What is BMI (body mass index)?
Body mass index, or BMI, is a quick way to check whether your weight sits in a healthy range for your height. You divide your weight by your height squared and compare the result against standard ranges published by the World Health Organization. Doctors and health agencies like the CDC have leaned on it for decades for a simple reason: it costs nothing and takes seconds.
Before you read too much into your score, it helps to know what BMI actually measures. It doesn't measure body fat. It can't see how much of your weight is muscle, and it has no idea where your body stores fat. Think of it the way your doctor does: a first check that tells you whether anything is worth a closer look.
This BMI calculator uses the WHO adult ranges, which apply from age 20 upward. Kids and teens are scored on growth percentiles instead, so this isn't the right tool for them.
How to calculate BMI: the formula in metric and imperial
The math behind BMI is short enough to do on the back of an envelope: weight divided by height squared. The imperial version multiplies by 703 to account for pounds and inches, but it lands on the same number.
Metric formula
BMI = kg ÷ m²
Say you weigh 70 kg at 175 cm tall. Square the height (1.75 × 1.75 = 3.06), divide, and you get 22.9. That sits comfortably in the healthy range.
Imperial formula
BMI = 703 × lbs ÷ in²
At 154 lbs and 5 ft 10 in (70 inches): square the height to get 4,900, then 703 × 154 ÷ 4,900 = 22.1. Also a healthy result.
No need to work any of this out by hand, of course. The calculator up top takes either unit system and switches between them on the spot.
BMI chart for adults (WHO categories)
Here are the ranges the WHO uses for adults. The cut-offs don't change with sex or age; the same table applies to every adult from 20 up.
| Category | BMI range (kg/m²) |
|---|---|
| Severe thinness | < 16.0 |
| Moderate thinness | 16.0 – 16.9 |
| Mild thinness | 17.0 – 18.4 |
| Normal (healthy) weight | 18.5 – 24.9 |
| Overweight (pre-obesity) | 25.0 – 29.9 |
| Obesity, class I | 30.0 – 34.9 |
| Obesity, class II | 35.0 – 39.9 |
| Obesity, class III | ≥ 40.0 |
One caveat worth knowing: a WHO expert group found that in many Asian populations, health risks start climbing from a BMI of about 23 rather than 25. If that applies to you, treat the upper part of the healthy band with some caution and bring it up with your doctor.
Healthy weight range by height
Because the formula only uses two numbers, you can run it backwards: take the healthy BMI range of 18.5 to 24.9 and work out what weight that means at your height. The calculator does this automatically for your exact height. The table below shows how it comes out at a few common ones.
| Height | Healthy weight (kg) | Healthy weight (lbs) |
|---|---|---|
| 152 cm (5′0″) | 42.7 – 57.5 | 94.7 – 127.5 |
| 160 cm (5′3″) | 47.4 – 63.7 | 104.5 – 140.6 |
| 168 cm (5′6″) | 52.2 – 70.3 | 114.6 – 154.3 |
| 175 cm (5′9″) | 56.7 – 76.3 | 125.3 – 168.6 |
| 183 cm (6′0″) | 61.9 – 83.4 | 136.4 – 183.6 |
| 191 cm (6′3″) | 67.5 – 90.8 | 148.0 – 199.2 |
A note on the numbers: the heights are rounded pairs, and the kg column is worked out from centimetres while the lbs column comes from inches, so the two can disagree by a hair.
BMI Prime and Ponderal Index
Your results panel shows two extra numbers, and they deserve a quick explanation.
BMI Prime is simply your BMI divided by 25, the top of the healthy range. It tells you at a glance how close to the line you are: under 1.0 means you're below the overweight threshold, over 1.0 means you're past it. A BMI of 22.9 works out to a BMI Prime of 0.92, so about 8% of headroom.
The Ponderal Index divides weight by height cubed instead of squared. That small change matters at the extremes, because standard BMI tends to read very tall people as heavier than they really are and very short people as lighter. At 70 kg and 1.75 m the Ponderal Index comes out at 13.1, and most healthy adults land somewhere between 11 and 15.
Limitations of BMI: what it can't tell you
BMI became the standard because it's easy, not because it's precise. Before you take your score too seriously, here's what it misses:
- It can't tell muscle from fat. Rugby players, lifters, and plenty of gym regulars score as overweight while carrying very little fat.
- It doesn't know where your fat sits. Fat packed around the organs in your belly does far more damage than fat on your hips, which is why doctors often measure waist circumference alongside BMI.
- Age changes what the number means. People lose muscle as they get older, so a 70-year-old and a 30-year-old with the same normal BMI can have quite different amounts of body fat.
- It has no place in pregnancy, and children need percentile charts rather than the adult table.
- The risk thresholds shift between populations, as the note on Asian cut-offs above shows.
If you recognise yourself anywhere in that list, don't lean on BMI alone. Check your body fat percentage or your lean body mass as well; together they say far more than any single number.
Health risks of a high or low BMI
What a high BMI (25 and above) is linked with:
- Type 2 diabetes and insulin resistance
- High blood pressure and unhealthy cholesterol levels
- Coronary heart disease and stroke
- Several cancers, including breast, colon, and kidney
- Sleep apnea and breathing problems
- Osteoarthritis and joint pain
What a low BMI (under 18.5) is linked with:
- Malnutrition and vitamin deficiencies
- Weakened immune function
- Osteoporosis and higher fracture risk
- Anemia and fatigue
- Fertility problems
- Slower recovery from illness or surgery
Two honest caveats here. These are patterns found across millions of people, not predictions about you personally. And BMI is only a rough stand-in for the body fat that actually drives the risk. Still, the links are strong enough that a score outside 18.5 to 24.9 is worth mentioning at your next check-up.
How to reach a healthy BMI
If your result wasn't what you hoped, the worst possible response is a crash diet. Weight lost fast has a habit of coming straight back. The CDC suggests losing no more than 1–2 lbs (roughly 0.5–1 kg) a week, and the WHO's baseline for adults is at least 150 minutes of moderate exercise spread across the week. Slow and a little boring, but it's the approach that actually holds up in long-term studies.
The most useful first step is finding out how much energy your body burns in a day. These tools will get you there:
Frequently asked questions about BMI
What is a healthy BMI for adults?
For adults aged 20 and over, WHO classifies a BMI of 18.5 to under 25 as normal weight. Below 18.5 is underweight; 25 to under 30 is overweight; 30 and above is obese.
What is the BMI formula?
BMI equals weight in kilograms divided by height in metres squared (kg/m²). In imperial units, it is weight in pounds divided by height in inches squared, multiplied by 703. Both versions give the same result.
Is BMI reliable for everyone?
BMI is a useful rough check, but it cannot tell muscle from fat. Athletes and very muscular people sometimes score overweight even with low body fat. Treat it as a starting point, not a diagnosis.
Is the BMI calculation different for men and women?
No. For adults aged 20 and over, the formula and the WHO categories are the same for men and women. Women typically carry more body fat than men at the same BMI, which is one reason BMI works best alongside other measures such as waist circumference.
How is BMI different from body fat percentage?
BMI is a height-to-weight ratio. Body fat percentage measures how much of your weight is fat. Two people with the same BMI can have very different body composition — which is why athletes may prefer a body fat calculator.
Can I use this calculator for children?
No. This tool uses adult WHO cut-offs only. Children and teens need age- and sex-specific BMI percentiles — ask a pediatrician or use a dedicated child BMI chart.
When should I talk to a doctor about my BMI?
See a healthcare provider if your BMI is outside the normal range, you have lost weight without trying, you are planning major diet or exercise changes, or you want personalised advice. Online tools cannot replace a clinical assessment.
How do I use this BMI calculator?
Enter your height and weight on the left, pick metric or imperial units, and click Calculate BMI. Your score and category show up on the right straight away.
Can I use kilograms or pounds?
Yes — switch between metric (kg and cm) and imperial (lbs and feet/inches) before you calculate. The BMI number comes out the same either way.
Sources and further reading
- World Health Organization — Obesity and overweight fact sheet
- CDC — About Body Mass Index (BMI)
- NIH National Heart, Lung, and Blood Institute — Calculate Your BMI
- WHO Expert Consultation — Appropriate BMI for Asian populations (The Lancet, 2004)
This page is for general information only and is not medical advice. Always consult a qualified healthcare provider about your weight and health.