Ideal Body Weight (IBW) formulas are relics of mid-20th century medicine that refuse to die because they remain useful — in clinical settings — for calculating drug doses. Devine's formula (1974) is the most cited: men = 50 kg + 2.3 kg per inch over 5 feet; women = 45.5 kg + 2.3 kg per inch over 5 feet. It wasn't developed for weight loss goals but for gentamicin dosing, and hospitals worldwide still use it that way.

The problem is that IBW formulas were derived decades ago on smaller, less muscular populations. Applied to modern adults, they tend to underestimate healthy weight — a 6-foot man returns 78 kg (172 lb) from Devine, which is at the lower end of normal BMI. Newer formulas — Robinson (1983), Miller (1983) — return slightly higher numbers, and the four-formula average gives a reasonable center of the healthy BMI range.

Frame size adjustment matters. Elbow breadth (measured with calipers), wrist circumference (with tape), or the height-to-wrist ratio classifies you as small, medium or large frame. Small frames sit 10% under IBW comfortably; large frames sit 10% above. A 6-foot man with a 6.5" elbow breadth (large frame) has an adjusted IBW of ~86 kg, not 78 kg. Skipping frame adjustment is why so many muscular people get told they're 'overweight.'

In pharmacology, IBW drives Adjusted Body Weight (ABW) for obese patients: ABW = IBW + 0.4 × (actual − IBW). Used for drugs that distribute partially into adipose tissue — vancomycin, tobramycin, some anesthetics. Total body weight would overdose these patients; IBW alone would underdose. ABW splits the difference based on the drug's distribution.

For personal use — 'what should I weigh?' — IBW is the wrong tool. Modern medicine uses the healthy BMI range (18.5–24.9) as the target zone, adjusted for age, muscle mass, waist circumference and metabolic health markers. IBW gives a single number that pretends to be an answer. The healthy range gives a band you can live inside comfortably for decades. Use IBW when your prescription requires it; use the healthy range when planning your own body composition goals.