Body mass index is one of the simplest numbers in health: your weight in kilograms divided by the square of your height in metres. A man who weighs 70 kg and is 1.70 m tall has a BMI of 70 ÷ 2.89 = 24.2. Under the World Health Organization's international table that is "normal weight". Under the lower cut-offs that were proposed for adults in the Asia-Pacific region, the same 24.2 is "overweight, at risk". This article explains where both tables come from, what BMI is good for, what it misses, how the calorie numbers that usually sit next to it (BMR and TDEE) are estimated, and the common mistakes. Every figure below comes from the BMI calculator's own logic. None of it is medical advice: it is background for a conversation with a health professional.
The formula, and one rounding rule
BMI = weight (kg) ÷ height (m)²Height is in metres, not centimetres: 170 cm is 1.70 m, and 1.70² = 2.89. For pounds and inches, the calculator converts with the exact definitions (1 lb = 0.45359237 kg, 1 in = 2.54 cm). 154 lb at 5 ft 7 in becomes 69.85 kg at 170.18 cm, a BMI of 24.12. The well-known shortcut 703 × lb ÷ in² gives 24.117; both round to 24.1.
BMI tables are written with one decimal (18.5 to 24.9, 25.0 to 29.9), so the calculator rounds BMI to one decimal first and then looks up the category. That matters at the edges: 76.5 kg at 1.75 m is 24.98. Shown as 25.0, it is classed as overweight, not as a "24.98 that is still normal". A tool that compares the unrounded value with 25 would show you "25.0, normal", which is confusing.
The WHO table
For adults (18 and over) WHO uses the same cut-offs for men and women and for all ages:
| BMI | WHO category |
|---|---|
| below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25.0 to 29.9 | Overweight (pre-obese) |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40 and above | Obesity class III |
Why Asian cut-offs are lower
Researchers found that in many Asian populations the risk of type 2 diabetes and cardiovascular disease starts to rise at lower BMIs than in European populations: at the same BMI, people tend to carry more body fat and more of it around the waist. In 2000, the report The Asia-Pacific perspective: redefining obesity and its treatment (WHO Western Pacific Region, with IASO and IOTF) proposed provisional cut-offs for adults in the region:
| BMI | Asia-Pacific category (2000, provisional) |
|---|---|
| below 18.5 | Underweight |
| 18.5 to 22.9 | Normal range |
| 23.0 to 24.9 | Overweight, at risk |
| 25.0 to 29.9 | Obese I |
| 30 and above | Obese II |
In 2004 a WHO expert consultation, published in The Lancet, looked at the evidence again. It kept the international table, noted that the BMI at which risk rises differs between Asian populations (from about 22 to 25), and added 23, 27.5, 32.5 and 37.5 as public health action points that countries can use. The calculator shows the 2000 table as a second reference next to WHO's, not as a verdict. Many clinics in Vietnam and the rest of East and Southeast Asia use the lower cut-offs, which is why a result can be "normal" in one column and "at risk" in the other.
Three worked examples
- 70 kg, 170 cm, man, 30. BMI 24.2: WHO normal, Asia-Pacific overweight (at risk). A normal BMI at 170 cm is 53.5 to 71.9 kg by WHO and 53.5 to 66.1 kg by the Asia-Pacific table. The calculator rounds the lower end up and the upper end down, so both ends really are inside the range.
- 58 kg, 160 cm, woman, 35. BMI 22.7: normal in both tables. Normal weight at 160 cm is 47.4 to 63.7 kg (WHO) or 47.4 to 58.6 kg (Asia-Pacific).
- 95 kg, 180 cm, man, 45. BMI 29.3: overweight by WHO, obese I by the Asia-Pacific cut-offs. A normal BMI at 180 cm is 60.0 to 80.6 kg (WHO).
BMR and TDEE: the calorie numbers
BMR (basal metabolic rate) is the energy your body uses at complete rest. The calculator uses the Mifflin-St Jeor equation, published in 1990; a 2005 systematic review for the American Dietetic Association (Frankenfield and colleagues) found it the most reliable of the common prediction equations for healthy adults, while still off by more than 10% for some people:
BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5 (men)
BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161 (women)For the first example: 700 + 1,062.5 − 150 + 5 = 1,617.5 kcal a day, shown as 1,618.
TDEE (total daily energy expenditure) adds everyday movement and exercise. It is estimated by multiplying BMR by an activity factor:
| Activity level | Factor | Example: BMR 1,617.5 |
|---|---|---|
| Sedentary (desk job, little exercise) | 1.2 | 1,941 kcal |
| Light (exercise 1 to 3 days a week) | 1.375 | 2,224 kcal |
| Moderate (3 to 5 days a week) | 1.55 | 2,507 kcal |
| Active (hard exercise 6 to 7 days a week) | 1.725 | 2,790 kcal |
| Very active (physical job, twice-daily training) | 1.9 | 3,073 kcal |
The calculator multiplies the unrounded BMR and rounds once at the end. For a woman of 95 kg, 180 cm and 45 years, BMR is 950 + 1,125 − 225 − 161 = 1,689 kcal, and sedentary TDEE is 1,689 × 1.2 = 2,026.8, shown as 2,027.
What BMI does not tell you
- Fat or muscle. BMI only knows weight. A rugby player and a sedentary person of the same height and weight have the same BMI.
- Where fat sits. Fat around the waist carries more risk than fat on the hips. Waist circumference adds information BMI lacks.
- Children and teenagers. Their BMI changes as they grow, so it is compared with growth charts for age and sex. The adult table does not apply under 18, and the calculator does not give adult categories or calorie figures below that age.
- Pregnancy, older age, illness. Weight gain in pregnancy is expected, and older adults can have a normal BMI with too little muscle.
Common mistakes
- Using centimetres in the formula. 70 ÷ 170² gives 0.0024, which is not a BMI. Convert to metres first.
- Mixing units. The 703 factor is for pounds and inches only. Using it with kilograms gives nonsense.
- Classifying the unrounded value. A BMI of 24.98 is shown as 25.0; it should be read as 25.0.
- Reading one table only. If you live in Asia, check both columns. "Normal" by WHO and "at risk" by the Asia-Pacific cut-offs is a common combination between 23 and 24.9.
- Treating TDEE as exact. It is an estimate from an equation and a rough activity factor. Real needs can differ by a few hundred kilocalories a day.
- Using BMI for a child or an athlete. Use growth charts for children and body composition measurements for athletes.
How to use the calculator
Open the BMI calculator, choose metric or imperial, and enter weight, height, age and sex. The result shows the BMI, both categories, the normal weight range for your height in both tables, your BMR and your daily energy needs at the activity level you pick, with all five levels in a table. Nothing is sent anywhere. If you are working out other everyday numbers, the age calculator gives an exact age in years, months and days. And whatever the number says, a health professional can put it in context with the things BMI cannot see.