BMI (Body Mass Index) is calculated by dividing weight (kg) by height (m) squared. Formula: BMI = weight(kg) ÷ height(m)². It is a widely used international indicator of obesity.

BMI Calculator — Free Body Mass Index Tool

Q: How is BMI classified?

According to the Japan Obesity Society (JASSO), for adults: BMI below 18.5 is Underweight, 18.5–24.9 is Normal weight, 25.0–29.9 is Obese class I, 30.0–34.9 is Obese class II, 35.0–39.9 is Obese class III, and 40 or above is Obese class IV. WHO classifies BMI 25+ as Overweight and BMI 30+ as Obese.

Q: Does high muscle mass give a high BMI?

Yes. BMI is calculated only from weight and height, so it cannot distinguish muscle from fat. Athletes with high muscle mass may appear overweight or obese on the BMI scale despite having low body fat.

Q: How is ideal weight calculated?

This tool uses the formula: Ideal weight (kg) = height(m)² × 22. BMI 22 is the value identified by JASSO as carrying the lowest risk of disease.

Q: Can BMI be used for children and older adults?

This tool is intended for adults aged 18 and over. For children, age- and sex-specific growth charts are used instead. For older adults, sarcopenia (muscle loss) means BMI alone may underestimate health risk; body composition assessment is recommended.

Enter your numbers

Height

Weight

BMI (Body Mass Index)

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Your input is calculated on your device only — nothing is sent externally

10 18.5 25 30 40+

Underweight <18.5 Normal 18.5–24.9 Obese Cl.I 25–29.9 Obese Cl.II+ ≥30

How to Use the BMI Calculator

BMI (Body Mass Index) is a worldwide measure of weight status calculated from height and weight. It is found as BMI = weight(kg) ÷ height(m)² and, being easy to compute, is widely used in health checks. This tool shows your BMI, weight category and ideal weight from just your height and weight.

How to use this tool

Simply enter your height (cm) and weight (kg). The result shows a weight category based on the Japan Society for the Study of Obesity criteria and an ideal weight based on BMI 22, the value associated with the lowest disease risk.

Reading the number with care

Because BMI uses only height and weight, it cannot tell fat from muscle. A well-trained person can show a high “obese” BMI despite low body fat, while someone at a normal weight may still carry excess fat—“hidden obesity” that BMI misses. Treat BMI as just one guide.

Tips

To understand body composition, also use the body fat calculator, and for cutting or bulking plans the TDEE calculator. Children and the elderly need different standards, so this tool is for adults.

Terms & FAQ

What is BMI?

BMI (Body Mass Index) is an international indicator of obesity calculated from weight and height.

Formula: BMI = weight(kg) ÷ height(m)²

Example — height 170 cm, weight 65 kg: 65 ÷ 1.70² ≈ 22.5

BMI is used by the WHO (World Health Organization) and the Japan Obesity Society (JASSO) to assess obesity in adults.

How is BMI classified?

Japan Obesity Society (JASSO) classification (adults, 18 and over):

BMI below 18.5 → Underweight
BMI 18.5–24.9 → Normal weight
BMI 25.0–29.9 → Obese (Class I)
BMI 30.0–34.9 → Obese (Class II)
BMI 35.0–39.9 → Obese (Class III)
BMI 40.0 or above → Obese (Class IV)

Note: WHO classifies BMI 25+ as "Pre-obese (Overweight)" and BMI 30+ as "Obese."

Does high muscle mass give a high BMI?

Yes. BMI is calculated from weight and height only, so it cannot distinguish fat from muscle.

Athletes and experienced lifters with high muscle mass may be classified as overweight or obese even with low body fat.

If you train regularly, assess body fat percentage and muscle mass alongside BMI — for example with an InBody body composition scanner.

How is ideal weight calculated?

This tool uses:

Ideal weight (kg) = height(m)² × 22

BMI 22 is identified by JASSO as the value carrying the lowest disease risk.

The normal weight range (BMI 18.5–24.9) is also shown for reference.

Can BMI be used for children and older adults?

This tool is intended for adults aged 18 and over.

Children under 18: Age- and sex-specific growth charts are used because children are still developing.

Older adults: Sarcopenia (muscle loss) can mean BMI appears normal while body fat is elevated. Body composition assessment is recommended alongside BMI.

Formula & references
· Japan Obesity Society (2022). Obesity Treatment Guideline 2022. Life Science Publishers.
· WHO (2000). Obesity: Preventing and managing the global epidemic. WHO Technical Report Series 894.
· Ideal weight (BMI 22 basis): based on JASSO recommendation

Before you use this tool
The results of this tool are estimates based on the BMI formula. BMI does not directly reflect body fat percentage, muscle mass or bone density. For a detailed health assessment, please consult a doctor or registered dietitian. People with pre-existing conditions, pregnant women or those with an illness should seek medical advice before using this tool. This service is for informational purposes only and is not a substitute for medical or nutritional advice.

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