Math, finance, health: find the calculator that fits you
Last updated: May 25, 2026
Body Mass Index (BMI) Calculator
How is BMI calculated?
Divide your weight in kg by the square of your height in metres: BMI = weight / (height x height). Example: 70 kg, 1.75 m → 70 / 3.0625 = 22.9. Normal range: 18.5-24.9.
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What Is Body Mass Index (BMI) and How to Calculate It
The Body Mass Index (BMI) is a parameter used to assess whether a person's weight is appropriate for their height. This free online BMI calculator allows you to instantly calculate your body mass index and discover which category you fall into: underweight, normal weight, overweight, or obese.
How Is BMI Calculated?
The BMI formula is simple: divide your weight (in kilograms) by the square of your height (in meters). In mathematical terms:
BMI = Weight (kg) ÷ Height² (m²)
For example, a person weighing 70 kg and standing 1.75 m tall will have a BMI of: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 (normal weight).
WHO BMI Category Table
The World Health Organization (WHO) has defined the following weight categories based on BMI values:
Category
BMI (kg/m²)
Health Risk
Severe Thinness
< 16.0
High
Underweight
16.0 – 18.4
Moderate
Normal Weight
18.5 – 24.9
Low
Overweight
25.0 – 29.9
Increased
Obesity Class I
30.0 – 34.9
High
Obesity Class II
35.0 – 39.9
Very High
Obesity Class III
≥ 40.0
Extremely High
What Do the BMI Calculator Results Mean?
In addition to the BMI value, our calculator provides additional information:
Weight category: indicates whether you are underweight, normal weight, overweight, or obese according to WHO guidelines
Healthy BMI range: shows the BMI range considered healthy (18.5-24.9)
Ideal weight: estimate of the optimal weight for your height
BMI Prime: ratio between your BMI and the overweight threshold (25). Between 0.74 and 1 indicates normal weight, below 0.74 underweight, 1 or more overweight
Ponderal Index (PI): alternative to BMI that considers height cubed, more accurate for very tall or short people
BMI Limitations: When It's Not Reliable
BMI is a useful screening tool but has important limitations:
Doesn't distinguish fat mass from lean mass: athletes and bodybuilders may be classified as "overweight" despite having low body fat. For a more accurate assessment, use our body fat calculator
Doesn't consider fat distribution: abdominal fat is more dangerous than hip fat
Variations by age and sex: women tend to have more body fat than men at the same BMI
Ethnic differences: some populations have different risks at the same BMI
Not suitable for children and adolescents: age-specific percentiles are used for minors
BMI and Health: Why It's Important to Monitor It
Maintaining a BMI in the normal weight range (18.5-24.9) is associated with a lower risk of numerous conditions:
Cardiovascular diseases (heart attack, stroke)
Type 2 diabetes
High blood pressure
Certain types of cancer
Joint problems and osteoarthritis
Sleep apnea
If your BMI indicates overweight or obesity, consider consulting a doctor or nutritionist. To start monitoring your diet, you can use our calorie calculator to estimate daily caloric needs.
How to Achieve and Maintain a Healthy Weight
To achieve a healthy BMI, it's essential to combine:
Frequently Asked Questions About BMI and the Calculator
What is the ideal BMI for an adult?
According to the WHO, the ideal BMI for an adult is between 18.5 and 24.9 kg/m². This range is associated with the lowest risk of chronic diseases. However, the "optimal" value may vary slightly based on age, sex, and body composition. Some studies suggest that a BMI between 20 and 22 is associated with maximum longevity.
Is BMI accurate for athletes and bodybuilders?
No, BMI is not accurate for athletes and bodybuilders. Since it doesn't distinguish between fat mass and muscle mass, very muscular people may be classified as "overweight" or "obese" according to BMI despite having very low body fat percentage. For these individuals, it's more appropriate to directly measure body fat percentage.
How is BMI calculated for children?
In children and adolescents (ages 2-18), BMI is calculated with the same formula as adults, but the interpretation is different. Age and sex-specific growth percentiles are used, comparing the child's BMI with that of peers. A BMI at the 85th-95th percentile indicates overweight, above the 95th percentile indicates obesity. Consult a pediatrician for proper evaluation.
How often should I check my BMI?
For most people, checking BMI once a month is sufficient to monitor weight changes. If you're following a weight loss or muscle gain program, you can check it every 1-2 weeks. Avoid weighing yourself too frequently (every day) because daily weight fluctuations can be misleading.
What is the difference between BMI and body fat?
BMI is a mathematical ratio between weight and height, it doesn't directly measure body fat. Body fat (or body fat percentage) indicates the actual amount of adipose tissue. A person can have a normal BMI but a high fat percentage (skinny fat), or vice versa. For a complete assessment, it's advisable to measure both parameters.
Does BMI change with age?
Yes, BMI tends to increase with age for several reasons: loss of muscle mass (sarcopenia), slowing metabolism, and redistribution of body fat. Some experts suggest that for the elderly (over 65) a slightly higher BMI (23-27) might be associated with better health outcomes. However, WHO guidelines remain the same for all adult ages.
What does "BMI Prime" mean in the results?
BMI Prime is the ratio between your BMI and the threshold value of 25 (upper limit of normal weight). A BMI Prime of 1.0 means you are exactly at the threshold. Values below 1.0 indicate normal weight or underweight, values above indicate overweight. It's useful for understanding how far you are from the threshold: for example, a BMI Prime of 1.2 means your BMI is 20% above the threshold.
Is the BMI calculator free and safe?
Yes, our BMI calculator is completely free and can be used without limits. All calculations are performed locally in your browser: your data (weight, height, age) is not sent to any server and remains completely private. We don't require registration or collect personal information.
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