What Your BMI Means, and What It Doesn't
BMI is your weight in kilograms divided by your height in metres squared, and for adults a result between 18.5 and 24.9 is classed as a healthy weight. It is a quick screening number, not a diagnosis: it can't tell muscle from fat or show where fat sits, so it misjudges some people. Used with one more measurement, your waist, it gives a reasonable first picture. This article explains the ranges, who BMI misleads, and what to look at alongside it.
This article is general information, not medical advice. If your weight worries you, talk to a doctor.
How is BMI calculated?
BMI = weight (kg) ÷ height (m)²
A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In pounds and inches the formula is 703 × weight (lb) ÷ height (in)², which gives the same answer.
BMI Calculator does this for you in either unit and shows the category, the healthy weight range for your height, and a few related estimates explained below.
What do the BMI ranges mean?
For adults, the World Health Organization uses these categories:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obesity, class I |
| 35 – 39.9 | Obesity, class II |
| 40 and above | Obesity, class III |
The ranges come from large population studies: across many people, health risks such as type 2 diabetes and heart disease rise as BMI moves above the healthy range, and some risks also rise below it. They describe risk across a population, not certainty for one person.
For people of Asian background, a WHO expert group suggested lower action points, because the same risks appear at lower BMIs: increased risk from about 23 and high risk from about 27.5. Some countries use these lower thresholds in their own guidance.
Children and teenagers are different: their BMI is compared with others of the same age and sex using growth charts (percentiles), and the adult table doesn't apply.
Who does BMI mislead?
BMI only knows weight and height, so it gets some people wrong:
- Muscular people. Muscle is dense. A rugby player or regular weightlifter can have a BMI over 25 with little body fat.
- Older adults. With age, muscle is often replaced by fat at the same weight, so a "healthy" BMI can hide more fat than it suggests.
- People with fat around the middle. Fat stored around the organs is linked with more health risk than fat on the hips and thighs, and BMI can't see where fat is.
- Pregnancy. BMI isn't meaningful during pregnancy; midwives and doctors use the pre-pregnancy BMI instead.
- Very tall or very short people, for whom the formula slightly over- or under-estimates.
What should I check alongside BMI?
Your waist
Measure around your middle at about the level of your navel, standing relaxed, after breathing out. Then divide by your height in the same units: that is your waist-to-height ratio. A widely used guide, for example in UK health guidance, is to keep your waist to less than half your height, a ratio under 0.5.
It catches what BMI misses: someone with a "healthy" BMI but a ratio of 0.55 may carry more fat around the middle than their BMI suggests. Enter your waist in BMI Calculator and it shows the ratio next to your BMI.
Change over time
One reading tells you little; the direction matters more. Measure at the same time of day, in similar clothes, every few weeks rather than every day: daily weight moves by a kilogram or more with water and food alone.
What else does the calculator estimate?
Beyond BMI, the calculator gives several estimates. They are formulas applied to your numbers, not measurements, so treat them as starting points:
| Estimate | What it is | Based on |
|---|---|---|
| BMR | Calories your body uses at complete rest | Mifflin–St Jeor equation (weight, height, age, sex) |
| Daily energy (TDEE) | BMR adjusted for five activity levels | Standard activity multipliers |
| Body fat % | An estimate, not a measurement | Deurenberg formula (BMI, age, sex) |
| Ideal weight | A single reference weight | Devine formula, originally made for drug dosing |
| Healthy weight range | The weights that give a BMI of 18.5–24.9 at your height | The BMI table above |
The healthy weight range is usually more useful than the single "ideal weight": many weights within it are fine.
What if my BMI is outside the healthy range?
A number outside the range is a reason to look more closely, not a verdict:
- If it's high, check your waist-to-height ratio too. If both are high, it's worth discussing with a doctor, who can look at blood pressure, blood sugar and cholesterol, which matter more than weight alone.
- If it's low, especially with unplanned weight loss, tiredness or loss of appetite, see a doctor.
- If you are muscular and your waist ratio is under 0.5, a BMI over 25 is unlikely to mean what the table says.
Lasting change usually comes from small habits kept up for months: regular meals with plenty of vegetables, fewer sugary drinks, more daily walking, and enough sleep. A doctor or registered dietitian can tailor that to you, particularly if you have a health condition.
Quick answers
Is BMI accurate? For groups of people, it's a good indicator of risk. For one person it's a rough screen that can be wrong for muscular, older or pregnant people.
What's a healthy BMI for women and men? The adult ranges are the same for both: 18.5 to 24.9. Women tend to carry more body fat at the same BMI, which is one reason BMI alone is limited.
Is waist size better than BMI? It adds what BMI lacks: where fat is. The two together are better than either alone.
Can I use the calculator for children? No. Children need age- and sex-specific growth charts; ask your child's doctor or school nurse.
What to do next
Work out your BMI and waist-to-height ratio with BMI Calculator. To convert between kilograms and pounds or centimetres and inches, use Unit Converter.