What BMI is, and how it is calculated
BMI compares your weight with your height. The metric formula is:
BMI = weight (kg) / height (m)2
In pounds and inches, multiply by 703: BMI = 703 × weight (lb) / height (in)2. The number has no unit, and it is the same whichever system you use.
Two worked examples. A person who is 1.75 m tall and weighs 70 kg has a BMI of 70 / (1.75 × 1.75) = 70 / 3.0625, which is about 22.9. Someone who is 5 ft 10 in (70 inches) and weighs 180 lb has a BMI of 703 × 180 / 4,900, about 25.8.
BMI was developed in the 19th century as a way to describe populations, not to diagnose individuals. Today it is used because it is cheap, needs only a scale and a tape measure, and correlates reasonably with body fat across large groups of people. Our BMI calculator does the arithmetic in either unit system and shows where you fall.
The standard categories
For adults aged 20 and over, the World Health Organization and the US Centers for Disease Control use the same cut-offs:
- Below 18.5: underweight
- 18.5 to 24.9: healthy weight range
- 25.0 to 29.9: overweight
- 30.0 and above: obesity, commonly split into class 1 (30.0 to 34.9), class 2 (35.0 to 39.9) and class 3 (40 or higher)
These are screening bands, not a pass or fail. A BMI of 24.8 and one of 25.1 are not meaningfully different people, even though they fall on opposite sides of a line. Treat the category as a prompt for a conversation, not a verdict.
You can also work backwards to find the weight range your height implies. At 1.75 m, a BMI between 18.5 and 24.9 corresponds to roughly 56.7 kg to 76.3 kg. If your weight is outside that, it does not automatically mean a problem, but it is a reason to look at other measures.
Where BMI misleads
BMI uses only two inputs, so it cannot see what your weight is made of or where it sits. The main blind spots:
- Muscle. Muscle is denser than fat. A strength athlete can land in the overweight or obese band with low body fat.
- Body fat distribution. Fat carried around the abdomen is linked to higher metabolic risk than fat on the hips and thighs, but BMI treats them the same.
- Age. Older adults tend to lose muscle and gain fat at the same weight. A "normal" BMI can hide low muscle mass.
- Ethnicity. Health risks at a given BMI differ between populations. The WHO has noted that for many Asian populations risk rises at lower BMI values, and some health bodies use lower action thresholds, commonly around 23 and 27.5.
- Height extremes. The formula tends to over-penalize very tall people and under-penalize very short ones.
- Pregnancy and growth. BMI is not meant for pregnant women. For children and teens, BMI is read as a percentile for age and sex, not with the adult bands.
In short, BMI is a decent first filter and a poor final answer.
Measures that add context
If your BMI is in an unexpected range, a few other numbers give a fuller picture:
- Waist circumference. Commonly cited thresholds for added risk are above about 102 cm (40 in) in men and 88 cm (35 in) in women, though thresholds vary by guideline and population.
- Waist-to-height ratio. Keeping your waist under half your height is a simple rule used in some guidelines.
- Blood pressure, blood sugar and cholesterol. These show how your body is actually functioning, which BMI cannot.
- Fitness and habits. Regular activity, sleep, diet quality and smoking status all matter, independently of weight.
A clinician can also use body composition methods such as skinfold measurement or DEXA scans, though these are not needed for most people.
What to do with your result
If your number is within the healthy range and you feel well, there is nothing to fix. Re-check occasionally, particularly if your weight changes by more than a few kilograms.
If it is above or below the range, treat it as a reason to look closer rather than as a diagnosis. Consider a routine check-up and ask your doctor whether your weight is a concern given your age, family history and other measurements. Aim for gradual changes you can maintain: crash approaches tend to be hard to sustain.
If you are trying to change your weight, energy balance is the lever. Our calorie calculator estimates your daily energy needs from your size and activity, which can help you set a sensible starting target. It is an estimate, and individual needs vary.
This guide is general information and not medical advice. For personal health decisions, speak with a qualified healthcare professional.
Frequently asked questions
Is BMI accurate for athletes?
Often not. Muscle weighs more than fat, so people with a lot of muscle can have a high BMI without excess body fat. Waist measurements or body composition tests give a better picture.
Is the BMI formula different for men and women?
No. The adult formula and categories are the same for both. Body fat percentages differ between sexes at the same BMI, which is one reason the number is only a rough guide.
What BMI is considered healthy?
For most adults the healthy range is 18.5 to 24.9. Some groups, including many people of Asian descent, may face higher risk at lower values, so ask your doctor what range suits you.
Can I use BMI for my child?
Not with adult categories. Children's BMI is plotted as a percentile for their age and sex on growth charts, and a pediatrician should interpret it.
How often should I check my BMI?
There is no need to check often. Every few months is plenty, or when your weight changes noticeably. Day-to-day weight swings are mostly water and do not matter.