BMI Calculator — Body Mass Index (Metric & Imperial)
Calculate your BMI with metric and imperial units. Learn what it measures, its key limitations, and why it's a screening tool, not a diagnosis.
BMI Calculator
What your BMI actually says
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How the BMI Formula Works
BMI is calculated by dividing weight in kilograms by height in metres squared. The formula is simple: BMI = weight (kg) / height (m)². For a worked example, a person weighing 70 kg at 1.75 m tall would calculate as follows: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. This falls into the "Normal" category.
The standard adult categories are:
- Below 18.5: Underweight
- 18.5 to 24.9: Normal weight
- 25 to 29.9: Overweight
- 30 and above: Obese
These thresholds are based on population-level data from the World Health Organization. The same formula applies whether you use metric or imperial units; the calculator converts automatically.
What BMI Cannot See
BMI is a ratio of two measurements—weight and height—and nothing else. It cannot distinguish muscle from fat. A muscular athlete who trains regularly may land in the "overweight" or even "obese" category while carrying very little body fat, because muscle tissue weighs more than fat tissue at the same volume. Conversely, someone with high body fat but low muscle mass may fall into the "normal" category despite having poor body composition.
BMI also says nothing about fat distribution. Where your body stores fat matters for health risk. Fat accumulated around the abdomen carries different health implications than the same amount of weight distributed elsewhere. BMI cannot account for this difference.
When Standard Categories Don't Apply
The adult BMI bands were designed for comparing populations, not diagnosing individuals. Several groups fall outside their scope:
- Children and teenagers are assessed using age- and sex-specific percentile charts, not the adult cutoffs.
- Pregnant people experience temporary weight gain that BMI treats as static.
- Older adults often have reduced muscle mass; the same BMI may carry different meaning as they age.
- People of certain ancestries may have different risk thresholds for the same BMI value, a factor not captured by the single standard scale.
BMI as a Screening Indicator
BMI is a screening indicator—a starting point for conversation—not a diagnosis. Whether a particular BMI warrants action depends on the whole picture: body composition, fitness level, family history, blood pressure, cholesterol, and more. Decisions about your health belong with a clinician who can look at all these factors together.
For related calculations, see our TDEE calculator for energy expenditure and macro calculator for nutrition planning.
How to use the BMI Calculator
Takes about a minute. No signup, no download, your data stays in your browser.
- 1Open the tool. Scroll up to the BMI Calculator above — it loads instantly in your browser, no install needed.
- 2Enter your values. The fields come pre-filled with realistic defaults so you can see how it works — replace them with your own numbers.
- 3Read the result. The output updates instantly. Copy or share it — nothing is uploaded to a server, everything stays on your device.
Frequently asked questions
Common questions about the BMI Calculator.
Is BMI accurate for athletes?
BMI cannot distinguish muscle from fat. Athletes with significant muscle mass often score higher than their actual body fat percentage warrants. A muscular person may appear overweight by BMI while having low body fat. This is why body composition assessment matters more than the BMI number alone.
What is a healthy BMI?
A healthy BMI for adults falls between 18.5 and 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. However, BMI is just one indicator of health. Your overall wellbeing depends on fitness level, diet, family history, blood pressure, and other factors.
Does BMI work for children?
No, adult BMI categories do not apply to children and teenagers. Pediatricians use age- and sex-specific percentile charts that account for growth and development. A BMI that looks high in a child may be perfectly normal depending on their age and sex.
Why is my BMI high when I look fine?
BMI is a simple ratio of weight to height and ignores body composition. If you have significant muscle mass from exercise, your BMI may be elevated even though your body composition is healthy. BMI also cannot see where your body stores fat or assess your overall fitness level.
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