BMI Calculator
BMI (Body Mass Index) is a quick screening tool to assess whether your weight falls within a healthy range for your height. Free and instant.
Built and reviewed by Jesse · Last reviewed
Fill in your details to see your BMI.
- Enter your height. In centimeters, or feet and inches.
- Enter your weight. In kilograms or pounds.
- Read your BMI and category. The result places you in the WHO weight category for adults.
BMI is calculated using a simple formula:
BMI = weight (kg) / [height (m)]²
For example, if you weigh 70 kg and are 1.75 m tall (175 cm), your BMI is 70 / (1.75 × 1.75) = 22.9, which falls in the "normal weight" category.
Want a goal weight instead? The ideal weight calculator works in reverse to show a healthy weight range for your height.
The World Health Organization classifies adult BMI as follows:
| Category | BMI range (kg/m²) | Notes |
|---|---|---|
| Underweight | Below 18.5 | May indicate malnutrition or underlying health issues. |
| Normal weight | 18.5–24.9 | Lowest health risk in population studies. |
| Overweight | 25.0–29.9 | Elevated risk of metabolic disease. |
| Obese | 30.0 and above | Higher risk of chronic disease; medical guidance advised. |
BMI is reported to one decimal place, which makes it look more precise than the measurements behind it. Height is squared in the formula, so an error in your height counts twice over. Taking a 70 kg adult at 175 cm — a BMI of 22.9 — here is what ordinary measurement slips do:
| Measurement slip | Change in BMI | What it means |
|---|---|---|
| Weighing in clothes and shoes (about 1 kg) | +0.33 | Reads high. Weigh in light clothing, or subtract an allowance. |
| A 2 kg swing across a week (water, food volume, glycogen) | +/- 0.65 | Moves either way at random. Use a weekly average if you are near a boundary. |
| Height recorded 2 cm short (shoes off vs on, or slouching) | +0.53 | Reads high, and by more than a kilogram of clothing does. |
| Height rounded down to the nearest 5 cm | +1.36 | Reads high enough on its own to change your category near a cut-off. |
| Both together: 1 kg of clothing and 2 cm of height | +0.87 | Nearly a full BMI point — enough to cross a boundary if you are near one. |
Computed with the same formula this calculator uses. The direction of each error is what matters more than its exact size.
The practical consequence: treat your BMI as accurate to about half a point. The WHO category boundaries are hard lines at 18.5 and 25, so if you land within half a point of one, you are on the boundary rather than inside a category. Measure your height once, carefully, without shoes, and reuse that figure; weigh at the same time of day in the same clothing.
Because height is fixed in the short term, one BMI point is a fixed number of kilograms for you — your height in metres, squared. That makes it easy to translate a BMI target into a weight target:
| Height | 1 BMI point | Width of the normal band (18.5 to 25) | Weight at BMI 25 |
|---|---|---|---|
| 155 cm | 2.4 kg | 15.6 kg | 60.1 kg |
| 160 cm | 2.6 kg | 16.6 kg | 64.0 kg |
| 165 cm | 2.7 kg | 17.7 kg | 68.1 kg |
| 170 cm | 2.9 kg | 18.8 kg | 72.3 kg |
| 175 cm | 3.1 kg | 19.9 kg | 76.6 kg |
| 180 cm | 3.2 kg | 21.1 kg | 81.0 kg |
| 185 cm | 3.4 kg | 22.2 kg | 85.6 kg |
| 190 cm | 3.6 kg | 23.5 kg | 90.3 kg |
Derived by solving BMI = weight / height squared for weight. The full table for every height and weight is on the BMI chart page.
Two things follow. A shorter adult crosses BMI boundaries on less weight change than a taller one — at 155 cm a single point is 2.4 kg, at 190 cm it is 3.6kg. And the normal band is wide: at most adult heights there is 15 to 20 kg between its two edges, so "normal weight" covers a much broader range of bodies than the single label suggests.
For the full grid of heights and weights, see the BMI chart by height and weight.
This is the most common technical objection to BMI: body mass is three-dimensional and height is one, so dividing by height squared is said to flatter short people and penalize tall ones. The objection has been tested directly.
Heymsfield and colleagues fitted the actual relationship between weight and height in the US and Korean national health surveys across eight sex and ethnic groups. Once age and adiposity were controlled for, the measured scaling power was about 2.12 in men and 2.02 in women, with no significant differences between ethnic groups. Their conclusion was that the squared denominator makes BMI a generalizable, height-independent measure of body shape across most populations.
So the exponent is not BMI's weak point. Its weak point is that shape is not composition: the formula has no way to tell muscle from fat, or belly fat from hip fat. That limitation is the subject of our guide on what BMI does and doesn't tell you, and we quantified it by finding bodies where BMI and a body-fat estimate reach opposite verdicts.
BMI is a useful screening tool, but it has important limitations:
- Doesn't measure body fat. BMI treats all weight equally. Two people with the same BMI can have very different body compositions — one might be muscular, the other high body fat. A body fat calculator gives a clearer picture.
- Not suitable for athletes.People with high muscle mass often have "overweight" or "obese" BMI despite being healthy and fit.
- Ignores fat distribution.Belly fat (visceral fat) is more harmful than fat stored elsewhere, but BMI doesn't account for this. Waist circumference is a better predictor of metabolic risk.
- Population averages. BMI cutoffs are based on population studies, not individuals. Your health risk depends on many factors beyond BMI: genetics, diet, activity, sleep, stress, and more.
BMI is best used as a quick health screening tool for the general population. It can help identify whether further assessment is needed, but it should not be the only metric you rely on. For a fuller picture, combine BMI with waist circumference, body fat percentage, blood markers (cholesterol, glucose), and how you feel physically.
If your BMI is outside the normal range and you're unsure what to do, consult a doctor or registered dietitian. They can assess your overall health and recommend personalized next steps.
This calculator implements the WHO adult classification, which is defined for non-pregnant adults aged 18 and over. It will return a number for any input, but the category label is not meaningful for everyone:
- Children and teenagers under 18. BMI is interpreted against age- and sex-specific growth percentiles, not fixed cut-offs. Use a pediatric BMI-for-age chart from a health authority instead.
- Pregnancy and breastfeeding. Weight gain is expected and monitored against pregnancy-specific guidance. Pre-pregnancy BMI is the figure clinicians use.
- Heavily trained athletes. Muscle is denser than fat, so a lean, muscular body can read as overweight. A body fat estimate is the more informative number.
- Older adults. Height is lost with age and muscle mass falls, so the same BMI can mean something different at 75 than at 35. Some guidance uses higher cut-offs for over-65s.
- Limb loss, significant edema, or large-volume ascites. Total body weight no longer reflects body composition, so BMI is not interpretable without clinical adjustment.
For adults outside these groups, BMI is a reasonable first screen — and the fastest one, since it needs nothing but a scale and a tape on the wall.
Body Mass Index
BMI = weight(kg) / height(m)^2- weight
- = body weight in kilograms
- height
- = height in meters
WHO adult categories: under 18.5 = underweight, 18.5-24.9 = normal, 25.0-29.9 = overweight, 30.0 and above = obese.
What is BMI?
BMI (Body Mass Index) is a simple calculation using your height and weight (BMI = kg/m²). It's used as a screening tool to categorize whether someone is underweight, normal weight, overweight, or obese.
How accurate is BMI?
BMI is a quick population-level screening tool, but it has limitations. It doesn't distinguish between muscle and fat, so athletes or very muscular people may be categorized as overweight despite being healthy. It also doesn't account for age, sex, or body composition. Use it as a starting point, not a definitive health assessment.
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered normal weight. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is classified as obese. These ranges are based on population studies linking BMI to health risk.
Does BMI apply to everyone?
No. BMI is not suitable for children, pregnant women, elderly adults, or highly trained athletes. It's designed for the general adult population. If you have significant muscle mass or a medical condition, consult a healthcare provider instead of relying on BMI alone.
Why does BMI matter?
Higher BMI is statistically associated with increased risk of type 2 diabetes, heart disease, high blood pressure, and certain cancers. Lower BMI (underweight) is associated with nutrient deficiencies and weakened immunity. BMI helps identify risk, but it's not a direct measure of health.
What if my BMI is too high or too low?
If your BMI is outside the normal range, consider lifestyle changes (diet, exercise) and consult a doctor or registered dietitian. Small, sustainable changes are more effective than extreme diets. BMI is a signal to investigate further, not a diagnosis.
Can I have a high BMI but still be healthy?
Yes. If you carry significant muscle mass (e.g., from strength training), your BMI may categorize you as overweight even though your body fat is low. In this case, other metrics like body fat percentage, waist circumference, or metabolic health markers are more informative than BMI alone.
How much weight do I need to lose to change BMI category?
It depends only on your height: one BMI point equals your height in metres, squared, expressed in kilograms. At 175 cm, one BMI point is 3.1 kg, so the whole normal band spans about 19.9 kg and the overweight threshold sits at 76.6 kg. Shorter adults need less weight change to move a point, taller adults need more.
How accurate does my height need to be?
More accurate than most people assume. At 175 cm and 70 kg, recording your height 2 cm short raises your BMI by 0.53, which is more than a kilogram of clothing does. Height is squared in the formula, so a small error in it counts twice over. Measure without shoes, standing straight, and reuse the same figure each time.
Is dividing by height squared actually correct?
Better than its critics suggest. Body weight is assumed to scale with the square of height, and a 2014 analysis of the US and Korean national health surveys measured the real scaling power at about 2.12 in men and 2.02 in women, with no significant differences between the ethnic groups studied once age and adiposity were accounted for. The authors concluded BMI works as a height-independent measure of body shape across most populations. BMI's real weakness is not the exponent, it is that shape is not composition.
Should I use the decimal place?
Not for decisions. Your recorded weight and height carry enough uncertainty to move BMI by several tenths of a point, so treat the number as being accurate to about half a point. If you land within half a point of a category boundary, the honest reading is that you are on the boundary.
How often should I check my BMI?
Monthly at most, unless you are actively changing your weight. BMI only moves when your weight does, and weekly checks mostly show water fluctuation. If you are in a deficit or a surplus, tracking body weight itself on a 7-day average is more informative than recalculating BMI.
References
- World Health Organization. Body mass index (BMI) classification for adults.
- Heymsfield SB, Peterson CM, Thomas DM, et al. Scaling of adult body weight to height across sex and race/ethnic groups: relevance to BMI. Am J Clin Nutr. 2014;100(6):1455-1461.
FitCalcs calculators provide general estimates for healthy adults and are not medical advice.
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