AkCalculators

Understanding BMI and Ideal Weight

Body Mass Index (BMI) and ideal weight estimates are widely used to quickly assess whether a person's weight is appropriate for their height. BMI is a simple ratio of weight to height that classifies individuals into categories associated with differing health risks. Ideal weight formulas (like Devine and Hamwi) provide another, height-based reference point. This article explains how BMI is calculated, how common ideal weight formulas work, their strengths and limitations, and how to interpret results.

How BMI is calculated

BMI uses metric units:

BMI = weight (kg) ÷ [height (m)]²

For imperial units:

BMI = 703 × weight (lb) ÷ [height (in)]²

BMI categories (WHO)

• Underweight: BMI < 18.5
• Normal weight: BMI 18.5–24.9
• Overweight: BMI 25–29.9
• Obesity class I: BMI 30–34.9
• Obesity class II: BMI 35–39.9
• Obesity class III: BMI ≥ 40

Common ideal weight formulas

These formulas give a quick estimate of an "ideal" or "reference" weight for adults. They are heuristic and don't replace clinical judgment.

• Devine formula (1974)
  Men: 50.0 kg + 2.3 kg × (height in inches − 60) Women: 45.5 kg + 2.3 kg × (height in inches − 60)
• Hamwi formula (1964)
  Men: 48.0 kg + 2.7 kg × (height in inches − 60) Women: 45.5 kg + 2.2 kg × (height in inches − 60)

Healthy weight range (using BMI)

A practical way to get a healthy weight range is to calculate the weights that correspond to BMI 18.5 and BMI 24.9 for your height:

Min weight (kg) = 18.5 × height(m)²
Max weight (kg) = 24.9 × height(m)²

Limitations

• BMI doesn't separate muscle from fat — athletic individuals may have high BMI but low body fat.
• Doesn't reflect fat distribution — abdominal fat poses higher risk than peripheral fat.
• Ethnic differences: some populations have higher risk at lower BMIs (e.g., South Asians).
• Not applicable to children — use BMI-for-age percentiles instead.

Practical examples

Example 1: 70 kg, 170 cm → BMI = 24.2 (normal). Devine ideal ≈ 62.0 kg (female example), healthy range ≈ 53.5–72.0 kg.

Example 2: 95 kg, 180 cm → BMI = 29.3 (overweight). Devine ideal ≈ 74.0 kg (male example), healthy range ≈ 59.9–80.8 kg.

How to use the results

Use BMI as a screening tool to determine whether further evaluation is needed. Combine BMI with waist circumference, WHtR, blood pressure, and metabolic markers (lipids, glucose) for a fuller risk profile. Discuss weight goals and strategies with a healthcare provider — personalized goals consider age, sex, medical conditions, and functional status.

Improving BMI and reaching a healthy weight

• Adopt a balanced, sustainable diet focusing on whole foods.
• Increase physical activity — aim for aerobic plus resistance training.
• Manage sleep, stress, and other lifestyle factors that impact weight.
• Seek medical advice for structured programs or medication if BMI is very high or if comorbidities exist.

Disclaimer: This article provides general information and does not replace medical advice. Consult a healthcare professional for personalized recommendations.