What is a healthy BMI?

For most adults, a BMI between 18.5 and 24.9 is classified as the normal-weight range by the World Health Organization. Values below 18.5 are classed as underweight, 25 to 29.9 as overweight, and 30 or above as obesity.

Is BMI accurate for athletes?

Often not. BMI cannot distinguish muscle from fat, so muscular people frequently score as overweight despite low body fat. If you train regularly, treat BMI as one data point and pair it with waist measurements, body-fat estimates, or progress photos.

Is BMI different for men and women?

The standard adult BMI formula and categories are the same for men and women. Women typically carry more body fat at the same BMI, which is one of the known limitations of using a single scale for everyone.

Does BMI apply to children and teenagers?

Not in this form. For anyone under 18, BMI is interpreted using age- and sex-specific percentile charts rather than the fixed adult cut-offs. This calculator is intended for adults.

How often should I recalculate my BMI?

BMI only changes when your weight changes, so checking every few weeks during an active weight-loss or muscle-gain phase is plenty. Day-to-day weight fluctuations are mostly water and food in transit, not fat.

BMI Calculator (Metric & Imperial)

Enter your height and weight, in metric or imperial units, to get your body mass index and where it falls on the WHO scale.

What is BMI?

Body mass index (BMI) is a simple screening number that relates your weight to your height. It was designed for studying populations, not diagnosing individuals, but it remains the most widely used first check for weight-related health risk because it requires nothing more than a scale and a tape measure. Health bodies such as the World Health Organization use BMI cut-offs to define underweight, normal weight, overweight, and obesity in adults.

BMI is best understood as a starting point. A result outside the normal range does not mean something is wrong, and a result inside it does not guarantee good health. It tells you whether your weight is unusual for your height — nothing more, nothing less. What you do with that information matters far more than the number itself.

How BMI is calculated

BMI is your weight divided by the square of your height. The same formula is expressed in both unit systems:

Metric

BMI = weight (kg) ÷ height (m)²

Imperial

BMI = 703 × weight (lb) ÷ height (in)²

For example, someone who is 175 cm tall and weighs 70 kg has a BMI of 70 ÷ 1.75², which works out to 22.9 — inside the normal-weight range. The calculator above does the same arithmetic instantly and shows where the result lands on the scale.

BMI categories for adults

The World Health Organization defines the following BMI ranges for adults aged 18 and over. The same cut-offs apply to men and women.

BMI range	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 health authorities use lower overweight thresholds for people of South and East Asian descent, because health risks can rise at lower BMI values in those populations. If that applies to you, discuss the right cut-offs with your doctor.

What BMI cannot tell you

BMI has well-documented blind spots, and knowing them is the difference between using the number sensibly and being misled by it:

It cannot separate muscle from fat. Strength athletes and very muscular people routinely register as overweight or even obese while carrying little body fat.
It ignores where fat is stored. Fat around the organs (visceral fat) carries more health risk than fat under the skin, and two people with identical BMIs can differ enormously here. Waist circumference is a useful companion measurement.
It flattens age differences. Older adults tend to have more body fat than younger adults at the same BMI, and modest extra weight may even be protective late in life.
It says nothing about behavior. Diet quality, training, sleep, and activity level all predict health independently of BMI.

Using BMI as part of a bigger picture

The most useful thing you can do with a BMI result is treat it as a trend, not a verdict. A single reading reflects one moment; the direction it moves over weeks and months reflects what you are actually doing. If your goal is to move your BMI in either direction, the levers are unglamorous and reliable: a consistent calorie intake matched to your goal, enough protein to protect muscle, and regular training.

That is the part ActiveDay is built for. Log meals with the AI food camera or barcode scanner, let workouts sync in from Apple Health, and watch weekly trends instead of single weigh-ins. You can also browse the food database to check the calories and macros of what you eat, or read more on the ActiveDay home page.

Track the trend, not the snapshot.

ActiveDay logs your food and workouts on iPhone, so your BMI moves in the direction you choose.

Frequently asked questions

What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is classified as the normal-weight range by the World Health Organization. Values below 18.5 are classed as underweight, 25 to 29.9 as overweight, and 30 or above as obesity.
Is BMI accurate for athletes?
Often not. BMI cannot distinguish muscle from fat, so muscular people frequently score as overweight despite low body fat. If you train regularly, treat BMI as one data point and pair it with waist measurements, body-fat estimates, or progress photos.
Is BMI different for men and women?
The standard adult BMI formula and categories are the same for men and women. Women typically carry more body fat at the same BMI, which is one of the known limitations of using a single scale for everyone.
Does BMI apply to children and teenagers?
Not in this form. For anyone under 18, BMI is interpreted using age- and sex-specific percentile charts rather than the fixed adult cut-offs. This calculator is intended for adults.
How often should I recalculate my BMI?
BMI only changes when your weight changes, so checking every few weeks during an active weight-loss or muscle-gain phase is plenty. Day-to-day weight fluctuations are mostly water and food in transit, not fat.

BMI Calculator