BMI is your weight in kilograms divided by your height in meters squared (or 703 × pounds ÷ inches²), and it sorts adults into four bands from underweight to obese. It's quick and useful for spotting trends across a population, but it was never meant to be a verdict on one person's health. This guide walks through a worked example, explains what the categories mean, and shows where the number falls short so you can read it wisely.
The formula and how to calculate it
BMI is your weight divided by your height squared. In metric units the formula is BMI = weight (kg) / height (m)². In US units it's BMI = 703 × weight (lb) / height (in)². The 703 factor simply converts pounds and inches into the metric result, so both formulas give the same number for the same person.
Here's a worked example. Someone who is 5 feet 9 inches (69 inches) tall and weighs 170 pounds calculates it as 703 × 170 / (69 × 69) = 703 × 170 / 4,761, which works out to about 25.1. In metric that same person is roughly 1.75 m and 77 kg: 77 / (1.75 × 1.75) = 77 / 3.06, which is about 25.1 as well. No special equipment required — just a scale and a tape measure.
What the categories mean
The World Health Organization groups adult BMI into four broad bands: below 18.5 is underweight, 18.5 to 24.9 is the normal or healthy range, 25.0 to 29.9 is overweight, and 30.0 and above is obese. The person in our example, at 25.1, sits just inside the overweight band — barely across the line from the healthy range, which shows how little the categories mean at the edges.
These cutoffs come from population studies linking BMI to health risks like heart disease and type 2 diabetes. They describe statistical averages across large groups, not guarantees for individuals. Two people with the same BMI can have very different health, which is exactly why the number is a starting point for a conversation, not a diagnosis.
Where BMI falls short
BMI only knows your height and weight — it can't tell muscle from fat. A muscular athlete and a sedentary person can share an identical BMI while having completely different body compositions. This is why many fit, strong people land in the overweight band despite low body fat, and why BMI alone can misjudge them.
It also ignores where fat is stored, which matters a great deal. Fat around the abdomen carries more health risk than fat on the hips and thighs, so two people with the same BMI can face very different risks. BMI is also less reliable for older adults, who tend to lose muscle, and it uses different percentile-based charts for children and teens rather than the fixed adult cutoffs.
What to measure alongside BMI
Waist circumference is a simple, powerful complement. A waist over about 40 inches (102 cm) for men or 35 inches (88 cm) for women signals higher risk regardless of BMI, because it captures the abdominal fat BMI can't see. Waist-to-height ratio — keeping your waist under half your height — is another quick, evidence-backed check.
For a fuller picture, look at body fat percentage and your resting metabolism. Body fat percentage separates lean mass from fat directly, and your basal metabolic rate (BMR) tells you how many calories your body burns at rest, which anchors any sensible nutrition plan. Used together, these numbers give context that BMI on its own simply can't provide.
Run the numbers yourself
Frequently Asked Questions
Is a BMI of 25 unhealthy?
Not necessarily. A BMI of 25 sits at the very bottom of the overweight band, one decimal above the healthy range. For a muscular or athletic person it may reflect lean mass rather than excess fat. Pair it with waist measurement and body fat percentage before drawing any conclusion.
Should athletes use BMI?
BMI is unreliable for athletes because it can't distinguish muscle from fat, and muscle is denser than fat. A lean, muscular athlete often shows an overweight or even obese BMI despite low body fat. A body fat percentage estimate is far more informative for them.
What's a healthy BMI range for adults?
For most adults, 18.5 to 24.9 is considered the healthy range. Below 18.5 is underweight and 25 or above is overweight. These are population guidelines, so use them as a rough guide alongside other measures rather than a strict target.