Ideal Weight Range vs Exact Number — Why a Range is Scientifically Sound

Why a single number is scientifically misleading

Every IBW formula produces a single number, but that number is not a prescription — it is a population estimate with substantial individual variation. For a 5'8" woman, the Devine formula yields 56.4 kg (124.4 lb) as a single target. But two women at that height and weight can have vastly different health profiles: one at 18% body fat with high muscle mass, another at 32% body fat with low muscle mass. Both would be 'at ideal weight' by Devine, yet have very different metabolic health, cardiovascular risk, and functional capacity. Research by Janssen et al. (2005) demonstrated that the relationship between BMI and mortality risk follows a J-shaped curve, with the lowest mortality risk across a range of BMI values (20–25), not at any single point. This evidence strongly supports a range-based approach.

The BMI range method: scientifically validated

The World Health Organization defines a healthy BMI range as 18.5–24.9 for adults. This range is not arbitrary — it is derived from extensive epidemiological data linking BMI to all-cause mortality, cardiovascular disease, diabetes, and cancer risk. To calculate your healthy weight range using BMI: Minimum weight = 18.5 × height(m)² and Maximum weight = 24.9 × height(m)². For a person who is 1.73 m (5'8") tall: minimum = 18.5 × 2.99 = 55.4 kg (122 lb), maximum = 24.9 × 2.99 = 74.5 kg (164 lb). This gives a 19 kg (42 lb) healthy range — demonstrating why a single ideal weight number is meaningless for weight management goals.

Individual variation in body composition

The same body weight can represent dramatically different body compositions depending on muscle mass, bone density, and fat distribution. Key sources of individual variation include: Bone density varies by up to 30% between individuals of the same height, contributing 2–4 kg of body weight variation; Muscle mass varies by 10–15 kg between sedentary and athletic individuals of the same height; Frame size (wrist circumference, elbow breadth) accounts for up to 10% of body weight variation independent of height; Visceral vs subcutaneous fat distribution has vastly different health implications at the same body weight; Age-related body composition changes shift the optimal weight range over the lifespan. These variations are why medical organizations universally recommend weight ranges rather than specific targets.

How to calculate your personal ideal weight range

The most practical approach combines BMI ranges with IBW formulas and individual factors. Step 1: Calculate your BMI-based healthy range (BMI 18.5–24.9). For 5'10" (1.78 m): 58.8–78.8 kg (129–174 lb). Step 2: Calculate IBW using multiple formulas (Devine, Hamwi). For a 5'10" man: Devine = 73.8 kg, Hamwi = 71.6 kg. These values typically fall in the middle of the BMI range. Step 3: Adjust for frame size. Small frame: use 90–95% of calculated values. Medium frame: use full values. Large frame: use 105–110% of calculated values. Step 4: Consider body composition. If you have above-average muscle mass, your optimal weight may be at the upper end of the range. If sedentary, aim for the middle to lower end.

Weight management within your range

• Within range: Focus on body composition improvement (more muscle, less fat) rather than scale weight
• Above range: Target weight loss of 5–10% of current body weight, which provides clinically meaningful health benefits even without reaching the range
• Below range: Prioritize healthy weight gain through caloric surplus and resistance training, aiming for the lower end of the range as a first milestone
• Maintenance: Once within range, shift from weight-focused to body composition-focused goals
• Key insight: A 10 lb weight difference within your healthy range is medically insignificant. Body composition at a given weight matters far more than the weight itself.