What is BMI?
Body Mass Index is a number calculated from your weight and height. It was designed to screen large groups of people, and doctors still use it as a simple first check because it needs no equipment beyond a scale and a tape measure.
BMI does not measure body fat directly and it cannot tell the difference between muscle and fat. It is best treated as one piece of information alongside waist size, activity, diet, blood pressure and other measures a healthcare professional might look at.
How BMI is calculated
Example: someone who is 175 cm (1.75 m) tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
FitNumbra converts imperial measurements to metric before calculating, so both give the same result. We also show BMI Prime — your BMI divided by 25, the upper limit of the healthy range. A BMI Prime of 1.00 or less means you are at or below that limit.
BMI categories for adults
FitNumbra uses the World Health Organization classification for adults aged 18 and over:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy 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 |
BMI for children and teenagers
Adult categories do not apply under 18. For children and teens, BMI is compared with growth charts for their age and sex and reported as a percentile. If you enter an age under 18, FitNumbra shows the BMI number but not an adult category. A paediatrician or school nurse can interpret a child’s BMI properly.
Limitations of BMI
- Muscle vs fat: muscular people can have a “high” BMI with low body fat.
- Age: older adults can lose muscle and have more body fat at the same BMI.
- Fat distribution: BMI does not show where fat is stored. Fat around the waist is linked with more health risk than fat elsewhere.
- Ethnicity: risk at a given BMI differs between populations.
- Pregnancy: BMI is not meaningful during pregnancy.
What to do with your result
A BMI outside the healthy range is a reason to look more closely, not a diagnosis. If you are concerned about your weight, a doctor or registered dietitian can assess your overall health and help you set a realistic plan. You can also use the healthy weight calculator to see the weight range for your height, or the calorie calculator to estimate your daily energy needs.
Frequently asked questions
Is BMI accurate?
BMI is a reasonable screening tool for groups of adults, but it is less accurate for individuals because it cannot distinguish muscle from fat or show where fat is stored. Use it alongside other measures such as waist size.
What is a healthy BMI?
For most adults, a BMI of 18.5 to 24.9 is classed as a healthy weight by the World Health Organization. Some populations are advised to use lower thresholds.
Is BMI different for men and women?
The adult BMI formula and categories are the same for men and women. Women tend to have more body fat than men at the same BMI, which is one of the limitations of BMI.
Can I use this BMI calculator for my child?
You can calculate the number, but adult categories do not apply under 18. Children’s BMI must be compared with age- and sex-specific growth charts, so ask a healthcare professional to interpret it.
Why is my BMI high when I am muscular?
BMI only uses height and weight. Muscle is dense, so athletes and people who strength-train can have a BMI in the overweight range while having low body fat.
Methods and sources
- World Health Organization — Obesity and overweight fact sheet (BMI classification) (opens in a new tab)
- U.S. Centers for Disease Control and Prevention (CDC) — Adult BMI categories and BMI-for-age growth charts
Content reviewed: by the FitNumbra editorial team.