Free BMI Calculator
Calculate your Body Mass Index instantly. Enter your height and weight to get your BMI, weight category, and healthy weight range.
What is BMI?
Body Mass Index (BMI) is a simple calculation that uses your height and weight to estimate whether you fall into an underweight, normal weight, overweight, or obese category. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and remains one of the most widely used screening tools in healthcare.
BMI does not measure body fat directly. It is a proxy — a quick, inexpensive way to flag potential weight-related health risks at a population level. For most people who do not strength train, BMI correlates reasonably well with body fat percentage. But it has well-known limitations, especially for athletes.
The formula
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = 703 × weight (lbs) / height (in)²
BMI limitations for lifters
If you lift weights regularly, BMI can be misleading. Muscle is denser than fat, so a muscular person will weigh more than a sedentary person of the same height — even if they carry less body fat. Many competitive powerlifters, bodybuilders, and dedicated gym-goers fall into the "overweight" or even "obese" BMI categories despite being lean and healthy.
For a more complete picture of your body composition, consider tracking body fat percentage, waist-to-hip ratio, or using a DEXA scan. If you are focused on strength, your one rep max and DOTS score are far better metrics for tracking progress than BMI.
When BMI is useful
Despite its limitations, BMI remains useful as a starting point. It is quick to calculate, requires no equipment, and correlates with health outcomes at a population level. For someone beginning their fitness journey who does not yet have significant muscle mass, BMI provides a reasonable baseline. Pair it with a TDEE calculator to set calorie targets based on your goals, and track your training with a structured program to make sure the weight you gain is muscle, not fat.
Track What Actually Matters
BMI is a starting point — not the full picture. Slate tracks your strength progress, estimated 1RM, and muscle recovery so you can see real results from your training.
Download Free for iOSFrequently Asked Questions
BMI is calculated by dividing your weight in kilograms by your height in meters squared. The formula is: BMI = weight (kg) / height (m)². For imperial units, the formula is: BMI = 703 × weight (lbs) / height (inches)². The result is a number that categorizes your weight status relative to your height.
The World Health Organization defines a healthy BMI as 18.5 to 24.9. Below 18.5 is considered underweight, 25 to 29.9 is overweight, and 30 or above is classified as obese. These ranges are general guidelines and don't account for muscle mass, bone density, age, or body composition differences between individuals.
BMI has significant limitations for muscular individuals. Because it only considers height and weight, it cannot distinguish between muscle and fat. A lean, muscular lifter at 5'10" and 200 lbs would have a BMI of 28.7 (overweight), despite potentially having a low body fat percentage. For strength athletes, body fat percentage, waist circumference, or a DEXA scan provide a more accurate picture of health than BMI alone.
A BMI of 30 or higher is classified as obese by the WHO. Obesity is further divided into three classes: Class I (BMI 30 to 34.9), Class II (BMI 35 to 39.9), and Class III (BMI 40 or higher). These classifications help medical professionals assess health risks, though BMI should be considered alongside other metrics like waist circumference and overall fitness level.
Both metrics have value, but they measure different things. BMI is a quick screening tool based on height and weight. Body fat percentage directly measures how much of your body is fat versus lean tissue. For the general population, BMI is a reasonable proxy. For anyone who lifts weights regularly, body fat percentage is more useful because muscle mass can push BMI into overweight territory even at healthy body fat levels.
The BMI formula itself doesn't account for age, but body composition changes as you age. Adults tend to lose muscle mass and gain fat over time, especially after age 30. This means two people with the same BMI at ages 25 and 55 likely have very different body compositions. Some health organizations suggest that slightly higher BMI ranges may be acceptable for older adults.