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⚖️ BMI Calculator

Calculate your Body Mass Index (BMI) using metric or imperial measurements. Find your health category, ideal weight range, and understand what your number means.

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What Is BMI and How Is It Calculated?

Body Mass Index (BMI) is a numerical value derived from your height and weight. It's the most widely used screening tool for assessing whether an adult has a healthy body weight relative to their height. While it's not a direct measure of body fat, it correlates strongly with more precise measurements and is a practical starting point for health risk assessment.

The formula is simple: divide your weight in kilograms by the square of your height in meters (BMI = kg/m²). In imperial units: BMI = 703 × lbs / inches².

BMI Categories: What Your Number Means

The World Health Organization defines four standard adult BMI categories:

  • Underweight — Below 18.5: May indicate nutritional deficiency, eating disorders, or underlying medical conditions. Associated with increased risk of osteoporosis, immune deficiency, and fertility problems.
  • Normal weight — 18.5 to 24.9: Associated with the lowest health risks for most adults. This is the target range for weight management goals.
  • Overweight — 25 to 29.9: Modestly elevated risk for cardiovascular disease, type 2 diabetes, and hypertension. Lifestyle changes can often bring this into the normal range.
  • Obese — 30 and above: Significantly higher risk for serious health conditions. Further classified into Class I (30–34.9), Class II (35–39.9), and Class III (40+) obesity.

Limitations of BMI: What It Doesn't Measure

BMI is a population-level screening tool, not a personal diagnostic. It has well-documented limitations: muscle mass — athletes and bodybuilders often register as "overweight" despite very low body fat; age — older adults typically have more body fat at the same BMI than younger adults; sex — women naturally carry more fat than men at the same BMI; and ethnicity — some populations (notably South Asian) have higher health risks at lower BMI values than the standard thresholds suggest.

For a more complete picture, pair BMI with waist circumference, body fat percentage (see our body fat calculator), and a discussion with your doctor.

How to Reach a Healthy BMI

If your BMI is above 24.9, the most evidence-backed approach is a modest calorie deficit of 500–750 kcal/day, which produces 1–1.5 lbs of weight loss per week. Use our calorie calculator to find your TDEE and set a target intake. Combined with regular physical activity and adequate protein (0.7–1g per lb of body weight), most people can reach a healthy BMI within months to a year depending on the gap to close.

BMI for Children and Teens

This calculator is designed for adults aged 18+. For children and adolescents (ages 2–19), BMI is interpreted using age- and sex-specific percentile charts because healthy ranges change significantly through development. A child at the 85th–94th percentile for their age/sex group is considered overweight; at or above the 95th percentile is obese.

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Frequently Asked Questions

Common questions about BMI accuracy, healthy ranges, and what to do once you know your number — answered clearly.

Is BMI an accurate measure of body fat?

BMI is a proxy for body fatness, not a direct measurement. It correlates reasonably well with body fat at the population level but can misclassify individuals — particularly athletes (who may be "overweight" BMI due to muscle), older adults (who have more fat at the same BMI), and certain ethnic groups. For greater accuracy, consider body fat percentage tests (DEXA scan, hydrostatic weighing, or the Navy method in our body fat calculator). That said, for most people without extreme muscle mass, BMI is a useful and quick screening tool.

What BMI is considered overweight?

A BMI of 25.0 to 29.9 is classified as overweight according to WHO standards. This range is associated with modestly elevated risk for cardiovascular disease, type 2 diabetes, sleep apnea, and joint problems — though risk is also strongly influenced by other factors like diet, physical activity, smoking status, and genetics. Being at the lower end of the overweight range (25–27) with high physical fitness carries much lower risk than being sedentary at normal BMI.

What is a healthy BMI for women vs men?

The WHO's standard healthy BMI range of 18.5–24.9 applies to both men and women. However, women naturally carry more body fat than men at the same BMI due to hormonal differences and reproductive needs (typically 6–11% more fat). Some researchers argue the normal range upper limit should be slightly higher for women. In practice, both sexes use the same 18.5–24.9 threshold clinically, though interpretation should account for individual context including age, muscle mass, and ethnic background.

Can you have a normal BMI but still be unhealthy?

Yes — a condition called "normal weight obesity" or "skinny fat" occurs when someone has a normal BMI but a high percentage of body fat relative to lean muscle mass. This often results from being sedentary (losing muscle without gaining weight), aging, or crash dieting. Conversely, you can be overweight by BMI but metabolically healthy if you're physically active and have good cardiovascular fitness. This is why BMI is best used alongside other markers: waist circumference, fasting blood glucose, blood pressure, and cholesterol.

How much weight do I need to lose to reach a normal BMI?

Calculate the target weight using the normal BMI upper limit of 24.9: Target weight = 24.9 × height(m)². For a 5'8" (1.73m) person, that's 24.9 × 1.73² = 74.5 kg (164 lbs). The difference between your current weight and this target is the minimum you'd need to lose to enter the normal range. Enter your measurements above — the calculator shows this gap automatically. A safe rate of loss is 0.5–1 kg (1–2 lbs) per week, achievable through a 500–750 kcal daily deficit.