BMI Calculator
Enter your height and weight in metric or US/UK units to get your Body Mass Index, the matching adult weight category and health-risk level, the normal BMI range, and your personal normal weight range — all computed in this browser tab.
Enter your height and weight (optionally your age), then press Calculate. Results appear here.
How it works: BMI = kg ÷ m² (metric) or lb ÷ in² × 703 (US/UK), rounded to one decimal; the category (Normal = 18.5–24.9) is read off the rounded value. All math happens in this browser tab — nothing is uploaded, stored, or shared. BMI is a screening number, not a diagnosis; it does not measure body fat directly, so discuss results with a clinician rather than treating the cutoffs as a verdict.
Pick units and enter measurements
Choose Metric (height in cm, weight in kg) or US/UK (height in feet and inches, weight in pounds). Age is optional — leave it out to use the adult categories.
Press Calculate
The BMI number is computed from the standard formulas and rounded to one decimal. Every field must be a positive number inside the documented ranges, or the calculator tells you exactly what to fix.
Read the full result
You get the BMI, the adult category and health-risk level, the fixed normal BMI range (18.5–24.9), and the weight range that range implies for your height.
BMI categories for adults (age 20 and above)
| BMI (kg/m²) | Category | Health risk |
|---|---|---|
| 18.4 or below | Underweight | Malnutrition risk |
| 18.5 – 24.9 | Normal weight | Low risk |
| 25 – 29.9 | Overweight | Enhanced risk |
| 30 – 34.9 | Moderately obese | Medium risk |
| 35 – 39.9 | Severely obese | High risk |
| 40 or above | Very severely obese | Very high risk |
BMI categories for kids (age under 20)
| BMI percentile | Category |
|---|---|
| Less than 5th percentile | Underweight |
| 5th to less than 85th percentile | Normal weight |
| 85th to less than 95th percentile | Overweight |
| 95th percentile and greater | Obese |
For ages under 20, BMI is judged against age-and-sex growth percentiles, not the fixed adult numbers. This calculator computes the BMI value for any age but does not look up percentiles, so it reports no category under 20 rather than applying the adult cutoffs.
BMI formulas
Metric: BMI in kg/m² equals mass in kilograms divided by the square of height in meters — BMI = kg ÷ m².
US/UK: BMI in kg/m² equals mass in pounds divided by the square of height in inches, times 703 — BMI = lb ÷ in² × 703.
How this calculator works
- How is the category chosen?
- The BMI is rounded to one decimal, and the category is read off that displayed value in the adult table above — so the number and the category you see can never disagree.
- What is 'your normal weight range'?
- It is the weight window implied by your height alone: 18.5 to 24.9 kg/m² converted back to kg or lb (for example, at 170 cm it is 53.5–72.0 kg). It does not depend on the weight you entered.
- Why is age optional, and what happens under 20?
- Leave age empty and the adult (20 and above) categories are used, with a note saying so. Enter an age under 20 and you get the BMI value plus a clear percentile-required notice — kids and teens need age-and-sex growth-percentile charts that this tool does not ship, so no category is guessed.
- What happens to values typed in the other unit system?
- They are ignored. Only the selected unit system’s fields are read — typing in hidden fields cannot mix into your result.
- Is BMI a health diagnosis?
- No. BMI is a screening ratio of weight to height; it does not measure body fat, muscle mass, or distribution, and it behaves differently across ages and body types. Treat the cutoffs as a prompt to talk with a clinician, not a verdict.
Privacy: this calculator runs entirely in your browser tab. Your height, weight, age, and results are never uploaded, stored, or shared.