What is BMI?
Body Mass Index (BMI) is a screening measure that relates weight to height: weight in kilograms divided by height in meters squared. It was designed for studying populations, and it remains the standard first-pass tool used by the World Health Organization and health services worldwide because it’s simple, free, and correlates reasonably well with body fat across large groups.
The formula: BMI = weight (kg) ÷ height (m)². A 70 kg person who is 1.75 m tall has a BMI of 70 ÷ 3.0625 ≈ 22.9.
What Does This Tool Do?
Enter your weight and height — in metric (kg/cm) or imperial (lb/ft+in) units — and the tool instantly shows your BMI, your WHO category, and the weight range that corresponds to the “normal” band for your height. The category table highlights where you fall.
How to Use This Tool
- Pick your unit system (metric or imperial).
- Enter weight and height.
- Read your BMI, category, and healthy weight range — all update as you type.
WHO Classification for Adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
These cut-offs apply to adults 20 and older. For children and teens, BMI is interpreted through age- and sex-specific percentiles instead of fixed bands — a pediatric growth chart, not this table.
What BMI Can and Can’t Tell You
BMI is a screening number, not a diagnosis, and its limits are well documented:
- It can’t see body composition. Muscle is denser than fat, so muscular athletes routinely score “overweight” while carrying little fat. Conversely, a sedentary person can have a “normal” BMI with high body fat.
- It ignores fat distribution. Abdominal fat carries more metabolic risk than fat elsewhere; waist circumference and waist-to-height ratio capture this, BMI doesn’t.
- Ethnicity shifts risk thresholds. Health bodies in several Asian countries use lower cut-offs (overweight from 23) because metabolic risk rises at lower BMIs in those populations.
- Age matters. Slightly higher BMIs in older adults aren’t associated with the same risks as in younger adults.
Used honestly — as one data point that flags whether a closer look is worth it — BMI is genuinely useful. Used as a verdict, it misleads. If your number surprises you in either direction, that’s a conversation with a health professional, not a conclusion.
Frequently Asked Questions
Is my data private?
Completely. Your weight and height are processed in your browser only — nothing is transmitted, logged, or stored.
What's the healthy weight range shown?
It’s the weight span that would put your BMI between 18.5 and 24.9 at your current height. For someone 175 cm tall, that’s roughly 56.7–76.3 kg. It’s a band, not a target — where a healthy weight sits within (or slightly outside) it depends on your build and composition.
How accurate is the imperial conversion?
Exact — pounds convert at 0.45359237 kg and feet/inches at 2.54 cm per inch, the international definitions. The BMI itself is always computed in metric under the hood.
Does BMI work for children?
Not with the adult table. Children’s BMI must be compared against growth percentiles for their age and sex. Use a pediatric calculator or growth chart for anyone under 20.
My BMI says overweight but I lift weights. Should I worry?
Probably not — this is BMI’s best-known blind spot. Muscle mass raises BMI without raising health risk. Measures like waist circumference, body-fat percentage, blood pressure, and blood markers describe an athletic body far better than BMI does.
Is BMI medical advice?
No. It’s an educational screening number. Decisions about weight and health belong with a doctor or registered dietitian who can see the whole picture.