About the BMI Calculator
Body Mass Index compares your weight to your height on a single scale, giving a number that clinicians use as a quick screening tool for weight-related health risk. It takes two measurements and places you in one of four broad categories, which is both its strength and the reason it should never be read in isolation.
The formula
BMI = weight (kg) / height (m)²Weight in kilograms divided by height in metres squared. The standard categories are under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, and 30 or above obese.
How to use this calculator
- 1Enter your Weight (kg). The field starts at
70, which you can overwrite. - 2Enter your Height (cm). The field starts at
170, which you can overwrite. - 3Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Weight (kg) | 70 |
| Height (cm) | 170 |
Result
BMI: 24.2 (Normal)
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
BMI was devised in the 1830s as a population statistic, not an individual diagnostic, and that origin explains its limitations. It cannot distinguish muscle from fat, so a well-trained athlete often registers as overweight or obese while carrying very little body fat. It also says nothing about where fat is distributed, and abdominal fat carries considerably more metabolic risk than the same weight around the hips.
Where BMI does work well is as a first filter across large groups, and it correlates reasonably with health outcomes at the extremes. A BMI of 35 or a BMI of 16 is meaningful information. In the middle of the range, waist circumference, waist-to-height ratio, blood pressure, and blood markers tell you far more than the index does.
Things worth knowing
- Waist-to-height ratio is a better single predictor of metabolic risk for most people. Keeping your waist under half your height is the usual guideline.
- BMI thresholds differ by ancestry. Lower cut-offs — overweight from 23 — are recommended for people of South and East Asian descent because risk rises at a lower index.
- Children and adolescents need age- and sex-specific BMI percentiles, not adult categories.
- Muscle is denser than fat, so gaining strength can raise your BMI while improving your health.
- A single reading matters less than the trend. Track over months rather than reacting to daily fluctuation.
Frequently asked questions
What is a healthy BMI range?+
For adults, 18.5 to 24.9 is classed as normal weight. Lower thresholds apply for people of Asian descent, where health risk begins to rise from around 23.
Why does BMI say I am overweight when I am fit?+
Because it cannot tell muscle from fat. Muscle is denser, so a muscular person weighs more for their height and registers a higher BMI despite low body fat. Body fat percentage is the more relevant measure.
Is BMI accurate for older adults?+
Less so. Muscle mass declines and height can decrease with age, so BMI may understate body fat. Some research suggests a slightly higher BMI is protective after 65, and waist measurement is more informative.
What should I use instead of BMI?+
Waist circumference and waist-to-height ratio for fat distribution, body fat percentage for composition, and blood pressure, blood glucose, and lipids for actual metabolic health. BMI is a screen, not an assessment.