Body Mass Index (BMI) is a single number that compares your weight to your height. It was designed as a quick population-level screening tool, and that is exactly how it is best used: as a starting point, not a diagnosis. Understanding what BMI does and does not tell you keeps you from reading too much — or too little — into one figure.
How BMI is calculated
The formula is simply your weight divided by your height squared:
If you think in pounds and inches, the same idea becomes:
For example, someone who weighs 70 kg and stands 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. Because height is squared, taller people need proportionally more weight to reach the same BMI — which is part of why the measure works reasonably well across a whole population but can mislead for individuals at the extremes of height.
The BMI categories
For adults, the World Health Organization uses these cut-offs (they are the same for men and women):
| Category | BMI range | What it suggests |
|---|---|---|
| Underweight | Below 18.5 | May signal under-nutrition or an underlying issue |
| Healthy weight | 18.5 – 24.9 | Lowest average health risk for most adults |
| Overweight | 25.0 – 29.9 | Slightly raised risk; context matters |
| Obese | 30.0 and above | Higher average risk of several conditions |
These boundaries are based on large studies linking BMI to average health outcomes across groups of people. They describe statistical risk for a population, not certainty for any one person.
Working out your healthy weight range
You can turn the categories around to find the weight range that would put you in the healthy band. Multiply the lower and upper healthy cut-offs by your height squared:
At 1.75 m, that is 18.5 × 3.06 = about 57 kg at the bottom and 24.9 × 3.06 = about 76 kg at the top — roughly 125 to 168 lb. The BMI calculator shows this range for your own height automatically, so you do not have to do the arithmetic.
Where BMI falls short
BMI is popular because it needs only two easy measurements. That simplicity is also its weakness — it knows nothing about what your weight is made of or where it sits on your body.
- It can't tell muscle from fat. Muscle is denser than fat, so a lean, muscular athlete can land in the "overweight" range despite low body fat. Bodybuilders and many strength athletes are the classic example.
- It ignores fat distribution. Fat carried around the abdomen carries more health risk than fat on the hips and thighs, but BMI treats them identically. A waist measurement adds useful context.
- It shifts with age. Older adults tend to lose muscle and gain fat at the same weight, so a "healthy" BMI later in life can hide a higher body-fat percentage.
- Population differences exist. Some health bodies use a lower overweight threshold (around 23) for people of South and East Asian descent, who can face higher risk at lower BMIs.
- It does not apply to children the same way. Children and teenagers are assessed with age- and sex-specific percentile charts, not the fixed adult bands above.
How to use BMI sensibly
Treat BMI as one input among several. If your number sits comfortably in the healthy range and you feel well, it is reassuring. If it sits near or beyond a boundary, that is a prompt to look at the fuller picture — waist circumference, activity levels, diet, family history — and, when it matters, to talk to a healthcare professional rather than relying on the number alone. Trends over time are usually more telling than a single snapshot: a steadily rising BMI is worth more attention than one reading slightly above a cut-off.
BMI and your calorie needs
BMI tells you where you are; it doesn't tell you how to change. If you want to lose, maintain or gain weight, the next step is understanding how many calories your body uses each day. That number — your Total Daily Energy Expenditure — depends on your size, age, sex and activity level. Our guide to TDEE and calorie needs explains how it is worked out, and the TDEE calculator estimates it for you.
Try it yourself
Enter your height and weight in the free BMI calculator to see your BMI, your category and the healthy weight range for your height, in either metric or imperial units. Then pair it with the TDEE calculator to plan around your daily calorie needs.
Frequently asked questions
- What is a healthy BMI range?
- For most adults, 18.5 to 24.9 is classed as healthy. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. The ranges are the same for men and women.
- How do you calculate BMI?
- Divide your weight in kilograms by your height in metres squared, or multiply 703 by your weight in pounds and divide by your height in inches squared. For example, 70 kg at 1.75 m is 22.9.
- Is BMI accurate for athletes?
- Not reliably. BMI can't tell muscle from fat, so a muscular athlete may read as "overweight" while carrying little body fat. Waist measurement and body-fat testing are more useful for very muscular people.
- What weight do I need to reach a healthy BMI?
- Multiply 18.5 and 24.9 by your height in metres squared for the bottom and top of your healthy range. At 1.75 m that's roughly 57 to 76 kg. The BMI calculator works this out for you.
This guide is for educational purposes only and does not provide medical advice. BMI is a screening tool, not a diagnosis — consult a qualified healthcare professional about your health.