What Is BMI?
Body Mass Index (BMI) is a single number that describes body weight relative to height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, which is why it is sometimes called the Quetelet Index. BMI is not a measure of body fat, and it does not diagnose anything on its own. What it does well is place a large population into broad weight categories quickly and cheaply, which is why clinicians, insurers and public-health bodies still use it as a first screening step.
The BMI Formula
BMI is weight divided by the square of height. The metric form is the definition; the imperial form is the same equation with a unit-conversion constant.
Metric: BMI = weight (kg) ÷ height (m)2
Imperial: BMI = 703 × weight (lb) ÷ height (in)2
For example, a person weighing 70 kg at 1.75 m has a BMI of 70 ÷ 1.752 = 70 ÷ 3.0625 = 22.9 kg/m². The same person in imperial units weighs about 154 lb at 69 inches: 703 × 154 ÷ 692 = 108,262 ÷ 4,761 = 22.7. The small difference is rounding in the unit conversion.
BMI Categories for Adults
The World Health Organization classification below applies to adults aged 20 and over, of both sexes and all body types.
| Category | BMI (kg/m²) | BMI Prime |
|---|---|---|
| Severe Thinness | under 16.0 | under 0.64 |
| Moderate Thinness | 16.0 – 17.0 | 0.64 – 0.68 |
| Mild Thinness | 17.0 – 18.5 | 0.68 – 0.74 |
| Normal | 18.5 – 25.0 | 0.74 – 1.00 |
| Overweight | 25.0 – 30.0 | 1.00 – 1.20 |
| Obese Class I | 30.0 – 35.0 | 1.20 – 1.40 |
| Obese Class II | 35.0 – 40.0 | 1.40 – 1.60 |
| Obese Class III | 40.0 and over | 1.60 and over |
BMI Weight Chart for Adults
The table below converts the BMI categories into actual weights at each height, so you can read your healthy weight range directly.
| Height | Underweight (lb) | Normal (lb) | Overweight (lb) | Obese (lb) |
|---|---|---|---|---|
| 4' 10" | under 89 | 89 – 120 | 121 – 144 | 145 and over |
| 4' 11" | under 92 | 92 – 124 | 125 – 149 | 150 and over |
| 5' 0" | under 95 | 95 – 128 | 129 – 154 | 155 and over |
| 5' 1" | under 98 | 98 – 132 | 133 – 159 | 160 and over |
| 5' 2" | under 101 | 101 – 137 | 138 – 164 | 165 and over |
| 5' 3" | under 104 | 104 – 141 | 142 – 169 | 170 and over |
| 5' 4" | under 108 | 108 – 146 | 147 – 175 | 176 and over |
| 5' 5" | under 111 | 111 – 150 | 151 – 180 | 181 and over |
| 5' 6" | under 115 | 115 – 155 | 156 – 186 | 187 and over |
| 5' 7" | under 118 | 118 – 160 | 161 – 192 | 193 and over |
| 5' 8" | under 122 | 122 – 164 | 165 – 197 | 198 and over |
| 5' 9" | under 125 | 125 – 169 | 170 – 203 | 204 and over |
| 5' 10" | under 129 | 129 – 174 | 175 – 209 | 210 and over |
| 5' 11" | under 133 | 133 – 179 | 180 – 215 | 216 and over |
| 6' 0" | under 136 | 136 – 184 | 185 – 221 | 222 and over |
| 6' 1" | under 140 | 140 – 189 | 190 – 227 | 228 and over |
| 6' 2" | under 144 | 144 – 195 | 196 – 234 | 235 and over |
| 6' 3" | under 148 | 148 – 200 | 201 – 240 | 241 and over |
| 6' 4" | under 152 | 152 – 205 | 206 – 246 | 247 and over |
| 6' 5" | under 156 | 156 – 211 | 212 – 253 | 254 and over |
| 6' 6" | under 160 | 160 – 216 | 217 – 260 | 261 and over |
BMI for Children and Teens
Between the ages of 2 and 19, BMI is interpreted differently. Children's body composition changes constantly with growth, and it differs between boys and girls, so a fixed cut-off such as 25 is meaningless. Instead, a child's BMI is compared against the BMI of other children of the same age and sex, and expressed as a percentile.
| Percentile range | Category |
|---|---|
| Below the 5th percentile | Underweight |
| 5th to below the 85th percentile | Healthy weight |
| 85th to below the 95th percentile | Overweight |
| 95th percentile and above | Obese |
A boy in the 60th percentile has a higher BMI than 60% of boys his age. The calculator above switches automatically to these percentile bands when the age entered is under 20, using the CDC growth-reference cut-offs.
Health Risks of Being Overweight
A persistently high BMI is associated with a measurably higher risk of a number of conditions. Association is not the same as causation, and BMI alone does not determine any individual's health, but the population-level pattern is consistent across large studies.
- High blood pressure, high LDL cholesterol and low HDL cholesterol
- Type 2 diabetes and insulin resistance
- Coronary heart disease, stroke and heart failure
- Osteoarthritis, particularly in the knees and hips, from sustained mechanical load
- Sleep apnea and other breathing problems
- Gallbladder disease and non-alcoholic fatty liver disease
- Several cancers, including endometrial, breast, colon, kidney and liver
- Complications during pregnancy and surgery
- Depression, anxiety and reduced quality of life, partly through social stigma
Health Risks of Being Underweight
The risks at the low end of the scale receive less attention but are real.
- Malnutrition, vitamin deficiencies and anemia
- Osteoporosis and a higher fracture rate from reduced bone density
- Weakened immune function and slower recovery from illness
- Fertility problems, including irregular or absent menstruation
- Growth and development problems in children and adolescents
- Higher surgical risk and, in some studies, higher all-cause mortality
- Possible indication of an underlying condition or an eating disorder
BMI Prime and the Ponderal Index
BMI Prime is simply BMI divided by 25, the upper limit of the normal range. A BMI Prime below 0.74 is underweight, 0.74 to 1.00 is normal, and above 1.00 is overweight. Because it is a ratio, it says immediately how far above or below the healthy limit someone is: a BMI Prime of 1.20 means 20% above the upper limit. It is also useful when comparing populations whose normal cut-offs differ.
The Ponderal Index divides weight by the cube of height rather than the square:
PI = weight (kg) ÷ height (m)3
Because human mass scales closer to the cube of height than the square at the extremes, the Ponderal Index is more reliable than BMI for very tall and very short people, where BMI systematically over- and under-estimates respectively. A normal Ponderal Index falls roughly between 11 and 15 kg/m³.
The Limitations of BMI
BMI is a useful screening tool, not a diagnosis. Its main weakness is that it cannot distinguish between the components of body weight.
- Muscle counts the same as fat. A trained athlete can register as overweight or obese while carrying very little body fat. Conversely a sedentary person of normal BMI may carry a high fat percentage — sometimes called "normal weight obesity".
- Fat distribution is ignored. Visceral fat around the abdomen carries far more metabolic risk than subcutaneous fat on the hips and thighs, but BMI treats them identically. Waist circumference and waist-to-height ratio capture this better.
- Age changes body composition. Older adults lose muscle and bone while gaining fat, so an unchanged BMI can conceal a substantial change in body composition.
- Sex differences are not accounted for. Women carry more body fat than men at the same BMI.
- Ethnic differences matter. The risk thresholds differ between populations. Several health authorities use a lower overweight cut-off of 23 kg/m² for people of South Asian, Chinese and other Asian descent, because cardiometabolic risk rises at a lower BMI.
- It does not apply to pregnancy. BMI is not meaningful during pregnancy; weight gain is assessed against pre-pregnancy BMI instead.
For these reasons BMI is best treated as one input among several. Waist circumference, body fat percentage, blood pressure, blood lipids, fasting glucose and physical activity levels together give a far more accurate picture of health than any single number.
Frequently Asked Questions
What is a healthy BMI?
For adults, 18.5 to 25 kg/m² is classified as normal by the WHO. For children and teens, the healthy band is the 5th to 85th percentile for their age and sex.
Does BMI differ for men and women?
The formula and the adult thresholds are identical. Women naturally carry a higher body fat percentage than men at the same BMI, which is one reason BMI is a weak proxy for body fat at the individual level.
Is BMI accurate for athletes?
Often not. Muscle is denser than fat, so a muscular person can have a BMI in the overweight or obese range with a low body fat percentage. A body fat measurement is more informative in that case.
How much should I weigh?
The calculator reports the weight range corresponding to a BMI of 18.5 to 25 for your height. That is a range, not a target — where you sit within it depends on your build, muscle mass and overall health.