What Your BMI Tells You (and What It Misses)

Body Mass Index (BMI) is your weight in kilograms divided by your height in meters squared. For adults, the WHO classes a BMI under 18.5 as underweight, 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight and 30 or above as obesity. It is a fast, free screening number that works reasonably well across large groups of people, but it cannot see muscle, fat distribution or fitness, so it can mislabel individuals.

This guide covers how the number is built, how to read it, and the situations where you should take it with a large grain of salt. The result is an estimate for general information, not a medical diagnosis or professional advice.

How is BMI calculated?

BMI = weight (kg) ÷ height (m)²

For someone who weighs 70 kg and is 175 cm tall:

1. Convert height to meters: 1.75 m.
2. Square it: 1.75 × 1.75 = 3.0625.
3. Divide weight by that: 70 ÷ 3.0625 = 22.9.

In imperial units, convert first. A person who weighs 154 lb and stands 5 ft 9 in is 69.85 kg and 1.7526 m tall, giving a BMI of 22.7. (One pound is exactly 0.45359237 kg, and one inch is exactly 2.54 cm.)

Because height is squared, BMI is very sensitive to height errors. Entering 170 cm instead of 175 cm for the same 70 kg moves the result from 22.9 to 24.2.

What do the BMI categories mean?

BMI	WHO category (adults)
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25 – 29.9	Overweight
30 and above	Obesity

These cut-offs come from population studies linking BMI ranges with health risks on average. They are the same for men and women, and they apply to adults only. Children and teenagers are assessed with age- and sex-specific growth charts instead.

What does the healthy weight range tell me?

Flip the formula around and you get the weight range that corresponds to a BMI of 18.5 to 24.9 at your height. For 175 cm, that is 56.7 to 76.3 kg. It is a wide band on purpose: build, frame and body composition vary a lot between people of the same height.

When is BMI misleading?

BMI only knows two things about you, weight and height. That leaves several blind spots.

• Muscular builds. A 188 cm, 95 kg rugby player gets a BMI of 26.9, "overweight", even with low body fat. Muscle is denser than fat, and BMI can't tell them apart.
• Older adults. People tend to lose muscle with age, so a "healthy" BMI can hide a high fat percentage.
• Fat distribution. Fat around the waist is linked to higher health risk than fat on the hips and thighs. Two people with the same BMI can have very different risk. Waist circumference is often used alongside BMI for this reason.
• Ethnicity. Some health bodies use lower thresholds for people of Asian descent, because health risks appear at lower BMIs in those populations.
• Pregnancy. BMI is not meaningful during pregnancy.

So treat BMI as a first look. If it lands outside the healthy range, or you are unsure what it means for you, a doctor can put it in context with other measurements.

How to use the BMI Calculator

The BMI Calculator works in both unit systems:

1. Pick Metric (kg, cm) or Imperial (lb, ft/in) in the Units menu.
2. Enter your Weight, then your Height: centimeters in metric, or feet and inches in separate fields in imperial.
3. Read your BMI (rounded to one decimal place), the Category (WHO, adults), and the Healthy weight range for this height. The range is shown in kilograms in both modes; the Weight Converter will turn it into pounds or stone.

The calculation runs in your browser and nothing you type is stored. For a fuller picture of your daily energy needs, the BMR Calculator estimates calories burned at rest and with activity.

Tips for a more reliable reading

• Weigh yourself at the same time of day, ideally in the morning, since weight can swing by a kilogram or more over a day.
• Measure height without shoes, standing straight against a wall.
• Track the trend over weeks rather than reacting to a single reading.

FAQ

Is a BMI of exactly 25 overweight?

Yes. The healthy range ends just below 25, so 25.0 falls in the overweight category. A result of 24.9 is still classed as a healthy weight.

Why does my BMI change when I only gained a little weight?

For an average adult height, each kilogram changes BMI by roughly a third of a point. At 175 cm, one kilogram is 1 ÷ 3.0625 ≈ 0.33 BMI points.

Should children use this calculator?

No. The categories here are for adults. For anyone under 20, BMI has to be compared with age- and sex-specific percentiles, which a pediatrician can do.

Check your number in either unit system with the BMI Calculator.