BMI Calculator
Calculate your Body Mass Index, see which category you fall into, and find the healthy weight range for your height. Works in both imperial (feet, inches, pounds) and metric (centimeters, kilograms). Uses the same formula the CDC and WHO use.
Quick answer
BMI = weight (kg) ÷ height (m)². A BMI of 18.5–24.9 is considered healthy, 25–29.9 is overweight, and 30+ is obese. Under 18.5 is underweight.
BMI Calculator
How BMI is calculated
Body Mass Index is a simple ratio of weight to height squared: BMI = kg / m².
For imperial units, the formula becomes BMI = (lbs / in²) × 703, where the 703
is a conversion factor that makes the two formulas equivalent. The CDC and WHO use identical
cutoffs: under 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, and 30 or
higher is obese. Obese is further subdivided into Class I (30–34.9), Class II (35–39.9),
and Class III / severe (40+).
BMI was developed in the 1830s by Belgian statistician Adolphe Quetelet as a population-level measure, not an individual diagnostic. It's a rough screening tool that correlates with body fat across most people, but it's not measuring body fat directly — it's measuring how heavy you are relative to how tall you are.
When to use it
BMI is useful as a first screen for weight-related health risk. If your BMI is well into the overweight or obese range, it's a reasonable signal to check in with a doctor about metabolic health — blood pressure, cholesterol, fasting glucose. It's also a common input on insurance applications, airline medical forms, and clinical screening questionnaires.
As a goal-setting tool, BMI gives you a healthy weight range for your height. A 5'10" adult has a healthy range of roughly 129–173 lbs — a 44-pound window, which is wide enough to accommodate different body types, genders, and muscle masses.
Common mistakes
- Treating BMI as a body fat measurement. BMI doesn't know if your weight is from muscle, fat, bone, or water. A lean 220-lb bodybuilder has the same BMI as a 220-lb couch potato of the same height.
- Using BMI on children or elderly adults. Adult BMI cutoffs don't apply to kids (use percentile charts instead) or very old adults (where slightly higher BMI is often associated with better outcomes).
- Obsessing over small changes. A 0.5 BMI change is roughly 3–4 pounds — normal day-to-day fluctuation from food, water, and glycogen. Look at trends over weeks or months, not daily measurements.
- Ignoring body shape. Waist-to-hip ratio and waist circumference are better indicators of metabolic risk than BMI alone. Belly fat is metabolically more dangerous than hip or thigh fat.
Frequently asked questions
What is a healthy BMI?
The CDC and WHO classify BMI 18.5–24.9 as healthy, 25–29.9 as overweight, and 30+ as obese. Under 18.5 is considered underweight.
How do I calculate BMI?
BMI = weight (kg) ÷ height (m) squared. For imperial: BMI = (weight in pounds ÷ height in inches squared) × 703.
Is BMI accurate for athletes?
No. BMI can't distinguish muscle from fat, so muscular athletes often score "overweight" or "obese" despite being lean and fit. Body fat percentage is a better metric for athletes.
What's the difference between BMI and body fat percentage?
BMI is a simple ratio of weight to height squared. Body fat percentage is the portion of your total weight that's fat tissue. Body fat is more accurate but requires calipers, DEXA, or circumference measurements.
Does BMI differ by gender?
The formula is the same, but women naturally carry a higher percentage of body fat than men at the same BMI. The CDC uses identical BMI cutoffs for both genders for adults.