What is BMI?
BMI stands for Body Mass Index. It's a number doctors and researchers have used for over 150 years to estimate whether someone's weight falls in a healthy range for their height.
The formula is simple. Take your weight in kilograms, divide by your height in meters squared. That's your BMI. The result lands in one of six categories, from underweight to severe obesity.
It's not a perfect measurement. It doesn't know if you're carrying muscle from years of lifting or fat from sitting at a desk. It doesn't account for bone density, ethnicity, or where you carry your weight. A 200-pound bodybuilder and a 200-pound office worker of the same height get the same BMI, even though their bodies are dramatically different.
So why use it? Because it's fast, free, and a reasonable starting point for most adults. It's the screening tool your doctor probably uses before ordering more detailed tests.
How BMI is calculated
The standard formula uses metric units: BMI = weight (kg) ÷ height² (m²). For imperial units, multiply weight (lbs) divided by height² (in²) by 703.
If you're 5'9" (175 cm) and weigh 170 lbs (77 kg), your BMI is about 25.1. That puts you in the overweight range, but only just — a few pounds either way and you're back in healthy.
What your BMI category means
Underweight (below 18.5): For some this is natural — fast metabolism, lean build, athletic background. For others, it can signal undereating or an undiagnosed condition.
Healthy weight (18.5–24.9): The range most associated with lowest risk for weight-related health conditions.
Overweight (25–29.9): The largest BMI category in the US adult population. A yellow flag, not a red one.
Class I obesity (30–34.9): The range where medical weight loss treatments start to show meaningful effects. GLP-1 medications can produce 10-15% body weight loss in this range.
Class II obesity (35–39.9): Higher health risks. GLP-1s showed 15-22% body weight loss over 68 weeks in clinical trials.
Class III obesity (BMI 40+): Highest health risk category. Real options exist that didn't five years ago.
Where BMI falls short
BMI was built for populations, not individuals. It doesn't measure body composition. It treats men and women the same. It doesn't account for ethnicity. It misses where fat sits. It ignores muscle. For these reasons, doctors increasingly use BMI alongside waist circumference, blood pressure, blood sugar, and cholesterol panels.
BMI and weight loss medications
Most telehealth weight loss programs in the US use BMI as a first-pass eligibility filter. BMI 30+ generally qualifies. BMI 27-29.9 qualifies if you also have a weight-related condition like type 2 diabetes, high blood pressure, sleep apnea, or high cholesterol.
GLP-1 medications work by mimicking a hormone your body naturally produces after eating. In clinical trials for semaglutide, participants lost an average of 15% of their body weight over 68 weeks. For tirzepatide, the average was over 20%.
When to talk to a doctor
Use this calculator as one data point. Talk to a doctor if your BMI is in Class I, II, or III obesity and you've tried weight loss without lasting results, or if you're in the overweight range and have any condition that worsens with weight. Our assessment takes two minutes and a US-licensed physician reviews everything within 24-48 hours.