What is BMI?
BMI stands for Body Mass Index. It is a single number that compares your weight to your height using a simple formula: your weight in kilograms divided by your height in metres squared (BMI = kg ÷ m²). Because it needs only two easy measurements, it is the most widely used screening tool for weight-related health risk.
BMI does not measure body fat directly and it cannot diagnose anything on its own. What it does well is flag whether your weight is likely too low, healthy, or too high for your height across a large population — a useful first signal worth acting on.
BMI categories
The World Health Organization defines these adult categories:
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obese | 30 and above |
The limitations of BMI
BMI is fast and free, but it is blunt. It cannot tell muscle from fat, so a lean, muscular athlete may read as “overweight” while carrying very little fat. It also says nothing about where fat sits — waist circumference is a better marker of cardiometabolic risk.
Standard BMI categories also do not apply to children and teenagers, who are assessed on age- and sex-specific percentile charts, and BMI is not valid during pregnancy. Older adults and some ethnic groups may have different healthy thresholds too. Use BMI as one data point, not the whole picture.
What to do with your number
If your BMI sits outside the healthy range, the lever that actually moves it is your daily energy balance. Work out your maintenance calories with the TDEE calculator, then set a sensible target using the calorie deficit calculator. Swoodie takes it from there — scan a plate or a barcode to log calories and macros, and follow a weight-loss plan built around foods you actually enjoy.
Frequently asked questions
What is BMI?
BMI (Body Mass Index) is a simple ratio of your weight to your height: weight in kilograms divided by height in metres squared. It is a quick screening tool to gauge whether your weight sits in a healthy range for your height across a population.
What is a healthy BMI?
For most adults a BMI between 18.5 and 24.9 is classed as the healthy range. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. This calculator also shows the weight range that keeps your BMI in the healthy band.
Is BMI accurate for athletes and muscular people?
Not always. BMI uses only height and weight, so it cannot tell muscle from fat. A muscular athlete can read as 'overweight' despite low body fat. Treat BMI as one rough signal alongside waist measurement, body composition, and how you feel.
How do I calculate BMI from pounds and inches?
Convert first: kilograms = pounds ÷ 2.205, and metres = inches × 0.0254. Then divide kilograms by metres squared. This calculator handles the conversion automatically when you switch to imperial units.
Should I use BMI for children or during pregnancy?
No. Standard adult BMI categories do not apply to children and teens, who use age- and sex-specific percentile charts, and BMI is not a valid measure during pregnancy. Speak to a healthcare professional for guidance in those cases.
This tool provides a general screening estimate for healthy adults and is not medical advice. BMI does not apply to children, pregnancy, or some clinical situations — consult a doctor or registered dietitian about your individual health.