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How BMI is calculated
BMI divides your weight by the square of your height:
- Metric: weight in kg ÷ (height in m)²
- Imperial: 703 × weight in lb ÷ (height in inches)²
For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
The formula is the same for everyone: there’s no adjustment for sex, and the Calorix app uses this exact formula wherever it shows BMI.
Adult BMI categories (20 and over)
These are the categories used by the CDC and the World Health Organization. They’re the same for women and men:
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to under 25 | Healthy weight |
| 25 to under 30 | Overweight |
| 30 to under 35 | Obesity, class 1 |
| 35 to under 40 | Obesity, class 2 |
| 40 or more | Obesity, class 3 |
The calculator rounds your BMI to one decimal place and uses that value to pick the category, so what you see always matches the label. It also shows the weight range that corresponds to a BMI of 18.5 to 24.9 at your height: at 175 cm, that’s about 57 to 76 kg.
BMI for children and teens (2 to 19)
Children’s bodies change as they grow, and boys and girls grow differently, so a child’s BMI is compared with children of the same sex and age. The CDC uses these percentile categories:
| BMI-for-age percentile | Category |
|---|---|
| Below the 5th | Underweight |
| 5th to under the 85th | Healthy weight |
| 85th to under the 95th | Overweight |
| 95th or above | Obesity |
Add an age between 2 and 19 and a sex in the calculator to see the percentile. It uses the full set of CDC 2000 growth chart parameters, month by month. A few details:
- Months matter. If you enter only years, the calculator uses the middle of that year of age.
- “Other” uses the average of the boys’ and girls’ charts.
- The healthy range shown is the weight between the 5th and 85th percentiles at that age and height.
Adult cut-offs can mislabel teens in both directions. A 13-year-old girl with a BMI of 17 is in the healthy range (around the 20th percentile), even though the adult cut-offs would call that underweight. A 13-year-old boy with a BMI of 23.5 is around the 90th percentile, which is overweight for his age, even though the adult cut-offs would call it healthy.
What BMI can’t tell you
The CDC describes BMI as a screening measure: one potential health indicator to consider with other factors. It has well-known limits:
- Muscle and fat look the same to BMI. Athletes and very muscular people can score high with little body fat.
- Body fat differs at the same BMI. In body-composition research, sex, age and ethnicity all predict body fat independently of BMI; at the same BMI, women tend to carry more body fat than men.
- Risk thresholds differ between populations. A WHO expert consultation found that many Asian populations face higher risks of type 2 diabetes and cardiovascular disease at BMIs below 25, and suggested additional public health action points at 23, 27.5, 32.5 and 37.5, while keeping the standard categories for international comparison.
- Not used in pregnancy. BMI isn’t used to judge weight during pregnancy.
If a BMI result worries you, a doctor or, for a child, a pediatrician can look at the whole picture.
Toasty’s view on BMI
You’ll notice Toasty looks the same whatever the result. That’s deliberate: in Calorix, Toasty celebrates habits like logging meals, drinking water and showing up, and never comments on bodies or weight. BMI is information you can use; it isn’t a grade.
Where to go next
If you’d like to change your weight, the calorie deficit calculator builds a plan with a safe weekly pace for adults, and the TDEE calculator shows the calories you burn each day.
Updated October 9, 2026
