BMI Calculator for Adults (US & Metric Units)

The BMI formula

Body mass index is weight divided by height squared. In metric units: BMI = kg ÷ m². In US units: BMI = 703 × lb ÷ in². The 703 is just the conversion factor that makes pounds and inches match the metric result.

Worked example: 5 feet 10 inches (70 inches) and 170 pounds. 70 squared is 4,900. 170 ÷ 4,900 × 703 = 24.4. In metric, 1.78 m and 77 kg gives 77 ÷ 3.17 = 24.3. Same answer, rounding aside.

What the categories mean

The CDC and WHO use the same adult cut-offs: under 18.5 is underweight, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 or above is obesity. Obesity is further split into class 1 (30 to 34.9), class 2 (35 to 39.9), and class 3 (40 and above).

For a 5-foot-10 adult, the healthy range works out to roughly 129 to 173 pounds. For 5 feet 4 inches, it is about 108 to 145 pounds. Those ranges are wide on purpose; BMI is a screening number, not a target weight.

Where BMI gets it wrong

BMI cannot tell muscle from fat. A 5-foot-10, 200-pound athlete with 12 percent body fat scores 28.7, technically overweight. Meanwhile someone with low muscle mass can sit at 23 and still carry a high body fat percentage, sometimes called normal-weight obesity.

It also ignores where fat sits. Visceral fat around the organs carries more health risk than fat on the hips. Waist circumference adds useful context: above 40 inches for men or 35 inches for women is associated with higher risk regardless of BMI. Waist-to-height ratio under 0.5 is another simple check.

BMI thresholds were built mostly on data from white European populations. Some health bodies recommend lower cut-offs for people of Asian descent, with risk rising from around 23 rather than 25.

Children and teens

Adult categories do not apply under age 20. For kids, BMI is compared against CDC growth charts for the same age and sex, and the result is a percentile. The 85th to 95th percentile is considered overweight; the 95th and above is obesity. Use a pediatric calculator or ask your pediatrician for those.

BMI and health risk: what the research shows

At a population level, BMI tracks risk well. Large studies consistently link a BMI of 30 and above with higher rates of type 2 diabetes, high blood pressure, heart disease, sleep apnea, and certain cancers. Risk climbs further at 35 and 40. That is why doctors, insurers, and public health agencies still use it: it is free, fast, and needs only a scale and a tape measure.

At the individual level, it is a starting point. Two people at BMI 27 can have very different health profiles depending on muscle mass, waist size, fitness, blood pressure, and blood sugar. If your BMI is borderline, those other numbers tell you far more than a decimal point of BMI.

Changing your BMI

Because height is fixed for adults, BMI moves only with weight. For a 5-foot-10 adult, each 7 pounds is about one BMI point. Going from 28 to 25 means losing about 21 pounds. At a sustainable pace of 1 pound a week, that is five months, which is a realistic timeline to plan around.

If you are building muscle through strength training, your BMI may stay flat or rise while your waist shrinks and your health markers improve. That is a good outcome, not a failure. Track waist measurement and how clothes fit alongside the scale.

How to use your number

Track the trend, not a single reading. Weigh at the same time of day, ideally in the morning, and look at the monthly average. If your BMI is outside the healthy range, it is worth a conversation with a doctor who can look at blood pressure, blood work, and body composition together. The calculation runs in your browser; your height and weight are never sent anywhere.