Human Body Mass Calculator: BMI & Ideal Weight

This calculator works out your Body Mass Index (BMI) and estimated ideal body weight range based on your height and weight. You enter your height in centimeters and your weight in kilograms, and you get your BMI value, the corresponding weight category, and a healthy weight range for your height.

Formula

The Body Mass Index (BMI) is calculated as weight in kilograms divided by the square of height in meters:

BMI = weight (kg) / [height (m)]²

Where:

• weight is your body mass in kilograms (kg).
• height is your body length in meters (m). If you measure in centimeters, divide by 100 first.

The result is expressed in kg/m². The World Health Organization (WHO) classifies adult BMI as follows:

• Below 18.5: Underweight
• 18.5 – 24.9: Normal weight
• 25.0 – 29.9: Overweight
• 30.0 and above: Obesity

The healthy weight range is derived by solving the BMI formula for weight using the lower and upper bounds of the normal category (18.5 and 24.9).

Worked Example

Using the default values: height = 170 cm, weight = 70 kg.

1. Convert height to meters: 170 cm ÷ 100 = 1.70 m
2. Square the height: 1.70 × 1.70 = 2.89 m²
3. Divide weight by height squared: 70 ÷ 2.89 = 24.2 kg/m²
4. The BMI is 24.2, which falls in the “Normal� category.
5. Healthy weight range: 18.5 × 2.89 = 53.5 kg (lower bound) and 24.9 × 2.89 = 72.0 kg (upper bound).

So a person 170 cm tall with a weight of 70 kg has a BMI of 24.2, which is within the normal range, and a healthy weight range for that height is about 53.5 to 72.0 kg.

When to Use This Calculator

Use this calculator to get a quick, standardized estimate of whether your weight is in a range associated with lower health risks for most adults. It is commonly used by doctors as a screening tool, by individuals tracking fitness goals, and by researchers studying population health. The BMI is especially useful because it requires only height and weight, which are easy to measure, and it correlates well with body fat percentage for most people.

However, BMI is not a direct measure of body fat. It does not distinguish between muscle and fat mass, nor does it account for fat distribution, age, sex, or ethnicity. For example, a muscular athlete may have a high BMI but low body fat, while an older adult with low muscle mass could have a normal BMI but excess fat. Therefore, BMI should be used as a general guide, not as a definitive diagnosis.

How to Read the Results

Your BMI value places you into one of four categories. The healthy range (18.5–24.9) is associated with the lowest risk of weight-related diseases such as type 2 diabetes, hypertension, and heart disease. If your BMI is below 18.5, you may be underweight, which can indicate malnutrition or other health issues. A BMI between 25 and 29.9 suggests overweight, and a BMI of 30 or higher indicates obesity, which increases health risks significantly.

The healthy weight range shown gives you a target interval for your height. For example, if you are 170 cm tall, staying between 53.5 kg and 72.0 kg keeps your BMI in the normal zone. Keep in mind that the lower end of the range may be too thin for some body frames, and the upper end may be too heavy for others.

Common Mistakes and Limitations

• Unit mix-ups: The formula requires height in meters and weight in kilograms. If you enter height in centimeters without dividing by 100, your BMI will be off by a factor of 10,000. Always convert units first.
• Rounding too early: If you round the height squared before dividing, you may get a slightly different result. Do the full calculation first, then round to one decimal place.
• Not accounting for muscle: BMI can overestimate body fat in athletes and underestimate it in older adults or those with low muscle mass. For a more accurate body composition, consider waist circumference, skinfold measurements, or bioelectrical impedance.
• Ignoring age and sex: Children and adolescents use different BMI percentiles, and older adults may have different healthy ranges. This calculator is intended for non-pregnant adults.

Alternatives to BMI

While BMI is the most common screening tool, other methods provide a more complete picture of body composition:

• Waist-to-hip ratio: Measures fat distribution. A ratio above 0.90 for men or 0.85 for women indicates abdominal obesity.
• Waist circumference: High values (above 102 cm for men, 88 cm for women) indicate excess abdominal fat.
• Body fat percentage: Measured via skinfold calipers, bioelectrical impedance scales, or DEXA scans. Healthy ranges differ by sex and age.
• Muscle mass and bone density: Useful for athletes and older adults to track changes in body composition.

For most people, combining BMI with waist circumference gives a better risk assessment than either alone.

Frequently Asked Questions

Is BMI accurate for athletes?

No, BMI can be misleading for athletes because it does not distinguish between muscle and fat. Muscle is denser than fat, so a muscular athlete may have a high BMI (over 25) yet have a very low body fat percentage. In such cases, body fat percentage or waist circumference is a better indicator.

What is a healthy BMI for a 50-year-old woman?

The same BMI categories apply to all adults, but some experts suggest that a slightly higher BMI (up to 27) may be acceptable for older adults, as a little extra weight can provide reserve during illness. However, the standard healthy range remains 18.5–24.9 for most adults.

Can BMI be too low?

Yes, a BMI below 18.5 is considered underweight and may be associated with nutritional deficiencies, weakened immune function, and osteoporosis. If you are underweight, it is important to consult a healthcare provider to identify the cause.

How do I calculate my BMI in pounds and inches?

If you use pounds and inches, the formula is: BMI = (weight in pounds ÷ (height in inches)²) × 703. For example, a person weighing 150 pounds and 5’6″ (66 inches) tall would calculate: (150 ÷ 4356) × 703 = 24.2. This calculator uses metric units, so you can also convert your measurements to kilograms and centimeters first.

Does BMI apply to children?

For children and teens, BMI is calculated the same way but interpreted using age- and sex-specific percentiles. A BMI below the 5th percentile is underweight, the 5th to 85th percentile is healthy, and above the 95th percentile is obesity. This calculator is only for adults.

What should I do if my BMI is in the overweight or obese range?

A BMI in these ranges indicates a higher risk for chronic diseases. Even modest weight loss (5–10% of body weight) can improve blood pressure, cholesterol, and blood sugar. Focus on a balanced diet, regular physical activity, and consult a healthcare professional for personalized guidance.

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in h | Height | cm | 170 | 0
in w | Weight | kg | 70 | 0
out bmi | BMI | kg/m² | w / ((h/100)^2) | 1
out category | Category | - | (bmi < 18.5 ? "Underweight" : bmi < 25 ? "Normal" : bmi < 30 ? "Overweight" : "Obese") | 0
out min_weight | Healthy min | kg | 18.5 * ((h/100)^2) | 1
out max_weight | Healthy max | kg | 24.9 * ((h/100)^2) | 1