Body Mass Index Calculator
Our human physiology calculator computes body mass index accurately. Enter measurements for results with formulas and error analysis.
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist
Medical disclaimer: This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Results are general estimates and may not reflect your individual circumstances. Always consult a qualified healthcare professional before making decisions about your health.
Body Mass Index Calculator
Calculator
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Formula: BMI = weight (kg) / height (m)^2
Worked example — BMI = 22.5 (Normal Weight, low health risk)
Formula
BMI = weight (kg) / height (m)^2
BMI is calculated by dividing body weight in kilograms by the square of height in meters. WHO categories: Underweight < 18.5, Normal 18.5-24.9, Overweight 25-29.9, Obese I 30-34.9, Obese II 35-39.9, Obese III 40+. BMI Prime = BMI / 25 (values above 1.0 indicate overweight).
Worked Examples
Example 1: Normal Weight BMI Calculation
Problem:Calculate BMI for a 65 kg person who is 170 cm tall.
Solution:BMI = weight / height^2 BMI = 65 / (1.70)^2 BMI = 65 / 2.89 BMI = 22.5 Category: Normal Weight (18.5-24.9)
Result:BMI = 22.5 (Normal Weight, low health risk)
Example 2: Overweight BMI with Ideal Weight
Problem:A 90 kg person is 175 cm tall. Calculate BMI and how much weight to reach normal range.
Solution:BMI = 90 / (1.75)^2 = 90 / 3.0625 = 29.4 Category: Overweight (25-29.9) Ideal weight upper limit: 24.9 x (1.75)^2 = 24.9 x 3.0625 = 76.3 kg Weight to lose: 90 - 76.3 = 13.7 kg
Result:BMI = 29.4 (Overweight), need to lose 13.7 kg to reach normal range
Example 3: Normal-range BMI example
Problem:A person weighs 65 kg and is 170 cm tall.
Solution:BMI = 65 / 1.70^2 = 22.5. That falls in the normal-weight range and supports a healthy-weight estimate for this height.
Result:BMI: 22.5 | Category: Normal Weight
Frequently Asked Questions
What is Body Mass Index (BMI)?
Body Mass Index is a numerical value calculated from a person's weight and height using the formula BMI = weight(kg) / height(m)^2. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the WHO as a simple screening tool for categorizing weight status. BMI ranges are: underweight (below 18.5), normal weight (18.5-24.9), overweight (25-29.9), and obese (30+). While BMI is widely used due to its simplicity, it does not directly measure body fat and has known limitations for athletes, elderly individuals, and different ethnic populations.
What are the limitations of BMI?
BMI has several important limitations. It does not distinguish between muscle mass and fat mass, so muscular athletes often have high BMIs despite being lean and healthy. It does not account for fat distribution, which is more important than total fat — abdominal fat is more dangerous than subcutaneous fat. BMI categories were developed primarily from European populations and may not apply equally to Asian, African, or other ethnic groups. Age and sex affect the relationship between BMI and body fat. For these reasons, BMI should be used alongside other measurements like waist circumference, body fat percentage, and waist-to-height ratio.
What is the waist-to-height ratio and why is it better than BMI alone?
The waist-to-height ratio (WHtR) is calculated by dividing waist circumference by height. A WHtR below 0.5 is considered healthy for both men and women of all ages and ethnicities. This simple metric has been shown in multiple studies to be a better predictor of cardiovascular disease, type 2 diabetes, and mortality than BMI alone. This is because WHtR captures central adiposity (belly fat), which is metabolically more dangerous than peripheral fat. The rule of thumb is simple: keep your waist to less than half your height.
How is body fat percentage estimated from BMI?
The Deurenberg formula estimates body fat from BMI: Body Fat % = (1.20 x BMI) + (0.23 x Age) - 16.2 for males, or - 5.4 for females. This formula explains about 80% of the variance in actual body fat as measured by underwater weighing or DEXA scans. However, it has limitations: it overestimates body fat in muscular individuals and underestimates it in the elderly. Healthy body fat ranges are typically 8-19% for men and 21-33% for women, though these vary by age. Direct measurement methods like DEXA, hydrostatic weighing, or bioelectrical impedance provide more accurate results.
What is the ideal weight range?
The ideal weight range corresponds to a BMI of 18.5 to 24.9, which is associated with the lowest health risks in population studies. For a person 170 cm tall, this range is approximately 53.5 to 72.0 kg. However, optimal weight depends on many individual factors including muscle mass, bone density, body frame, age, and overall health markers like blood pressure, cholesterol, and blood sugar. Some research suggests that a slightly higher BMI (25-27) may be associated with lower mortality in people over 65. The ideal weight concept should be personalized with input from healthcare professionals.
What is the difference between mass and weight?
Mass measures the amount of matter in an object (kilograms) and is constant everywhere. Weight is the gravitational force on that mass (newtons) and varies by location. On Earth, a 1 kg object weighs about 9.8 newtons. In everyday use, the terms are often used interchangeably.
References
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist · Editorial policy
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