BMI Calculator
Calculate your Body Mass Index and see which weight category it falls into.
Enter your height and weight in either metric (cm/kg) or imperial (ft-in/lb) units, and this tool calculates your Body Mass Index along with the standard World Health Organization category it falls into. BMI is a general screening measure and doesn't account for muscle mass, so it's best used as a rough guide rather than a full health assessment.
Where BMI actually came from
BMI was developed in the 1830s by a Belgian mathematician, Adolphe Quetelet, not as a personal health metric at all but as a way to study average characteristics across large populations for statistical purposes. It was adopted decades later as a quick individual health screening tool mostly because it's simple to calculate from just height and weight, not because it was originally designed with individual clinical assessment in mind — a history worth knowing when weighing how much to rely on it personally.
Why BMI's simplicity is both its strength and its weakness
Because BMI uses only height and weight, it can be calculated instantly without any specialised equipment, which is exactly why it remains so widely used for large-scale screening. But that same simplicity means it can't distinguish between weight from muscle and weight from fat, can't account for where fat is distributed on the body (which matters for health risk), and uses the same categories regardless of age, sex, or ethnicity, all of which more detailed health assessments do take into account.
Why BMI categories are still useful despite these limitations
For the majority of the general adult population who aren't unusually muscular or unusually short or tall, BMI does correlate reasonably well with body fat percentage and associated health risks at a population level, which is why it remains a standard first-pass screening tool used by healthcare providers worldwide. It's the starting point for a conversation about health, not a complete diagnosis in itself.
How to use it
- Choose metric or imperial units.
- Enter your height.
- Enter your weight.
- Your BMI and category appear instantly.
Common ways people use this tool
Getting a quick health screening baseline
Check your current BMI category as a starting point before discussing weight or health goals with a doctor.
Tracking BMI changes alongside a fitness goal
Recalculate BMI periodically alongside other measurements while working toward a weight or fitness goal.
Converting between metric and imperial for a known BMI target
Use the tool to check what weight, in either unit system, would correspond to a specific target BMI at your height.
Tips for getting the best results
- Treat your BMI result as one data point among several, not a complete picture of health on its own.
- If you're notably muscular or an athlete, be aware BMI may overestimate your health risk relative to actual body fat percentage.
- Recalculate periodically rather than once, since a trend over time is often more informative than a single snapshot.
- Discuss any health concerns related to weight with a healthcare professional rather than relying on BMI alone for decisions.
Frequently asked questions
What BMI ranges count as each category?
Under 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30 and above is classified as obese, per WHO standards.
Is BMI accurate for athletes or very muscular people?
Not really — BMI doesn't distinguish muscle from fat, so muscular individuals can show a higher BMI despite low body fat.
Should I make health decisions based on BMI alone?
BMI is a useful quick screening number, but a full picture of health should come from a healthcare professional considering other factors too.
Does BMI account for age or sex differences?
No, the standard BMI formula and categories apply the same calculation regardless of age or sex, which is one of its most commonly cited limitations.
Are there alternative measurements considered more accurate than BMI?
Yes, measures like waist-to-hip ratio, body fat percentage from skinfold or bioelectrical impedance measurement, and waist circumference alone are sometimes considered more directly informative about health risk than BMI.
Why do some countries use different BMI category cutoffs?
Some research suggests certain populations, particularly some Asian populations, may face elevated health risks at lower BMI thresholds than the standard WHO cutoffs, leading some health authorities to recommend adjusted category boundaries for those populations.
Can children use the same BMI categories as adults?
No, BMI for children and teenagers is typically interpreted using age- and sex-specific percentile charts rather than the fixed adult category thresholds used here.
Does BMI change meaningfully throughout a single day?
No, BMI is based on height and weight, and weight fluctuations throughout a day (from food, water, and normal bodily changes) are generally too small to shift BMI into a different category.
Is it normal for BMI to fluctuate week to week even without real fat loss or gain?
Yes, short-term weight changes from water retention, digestion, and other everyday factors are completely normal and don't necessarily reflect an actual change in body fat, which is why tracking a longer-term trend matters more than any single reading.
Should pregnant women use standard BMI categories?
No, standard BMI categories aren't designed for pregnancy, since expected weight gain during pregnancy is healthy and normal — a healthcare provider would use pregnancy-specific guidance rather than standard adult BMI categories.