BMI calculator online

How to use the BMI calculator

1. Choose metric (kilograms, centimeters) or imperial (pounds, inches) units.
2. Enter your weight.
3. Enter your height.
4. Read your BMI value and weight category, which update as you type.
5. Discuss the result with a healthcare provider if you have questions about what it means for you specifically.

BMI uses only weight and height. It does not account for muscle mass, bone density, age, sex, or body composition, all of which affect how meaningful a given BMI number is for a specific person.

Supported formats

Weight and height accept positive decimal values in either metric (kilograms and centimeters) or imperial (pounds and inches) units, selected with the units dropdown. Switching units does not automatically convert an already-entered value; re-enter your weight and height after switching units.

What BMI is and how it's calculated

BMI is calculated as weight in kilograms divided by height in meters squared (BMI = kg / m²). For imperial input, this calculator converts pounds to kilograms (× 0.453592) and inches to centimeters (× 2.54) first, then applies the same formula. The result is compared against the World Health Organization's standard adult categories: below 18.5 is underweight, 18.5 to just under 25 is normal weight, 25 to just under 30 is overweight, and 30 and above is obese. These category boundaries are widely used but were developed primarily from population-level data, which is part of why BMI works better as a screening tool across large groups than as a precise individual health measure.

Best practices

• Use BMI as one general indicator, not a complete health assessment. It doesn't distinguish muscle from fat, so a very muscular person can show a higher BMI without carrying excess body fat.
• Re-check your result periodically rather than reacting to a single reading, since weight naturally fluctuates day to day.
• Discuss your specific result with a healthcare provider, especially if it falls outside the "normal weight" range, since they can factor in your full health picture.

Common mistakes to avoid

• Mixing units — entering weight in pounds while height is still set to centimeters from a previous calculation. Always re-check both fields after switching the units dropdown.
• Treating BMI as diagnostic on its own. Two people with the same BMI can have very different body compositions and health profiles; BMI alone doesn't capture that difference.
• Applying adult BMI categories to children or teenagers. Growing children need age- and sex-specific growth charts, not the fixed adult category boundaries this calculator uses.

Real-world use cases

• General weight-status screening as a starting point for a conversation with a healthcare provider.
• Tracking a general trend over time alongside other health measures, not as a standalone metric.
• Understanding a BMI figure mentioned on a medical form or health app that doesn't explain the category boundaries.
• Quick unit conversion between metric and imperial while calculating, useful for people used to a different measurement system than a form requires.

Worked example: same BMI, different unit systems

A person weighing 70 kg at 175 cm and someone weighing 154.3 lb at 68.9 in are the same person expressed in different units, and both produce the same BMI:

Why BMI is widely used despite its limitations

Given how many individual factors BMI doesn't capture, it's a fair question why it remains so widely used in healthcare settings. The main reason is practicality at scale: BMI needs only two easily measured numbers, requires no special equipment beyond a scale and a tape measure, and correlates reasonably well with health risk across large populations, which makes it useful for public health screening and tracking trends over time in ways that more precise but more expensive measures (like body fat percentage from a DEXA scan) aren't practical for. The tradeoff is that this population-level usefulness doesn't always translate cleanly to an individual result, which is exactly why healthcare providers use BMI as one input among several, not a standalone verdict.

Limitations

This calculator computes standard adult BMI only; it is not calibrated for children, teenagers, pregnant women, or highly muscular individuals, all of whom need different assessment approaches. It does not account for body composition, waist circumference, age, or sex, all of which affect how a given BMI number should be interpreted for an individual. This is a general screening figure, not a medical diagnosis, and should not replace a healthcare provider's assessment.

Privacy

All calculation happens locally in your browser as you type. Nothing is sent to a server. See the privacy policy for the site's full data handling approach.

Troubleshooting

• If the result looks wrong after switching units, re-enter both weight and height, since values aren't automatically converted when you change the units dropdown.
• If you're calculating for a child or teenager, this adult-focused calculator's categories don't apply; consult growth-chart-based tools or a pediatrician instead.
• If you need unit conversion for other measurements, see the unit converter.

Conclusion

Use this calculator for a quick, general BMI screening figure in either unit system, and treat the result as a starting point for a conversation with a healthcare provider rather than a diagnosis. For other measurement conversions, see the unit converter.