Body Mass Index (BMI) is a single number that compares your weight to your height. Clinicians and public-health bodies use it as a quick screening figure — not a diagnosis — because it can be calculated from two measurements almost anyone can take at home.
This guide shows the metric and imperial formulas, works through several examples by hand, explains what the standard categories mean, and covers the situations where BMI gives a misleading picture.
You can check any figure in this article with the BMI Calculator.
The BMI Formula
BMI is mass divided by the square of height.
Metric
Imperial
The 703 factor simply converts pounds and inches into the metric result, so both formulas produce the same number for the same person.
What the variables mean
- Weight — body mass, measured in kilograms or pounds. Use a morning weight before eating for consistency.
- Height — standing height without shoes, in metres or inches.
- Height squared — the reason BMI rises quickly with small height errors. A 2 cm measurement error changes BMI by roughly 0.5 for an average adult.
BMI has units of kg/m², but it is almost always quoted as a bare number.
Worked Example 1: Metric
A person weighs 72 kg and is 1.75 m tall.
BMI = 23.5, which sits inside the normal-weight band for adults.
Worked Example 2: Imperial
A person weighs 185 lb and is 5 ft 11 in tall.
BMI = 25.8, just inside the overweight band.
Worked Example 3: Height Measured in Centimetres
A common mistake is dividing by centimetres instead of metres. For 68 kg and 162 cm:
Dividing by 162² instead would have produced 0.0026 — an obvious sign the height unit was wrong.
Standard Adult BMI Categories
The World Health Organization classification for adults aged 20 and over:
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
Some countries apply lower thresholds for people of South Asian, Chinese and other Asian ancestry, where cardiometabolic risk appears at a lower BMI. Health services in those regions often treat 23 as the overweight threshold and 27.5 as the obesity threshold. If you are unsure which classification applies to you, ask a clinician rather than assuming.
Reference Table: Weight Ranges by Height
Weight range corresponding to a BMI of 18.5 to 24.9:
| Height | Normal-weight range (kg) | Normal-weight range (lb) |
|---|---|---|
| 1.55 m (5'1") | 44.4 – 59.8 | 98 – 132 |
| 1.60 m (5'3") | 47.4 – 63.7 | 104 – 140 |
| 1.65 m (5'5") | 50.4 – 67.8 | 111 – 149 |
| 1.70 m (5'7") | 53.5 – 71.9 | 118 – 159 |
| 1.75 m (5'9") | 56.7 – 76.3 | 125 – 168 |
| 1.80 m (5'11") | 59.9 – 80.7 | 132 – 178 |
| 1.85 m (6'1") | 63.3 – 85.2 | 140 – 188 |
These ranges are calculated directly from the BMI formula and rounded to one decimal place.
Calculating BMI for Children and Teenagers
For anyone under 20, the number is calculated the same way but interpreted differently. A child's body composition changes constantly with age, so the raw BMI is plotted on an age-and-sex-specific growth chart and reported as a percentile.
- Below the 5th percentile — underweight
- 5th to 84th percentile — healthy weight
- 85th to 94th percentile — overweight
- 95th percentile and above — obesity
An adult category table should never be applied to a child's BMI.
Assumptions Behind BMI
BMI is a ratio, and every ratio carries assumptions:
- It assumes body mass scales with the square of height. That holds reasonably across a population but not for every individual.
- It makes no distinction between muscle, fat, bone and water.
- It says nothing about where fat is stored, which matters more for metabolic risk than the total.
- It assumes a standing measurement is available, which excludes many people with limited mobility or spinal conditions.
When BMI Is Misleading
Muscular people. A trained athlete carrying substantial lean mass can score 27 or 28 with very little body fat. The number is arithmetically correct and clinically meaningless.
Older adults. Muscle mass declines with age while fat increases, so an unchanged BMI can hide a real change in body composition.
Pregnancy. BMI is not used to assess weight during pregnancy; clinicians track gestational weight gain against pre-pregnancy BMI instead.
Very tall or very short people. Because height is squared, BMI slightly overstates the figure for tall people and understates it for short people compared with a cube-based measure.
Limb loss or significant oedema. Both distort mass relative to height.
Common Mistakes
- Mixing units — pounds with metres, or kilograms with inches.
- Dividing by centimetres rather than metres.
- Squaring the weight instead of the height.
- Using 703 in a metric calculation, or omitting it in an imperial one.
- Treating a category boundary as a hard clinical line. A BMI of 24.9 and 25.1 describe practically the same body.
Interpreting Your Result
Treat BMI as one input among several. A more complete picture combines it with:
- Waist circumference or waist-to-height ratio, which reflect abdominal fat
- Blood pressure, lipid and glucose readings
- Activity level, diet and family history
If your BMI falls outside the normal range, or you are unsure how to interpret it alongside other measurements, speak to a doctor or registered dietitian. This article is educational and is not medical advice or a diagnosis.
Related Calculations
- BMI Calculator — instant BMI in metric or imperial units
- BMR Calculator — resting energy expenditure
- Calorie Calculator — daily energy needs from activity level
- Body Fat Calculator — circumference-based fat estimate
- Ideal Weight Calculator — target weight ranges by height
Frequently Asked Questions
What is a healthy BMI for an adult?
For most adults, 18.5 to 24.9 is classified as normal weight. Some health services use 23 as the overweight threshold for people of Asian ancestry.
Does BMI differ for men and women?
The formula and the adult category table are identical. Women typically carry a higher proportion of body fat at the same BMI, which is one reason BMI alone cannot describe body composition.
Can I have a normal BMI and still be unhealthy?
Yes. A normal BMI with a high waist measurement, low activity level or poor metabolic markers still carries risk. BMI screens for weight relative to height, nothing more.
Should I recalculate BMI often?
Height changes very little in adulthood, so BMI moves only with weight. Monthly is usually enough to see a trend without reacting to daily fluctuation.
Is a BMI over 25 always a problem?
No. It signals that a closer look is worthwhile — particularly at waist size and metabolic markers — not that a health problem exists.
Summary
BMI = weight ÷ height², or 703 × pounds ÷ inches² in imperial units. It takes two measurements, a few seconds of arithmetic, and produces a screening number that is useful at population scale and only indicative for an individual. Calculate it with the BMI Calculator, then read it alongside waist measurement and clinical markers rather than on its own.