BMI Calculator

A BMI calculator is a quick screening tool that turns your height and weight into a single number, your Body Mass Index, and sorts that number into a category: underweight, normal weight, overweight, or obese. It's the same formula doctors, nurses, and insurers have used for decades, packaged into a free BMI calculator you can run from any phone or computer in a few seconds.

This body mass index tool is useful for anyone who wants an informal check on their weight status: people starting a fitness plan, patients preparing for a doctor's visit, or anyone curious how their numbers line up against the standard BMI chart. It's a screening number, not a diagnosis, and it doesn't replace a conversation with a healthcare provider, but it gives you the same starting figure a clinic would calculate at a checkup.

Using an online calculator beats doing the math by hand because it removes the risk of a squared-height error, converts between metric and imperial units automatically, and shows your category instantly instead of making you cross-reference a healthy weight chart afterward. Enter your numbers once and you have a result you can save and track over time.

The BMI formula is straightforward: divide your weight in kilograms by your height in meters, then divide that result by your height in meters again. In symbols, BMI = weight (kg) ÷ height (m)². If you only have pounds and inches, the imperial version is BMI = 703 × weight (lb) ÷ height (in)². Both formulas land on the same category when you check the result against the standard BMI chart.

Take someone who weighs 70 kg and stands 1.75 m tall. Height squared is 1.75 × 1.75 = 3.0625. Divide 70 by 3.0625 and you get a BMI of 22.9, which falls in the normal weight range. In pounds and inches, a person weighing 154 lb at 5 feet 9 inches (69 inches) gets 703 × 154 ÷ (69 × 69) = 22.7 — the small gap comes from rounding during the unit conversion.

• Metric formula: BMI = weight (kg) ÷ [height (m)]²
• Imperial formula: BMI = 703 × weight (lb) ÷ [height (in)]²
• Keep height to two decimal places in meters to avoid rounding errors
• An online BMI calculator applies the same formula instantly and skips the manual rounding

The World Health Organization divides adult BMI into four main categories that apply to both men and women aged 20 and over. A BMI under 18.5 is classified as underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obese. These are the same cutoffs a BMI calculator uses to label your result, and they're the version most clinics in the United States and internationally reference.

Obesity itself is split into three classes because health risk climbs with each step up. Class I (30–34.9) carries a moderate increase in risk for conditions like type 2 diabetes and high blood pressure, Class II (35–39.9) raises that risk further, and Class III (40 and above), sometimes called severe obesity, is linked to the highest risk and is often the threshold doctors cite when discussing more intensive treatment options.

• Underweight: below 18.5
• Normal weight: 18.5–24.9
• Overweight: 25–29.9
• Obesity Class I: 30–34.9
• Obesity Class II: 35–39.9
• Obesity Class III (severe): 40 and above

BMI has one well-known blind spot: it can't distinguish muscle from fat, because it only uses weight and height. Muscle is denser than fat, so a bodybuilder or competitive athlete can weigh more per inch of height than an average person while carrying very little body fat. The formula still divides that heavier weight by height squared and returns a number that can look overweight or even obese.

A real-world example: a 6-foot (183 cm) rugby player weighing 220 lb (100 kg) comes out to a BMI of about 29.9, technically overweight, even with a lean body fat percentage around 12%. For athletes, weightlifters, and anyone with above-average muscle mass, a body fat percentage calculator or waist-to-height ratio paints a more honest picture than BMI alone. This tool's results are informational only and are not a substitute for medical advice.

The standard 18.5–24.9 BMI chart only applies to adults aged 20 and up. For children and teens, doctors use BMI-for-age percentiles from CDC or WHO growth charts, because a healthy BMI shifts as kids grow. A BMI of 19 might be normal for a 15-year-old but underweight for a 25-year-old, so a BMI calculated for a child should be compared to a pediatric growth chart, not the adult categories.

Older adults face the opposite issue. After age 65, people naturally lose muscle mass even if their weight stays the same, so a normal BMI can mask a low-muscle, higher-fat body composition. Some geriatric guidelines suggest a slightly higher healthy range, around 22 to 27, for seniors. Pregnant women should also skip BMI-based weight goals entirely and follow their doctor's pregnancy-specific weight gain recommendations instead.

BMI and body fat percentage measure different things. BMI is a ratio of weight to height that takes seconds to calculate and needs no special equipment, which is why it's used for population-level health screening. Body fat percentage, measured through methods like the U.S. Navy tape-measurement method, skinfold calipers, or a DEXA scan, tells you what portion of your weight is actually fat versus muscle, bone, and water.

Neither number tells the whole story on its own. BMI is the faster, free starting point and works well for tracking trends over months, while body fat percentage is more informative for assessing fitness and health risk, especially for muscular or older adults. Many people use a BMI calculator for a quick check and a body fat calculator for a more detailed picture before changing their diet or training plan.