What BMI measures, and what it does not
Body Mass Index (BMI) compares your weight with your height in a single number. It was developed from the work of the Belgian statistician Adolphe Quetelet in the nineteenth century, and it survives because it is free, instant and needs no equipment, which is why health surveys and clinics use it as a first screen. What it measures is how heavy you are for your height. What it does not measure is how much of that weight is fat, where the fat sits, or how healthy you are. Treat it as a prompt for a conversation, not a verdict.
The formula
BMI = weight in kilograms ÷ (height in meters)²
The height must be converted from centimeters to meters before squaring. A person 170 cm tall is 1.70 m, and 1.70 × 1.70 = 2.89. If you work in pounds and inches, the equivalent is 703 × weight (lb) ÷ height (in)². This calculator takes metric input, so convert first if needed (1 kg ≈ 2.205 lb, 1 in = 2.54 cm).
Categories used by this calculator (WHO adult classification)
This calculator applies the World Health Organization's international adult cut-offs. They are the most widely used worldwide, and the most common reference in English-language material.
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25 to 29.9 | Overweight (pre-obese) |
| 30 and above | Obese |
The WHO scheme further divides obesity into class I (30 to 34.9), class II (35 to 39.9) and class III (40 and above). The calculator reports the four broad groups above.
Weight ranges by height
The table inverts the formula (weight = BMI × height²) to show where each WHO boundary falls at common heights. Find your height, and the two normal-range columns give the weight window that corresponds to a BMI of 18.5 to 24.9. The last column shows how many kilograms equal one BMI point at that height, which is handy for judging whether a change is meaningful.
| Height | Normal from (kg) | Normal up to (kg) | In pounds | Overweight from (kg) | Obese from (kg) | kg per BMI point |
|---|---|---|---|---|---|---|
| 150 cm (4'11") | 41.6 | 56.0 | 92 to 124 lb | 56.3 | 67.5 | 2.3 |
| 155 cm (5'1") | 44.4 | 59.8 | 98 to 132 lb | 60.1 | 72.1 | 2.4 |
| 160 cm (5'3") | 47.4 | 63.7 | 104 to 141 lb | 64.0 | 76.8 | 2.6 |
| 165 cm (5'5") | 50.4 | 67.8 | 111 to 149 lb | 68.1 | 81.7 | 2.7 |
| 170 cm (5'7") | 53.5 | 72.0 | 118 to 159 lb | 72.2 | 86.7 | 2.9 |
| 175 cm (5'9") | 56.7 | 76.3 | 125 to 168 lb | 76.6 | 91.9 | 3.1 |
| 180 cm (5'11") | 59.9 | 80.7 | 132 to 178 lb | 81.0 | 97.2 | 3.2 |
| 185 cm (6'1") | 63.3 | 85.2 | 140 to 188 lb | 85.6 | 102.7 | 3.4 |
| 190 cm (6'3") | 66.8 | 89.9 | 147 to 198 lb | 90.3 | 108.3 | 3.6 |
Worked examples
Example 1. Height 170 cm, weight 65 kg: 65 ÷ 2.89 = 22.5, comfortably inside the normal range. At the same height 70 kg gives 24.2 (upper end of normal) and 75 kg gives 26.0 (overweight band). Because one BMI point is about 2.9 kg at 170 cm, those 5 kg steps move the number by roughly 1.7 points each.
Example 2. Height 160 cm, weight 80 kg: 160 cm is 1.60 m, 1.60 × 1.60 = 2.56, and 80 ÷ 2.56 = 31.3, which is in the obese group. To reach the top of the normal range (24.9) at this height you would need to be about 63.7 kg, so the gap is about 16 kg. A first target such as moving down to BMI 30 (76.8 kg) is a gap of just over 3 kg and is a more manageable milestone.
Example 3. Height 180 cm, weight 90 kg: 90 ÷ 3.24 = 27.8, overweight by the WHO table. If this person lifts weights several times a week and has a lean waistline, the number may overstate their body fat, which is exactly the situation described below.
The reference weight shown in the result
Besides the category, the result displays a reference weight calculated as 22 × height². BMI 22 sits in the middle of the normal range and is commonly used as a reference point in several countries, but it is a convention, not a target everyone should reach. At 170 cm it gives 63.6 kg. Anyone whose weight is anywhere inside the normal-range window in the table above is generally not considered to need a change on BMI grounds alone.
Different countries, different cut-offs
The calculator uses WHO values, but several Asian authorities set lower boundaries because health risks associated with excess weight can appear at lower BMI values in some Asian populations. Asia-Pacific guidance, for instance as applied by the Korean Society for the Study of Obesity, treats 23 to 24.9 as an at-risk band and 25 or more as obese. Chinese adult guidance uses 24 and 28, and the Japan Society for the Study of Obesity uses 25 as the point where obesity begins. The table shows how the same BMI is labelled under each scheme. If you were measured at a clinic abroad, ask which scale was used before comparing numbers.
| BMI | WHO (used here) | Asia-Pacific | China | Japan |
|---|---|---|---|---|
| 17 | Underweight | Underweight | Underweight | Underweight |
| 18.5 | Normal | Normal | Normal | Normal |
| 22 | Normal | Normal | Normal | Normal |
| 23 | Normal | At risk (overweight) | Normal | Normal |
| 24 | Normal | At risk (overweight) | Overweight | Normal |
| 25 | Overweight | Obese I | Overweight | Obese (class 1 and above) |
| 28 | Overweight | Obese I | Obese | Obese (class 1 and above) |
| 30 | Obese | Obese II or higher | Obese | Obese (class 1 and above) |
| 35 | Obese | Obese II or higher | Obese | Obese (class 1 and above) |
When BMI can mislead
- Muscular people. Muscle is denser than fat, so athletes and regular lifters can land in the overweight band with low body fat.
- Older adults. Muscle tends to decline with age, so the same weight can carry more fat. Some research suggests a slightly higher BMI is not necessarily harmful in later life, so avoid deciding on weight loss from the number alone.
- Pregnancy and breastfeeding. Weight changes are expected, and the standard adult categories are not meant to be applied.
- Under 18. Growth is still under way. Children and teenagers are assessed with age- and sex-specific growth charts (percentiles), not adult cut-offs.
- Fat distribution. Two people with the same BMI can differ in how much fat is stored around the abdomen, which is generally associated with higher metabolic risk than fat stored elsewhere.
- Different body types and ethnic groups. The same BMI can reflect different body composition between groups, which is one reason regional cut-offs exist.
What to look at alongside BMI
Waist circumference is the simplest add-on: a tape measure at home can reveal abdominal fat that BMI hides. Body composition scans (bioimpedance scales, DEXA) estimate fat and muscle directly. A clinician will usually also look at blood pressure, blood glucose and cholesterol, family history, and how you feel in daily life. If you want a quick next step, try the Body Fat Calculator, which estimates body fat percentage from tape measurements.
What to do with your result
- Below 18.5: If the number reflects unintended weight loss, loss of appetite or fatigue, a medical check-up is more useful than a diet tweak.
- 18.5 to 24.9: There is nothing to correct on BMI grounds. Small day-to-day swings from water and food are normal; compare weekly averages, not single readings.
- 25 and above: A gradual change is easier to sustain than a crash plan. Use the BMR / TDEE Calculator to estimate daily energy needs, and consider talking to a doctor or dietitian, especially with a BMI of 30 or more or any existing condition.
- Tracking over time: Measure at the same time of day, with similar clothing, and judge the trend over several weeks.
Important notice
The results of this calculator are general information and are not medical advice, a diagnosis or a substitute for a professional assessment. Your height and weight stay in your browser. If you have concerns about your weight, eating, or health, speak with a doctor, a registered dietitian or another qualified professional.
Frequently Asked Questions
Can my BMI be normal even if my body fat percentage is high?
Yes. BMI does not distinguish muscle from fat, so a person with little muscle and a lot of fat can still have a normal BMI, and a muscular person can have a high BMI with low body fat. A body composition measurement gives a more accurate picture.
How is the reference weight calculated?
It uses a BMI of 22: reference weight (kg) = 22 × height (m)². At 170 cm that is 22 × 1.7² = 63.6 kg. It is a convention for illustration, and any weight in the normal range for your height is generally considered acceptable on BMI grounds.
Does this apply to children and teenagers?
No. These are adult cut-offs. For anyone under 18, growth charts that account for age and sex (percentile-based) are used instead, and a pediatrician can interpret them.
How much weight is one BMI point?
It depends on height, because a change in weight is divided by height squared. At 160 cm one point is about 2.6 kg, at 170 cm about 2.9 kg and at 180 cm about 3.2 kg. Taller people need a larger change to move the same amount.
Why does another website give me a different category?
Usually because it uses a different set of cut-offs. This page uses the WHO adult scale, while some Asian guidelines label BMI 23 to 24.9 or 24 to 27.9 differently, as the comparison table shows. The BMI number itself should be identical.