About This Calculator
BMI (Body Mass Index) is a widely used screening tool that estimates whether your body weight is appropriate for your height. It was developed in the 1830s by Belgian mathematician and statistician Adolphe Quetelet as a population-level measure and is now used by the World Health Organization, the CDC, and most national health systems as a quick way to identify weight categories.
This calculator uses the standard formula adopted by the WHO and shown below. Enter your weight and height in either metric or imperial units to get your BMI and category.
🧮 Calculate Your BMI
📊 BMI Categories (WHO Standard)
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy Weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Obese (Class III) |
Categories follow the World Health Organization classification used internationally. A BMI in the healthy weight range does not guarantee good health, and a BMI outside it does not necessarily indicate poor health.
About the BMI Formula
BMI is calculated by dividing weight by the square of height. The same formula is used worldwide; only the units differ. Both versions produce identical results when converted properly.
The factor 703 in the imperial formula converts pounds and inches to the equivalent kilograms-and-metres result. The calculator above handles unit conversion automatically.
Originally proposed by Adolphe Quetelet in the 1830s as the "Quetelet Index," the formula was renamed Body Mass Index in 1972 by Ancel Keys, who recommended it for population-level health studies. The WHO adopted the current category thresholds in 1995 and has refined them since. These thresholds are based on observed associations between BMI and mortality risk across large populations.
What To Do With Your BMI Result
Underweight (below 18.5): Speak to a doctor or registered dietitian to rule out underlying causes and ensure adequate nutrition. Underweight individuals can be at increased risk of nutrient deficiencies, reduced bone density, and immune system issues.
Healthy Weight (18.5 – 24.9): This range is associated with the lowest health risks for most adults. Focus on maintaining a balanced diet and regular physical activity.
Overweight (25.0 – 29.9): Gradual changes to diet and physical activity are generally the most sustainable path to improvement. Tracking your daily calorie intake is one of the most effective tools for weight management — research consistently shows that people who monitor what they eat lose more weight and keep it off longer.
Obese (30.0 and above): A healthcare professional can help develop a structured plan. The higher the obesity class, the more important it becomes to work with a doctor, dietitian, or other qualified professional rather than going it alone.
Use our free Food Tracker to log your daily intake and Daily Calorie Needs Calculator to find your TDEE — the calorie target that supports your goal.
Limitations of BMI
BMI is useful at the population level but has well-documented limitations as an individual measure. Understanding these matters more than the number itself.
It does not distinguish between fat and muscle. A highly muscular person may register as overweight or obese by BMI despite having very low body fat. This is why elite athletes — particularly in strength sports — frequently show BMI values in the overweight range despite excellent body composition.
It does not account for fat distribution. Where fat is stored matters for health risk. Visceral fat around the abdomen is more strongly associated with cardiovascular and metabolic disease than fat stored elsewhere, but BMI does not capture this. Waist circumference and waist-to-hip ratio are useful complementary measurements.
It varies in accuracy across populations. Research has shown that health risks associated with excess body fat begin at lower BMI thresholds for some ethnic groups — South Asian, East Asian, and certain Pacific Islander populations among them. Some health bodies, including the WHO and NHS, recommend adjusted cutoffs for these groups.
It is less reliable at the extremes of age. BMI is not a recommended assessment tool for children, teenagers (who use BMI-for-age percentiles), pregnant women, or the elderly. For these groups, alternative measures or clinical assessment are more appropriate.
Despite these limitations, BMI remains useful as a free, fast, and widely understood screening tool. The key is interpreting it in context — as one signal alongside body composition, fitness level, lifestyle, family history, and other health indicators.
❓ Frequently Asked Questions
A BMI between 18.5 and 24.9 is classified as healthy weight for most adults. Below 18.5 is underweight, 25-29.9 is overweight, and 30 or above is classified as obesity.
These ranges apply to most adults regardless of sex, but BMI has known limitations — particularly for very muscular individuals and certain ethnic groups. Asian populations may face health risks at lower BMI thresholds (23+ overweight, 27.5+ obese).
No. BMI doesn't distinguish between muscle and fat, so muscular individuals often score as overweight or obese while having very low body fat.
A bodybuilder weighing 200 pounds at 6 feet would have a BMI of 27 (overweight) despite very low body fat. For muscular people, body fat percentage and waist circumference are more meaningful health metrics.
BMI is weight in kilograms divided by height in metres squared (kg/m²). For pounds and inches, the formula is weight in pounds divided by height in inches squared, then multiplied by 703.
The same formula and ranges apply to all adults regardless of sex, age, or body composition. This universality is BMI's main strength as a population-level screening tool, but also its main weakness for individual assessment.
BMI is a screening tool that correlates with risk for several health conditions including type 2 diabetes, heart disease, high blood pressure, and certain cancers.
The relationship is strongest at the extremes — very low and very high BMI both increase mortality risk. BMI doesn't diagnose anything; it's a starting point for further evaluation by a healthcare provider.
BMI is a ratio of weight to height — it tells you nothing about what your weight is made of. Body fat percentage measures the proportion of your body that's fat tissue versus everything else (muscle, bone, organs, water).
Two people with identical BMI can have very different body fat percentages, making body fat a more direct measure of body composition. Use BMI as a quick screen and body fat percentage for more detailed assessment.
No, the adult BMI ranges don't apply to children and teenagers. For people under 20, BMI is interpreted using age- and sex-specific percentile charts from the CDC.
A child's BMI is compared to others of the same age and sex, with classifications based on percentile rather than the adult cutoffs. Consult a pediatrician for assessment of children's weight status.
BMI is a screening tool, not a diagnosis. If your BMI is in the overweight range (25-29.9) but your blood pressure, cholesterol, blood sugar, and waist circumference are all healthy — and you're physically active — your actual health risk may be low.
Discuss your full health picture with your doctor rather than acting on BMI alone. Many people in the 25-27 BMI range have excellent metabolic health, particularly if they're active and muscular.
If your BMI is above the healthy range and you want to lower it, a moderate calorie deficit combined with regular exercise is the most effective approach. Aim for 1-2 pounds of weight loss per week through a 500-1,000 calorie daily deficit.
Resistance training preserves muscle so the weight you lose is mostly fat. Crash diets typically result in weight regain — sustainable habits beat aggressive short-term changes.
Knowing your BMI is just a starting point. Our AI tracker logs entire meals from a quick description — "oatmeal with berries, coffee" — and instantly calculates calories and macros toward your goal. 100% free.
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