What Is BMI?
Body Mass Index, or BMI, is a calculation that compares your weight with your height. For adults, it is commonly used as a simple screening measure to group BMI values into categories such as underweight, healthy weight, overweight and obesity.
How BMI is calculated
In metric units, BMI is your weight in kilograms divided by your height in metres squared. In imperial units the same relationship is expressed in pounds and inches using a conversion factor of 703. Both formulas are used by this calculator depending on the units you choose, and both produce the same BMI for the same body.
Metric: BMI = weight (kg) / height (m)^2
Imperial: BMI = 703 x weight (lb) / height (in)^2
If you enter feet and inches, the height is converted to total inches first. Nothing else goes into the calculation: age, sex and activity level do not affect an adult BMI figure.
BMI Categories for Adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5-24.9 | Healthy weight |
| 25.0-29.9 | Overweight |
| 30.0-34.9 | Obesity - Class 1 |
| 35.0-39.9 | Obesity - Class 2 |
| 40.0 or higher | Obesity - Class 3 |
These ranges are used for adults. BMI assessment for children and teenagers uses age- and sex-specific growth references instead.
The same numbers you get in the app
The BMI shown on this page is calculated the same way as in the GATA app: the same formula and the same adult category thresholds. Entering the same height and weight in either place gives you the same reading, aside from small differences from rounding or the units you enter.
The difference is what happens next. This page gives you one reading today, while the app keeps your weight history so you can see the trend and pairs it with a daily calorie and protein target.
What BMI does not tell you
BMI does not distinguish muscle from body fat. Because it uses only height and weight, a muscular person and a person with more body fat can share the same BMI while having very different body compositions.
It was designed to describe populations rather than individuals. As a quick screen across a large group it works well; applied to one person it can be misleading, which is why it should be read alongside other information rather than on its own.
It also says nothing about where body fat sits, and nothing about diet, fitness, blood markers or sleep. Pregnancy, growth during childhood and adolescence, and some health conditions require different approaches to evaluating weight. If you are concerned about your weight or health, discuss your individual situation with a qualified healthcare professional.
BMI vs Calorie Needs
BMI and daily calorie needs answer different questions. BMI compares your weight with your height. Your calorie needs estimate how much energy your body uses each day.
If your goal is to maintain, lose or gain weight, your estimated daily energy needs are usually more useful for setting a calorie target than BMI alone.
Frequently Asked Questions
Where to go from here
If you want to act on your BMI reading, the next step is a daily calorie target and a way to see what you actually eat. GATA logs meals from a photo, recognises a full plate as separate foods with their own calories, and updates your daily target as your weight changes. GATA AI Pro is a subscription; new users can start with a 3-day free trial, once per user. There is no free plan.
- Daily calorie calculator
- GATA: AI calorie tracker and weight loss app
- Calorie calculator for women: daily calories by age, activity and goal
- Maintenance calorie calculator: how many calories do you need to maintain weight?
- Best protein tracker app: track protein and macros from one photo
- Am I eating okay? 7 signs your eating pattern is working