Ideal body weight calculator

Ideal body weight formulas: clinical tools, not fitness targets

Ideal body weight (IBW) formulas are mathematical relationships between height and a “target” weight. They exist primarily for clinical pharmacology — drug dosing, ventilator settings, and critical care calculations — not for determining healthy weight goals for fitness or aesthetics.

The Devine formula (1974): where it all started

B.J. Devine published a paper in 1974 titled “Gentamicin therapy” that included a simple rule-of-thumb for dosing the antibiotic gentamicin in obese patients. The formulas were:

• Men: 50 kg + 2.3 kg per inch over 5 feet
• Women: 45.5 kg + 2.3 kg per inch over 5 feet

Devine derived these from actuarial data and clinical convention, not from a rigorous population study. They were designed to give a practical number for drug dosing, not to define health or aesthetics. However, the simplicity of the formula led to its widespread adoption in clinical practice over the next 50 years.

Robinson, Miller, Hamwi: variations on a theme

Robinson (1983) and Miller (1983) independently derived similar formulas using actuarial insurance data, producing slightly higher IBW estimates (approximately 5–8% higher than Devine at typical heights). Hamwi (1964) used a rougher approximation:

• Men: 106 lbs + 6 lbs per inch over 5 feet
• Women: 100 lbs + 5 lbs per inch over 5 feet

Hamwi was designed as a mental math shortcut for clinicians to quickly estimate weight in patients without scale access. The four formulas typically agree within 5–10 lbs at average heights but diverge more at extremes.

Adjusted body weight for clinical dosing in obesity

When a patient's actual body weight exceeds IBW by more than 30%, some drugs are dosed using adjusted body weight (ABW):

ABW = IBW + 0.4 × (actual weight − IBW)

The 0.4 coefficient comes from the estimated proportion of excess weight that is metabolically active tissue (lean mass gained alongside fat). This avoids underdosing (using IBW alone ignores the real lean mass increase) and overdosing (using actual weight for drugs that don't distribute into fat).

Why IBW is outdated for personal health goals

These formulas have three fundamental problems for personal use:

• No body composition — the same height and sex produces the same IBW regardless of whether you are 10% body fat or 40% body fat. Two people at identical “ideal weight” can have radically different health profiles.
• Population averages, not optima — the source data reflected 1960s–1970s insurance populations, skewed toward sedentary adults without accounting for athletes or highly muscular individuals.
• Linear at extremes — the formulas behave poorly below 5'0” and above 6'4” because they assume a constant increment per inch across all heights, which doesn't match real body weight scaling.

A more useful framing: body fat percentage targets

For fitness-oriented goals, target a body fat range rather than a scale weight:

• Men: Essential fat: 2–5%. Athletic: 6–13%. Fit: 14–17%. Average: 18–24%. Obese: 25%+.
• Women: Essential fat: 10–13%. Athletic: 14–20%. Fit: 21–24%. Average: 25–31%. Obese: 32%+.

Body recomposition — simultaneously gaining muscle and losing fat through training and adequate protein — can significantly improve health and body composition while the scale weight barely moves. The scale is the wrong instrument for measuring progress toward a body composition goal.