muscular people can have a high BMI without excess fat.
Are my height and weight stored?
No. The calculation happens in your browser and nothing is saved or sent.