BMI-Based Ideal Weight vs Formula-Based Ideal Weight

The BMI method: calculating ideal weight from BMI ranges

The BMI method calculates an ideal weight range by working backward from the healthy BMI range (18.5–24.9). Since BMI = weight(kg) / height(m)², you can rearrange to find weight = BMI × height². For a person who is 1.75 m tall: the minimum healthy weight = 18.5 × 1.75² = 56.4 kg (124 lb) and the maximum healthy weight = 24.9 × 1.75² = 75.9 kg (167 lb). The midpoint ideal weight (BMI = 21.7) = 66.0 kg (145.5 lb). This method has three key advantages: it is grounded in large-scale epidemiological data linking BMI to mortality risk, it produces a range rather than a single number (acknowledging individual variation), and it is simple to calculate. Its primary limitation is that BMI, like all height-based methods, cannot distinguish between muscle and fat.

The formula method: traditional IBW equations

Traditional IBW formulas (Devine, Hamwi, Robinson, Miller) calculate a single ideal weight number based on height, gender, and empirically derived coefficients. For a 5'9" (1.75 m) man using the Devine formula: 50 kg + (9 inches × 2.3 kg) = 70.7 kg (155.9 lb). For a 5'5" (1.65 m) woman using Devine: 45.5 kg + (5 inches × 2.3 kg) = 57.0 kg (125.7 lb). The formula method was developed for specific clinical purposes — drug dosing (Devine), nutritional assessment (Hamwi), or anthropometric research (Robinson, Miller). Its advantages include gender-specific estimates that better reflect biological differences, and a single actionable number. Its limitations include: no range (ignoring individual variation), development based on 1960s–1970s population data that may not reflect modern body compositions, and no validation against health outcomes at the individual level.

Side-by-side comparison for the same person

Consider a 5'10" (1.78 m) woman who currently weighs 68 kg (150 lb): BMI method: BMI = 21.6 (within healthy range 18.5–24.9). Ideal weight range = 58.7–78.8 kg (129–174 lb). Midpoint = 68.8 kg (151.7 lb). Result: current weight is right at the midpoint ideal. Formula method (Devine): IBW = 45.5 + (10 × 2.3) = 68.5 kg (151 lb). Result: current weight is essentially at IBW. For this person, both methods agree closely. Now consider a 5'10" man at 85 kg (187 lb): BMI method: BMI = 26.9 (slightly overweight). Healthy range = 58.9–78.9 kg (130–174 lb). His weight is 6.1 kg above the range maximum. Formula method (Devine): IBW = 50 + (10 × 2.3) = 73 kg (161 lb). His weight is 12 kg above IBW. The formula method is stricter and produces a larger deviation from current weight.

Which method is more appropriate?

The answer depends on your purpose. For general health assessment: the BMI method is preferred because it is validated against population-level health outcomes and provides a range that acknowledges individual variation. Major health organizations (WHO, CDC, NHS) use BMI ranges as their primary weight classification tool. For drug dosing: use the Devine formula exclusively — it is the clinical standard and is embedded in all major pharmacological dosing references. For weight management goals: use the BMI method to establish your healthy range, then select a target weight within that range based on how you feel, your body composition, and your fitness level. For nutrition planning: Hamwi or Devine are commonly used to estimate maintenance calories and macronutrient targets. For athletes and muscular individuals: neither method works well — use body fat percentage and FFMI instead.

Combining both methods for the most accurate picture

• Step 1: Calculate your BMI-based healthy weight range (18.5–24.9 BMI)
• Step 2: Calculate your IBW using 2–3 formulas (Devine, Hamwi, Robinson)
• Step 3: Note where the IBW values fall within your BMI range — they should fall in the lower to middle portion
• Step 4: Assess your current body fat percentage to determine if your weight is appropriate for your body composition
• Step 5: Set a target weight range (not a single number) that falls within the BMI range and accounts for your frame size and muscle mass
• Key principle: A 5–10% difference between methods is normal and reflects their different development purposes. Use both as reference points, not as absolute truth.