Reviewed by Arisa Tanaphon, Certified Tai Chi Instructor, Mindful Movement Specialist
Key takeaways
- This BMI calculator is for adults aged 18 and over. BMI is weight in kilograms divided by height in metres squared; it does not directly distinguish muscle from body fat.
- Adult BMI categories are screening ranges, not a diagnosis or an exercise prescription. Age, ethnic background, waist measurement, and medical history can affect interpretation.
- Tai Chi for Seniors & Beginners has standing and chair plans across 5 levels. Choose by balance, mobility, and comfort rather than matching a BMI category to an app level.
- The app’s videos, pause, and adjustable rest help you inspect movements. A completed session records practice, not a change in body composition.
Who this calculator is for
Use this service to check the BMI of an adult aged 18 and over and get information about what to do next.
You should not use this tool to diagnose symptoms. If you are worried about your weight, speak to your local pharmacist or GP surgery.
Who should not use this tool
Do not use this tool if you, or the person you are checking:
- are under 18 years old
- are pregnant
- have been diagnosed with an eating disorder, or think you may have one
- have a condition that affects your height
If you or your child are aged between 2 and 17, use a BMI calculator designed for children and teenagers.
What you will need
To use the calculator, you need:
- height
- weight
- ethnic background information
The calculator asks about ethnic background because some groups, including Asian, Black African, African-Caribbean, and Middle Eastern populations, may have higher health risk at lower BMI values.
BMI Calculator (Adults 18+)
Calculate your body mass index and check your weight category.
Formula: BMI = weight(kg) / height(m)²
This calculator is for adults aged 18+ and is for guidance only.
For adults, BMI uses the same formula for female and male users.
Do not use this tool if you are pregnant, under 18, have (or may have) an eating disorder, or have a condition that affects height.
Your BMI result categories
Your result is shown as a number and one of these categories:
- underweight
- a healthy weight
- overweight
- obese
After your BMI result, you may also be asked to measure your waist. This can help assess whether you are carrying too much weight around your tummy.
How BMI is calculated
BMI is calculated as weight in kilograms divided by height in meters squared: BMI = kg/m².
The formula is simple and useful at population level, but it does not directly measure body fat percentage.
What your result means
A higher BMI is associated with increased risk of long-term conditions such as type 2 diabetes and heart disease.
However, BMI is only one measure of health. It cannot distinguish muscle from fat. For example, people with high muscle mass can be classified as overweight despite low body fat.
That is why waist measurement and clinical context can provide a better picture of overall health risk.
FAQ
No. This calculator is for adults aged 18 and over. Children and teenagers require age-specific BMI interpretation.
No. BMI is a screening measure and cannot diagnose medical conditions on its own.
Some ethnic groups can have elevated health risk at lower BMI values, so thresholds may be adjusted to improve risk guidance.
Use your result as a starting point. If you have concerns, discuss your weight and health with a pharmacist or GP.
References
Updated: April 15, 2026
