Understanding Ideal Body Weight (IBW) Formulas
Ideal Body Weight is a clinically derived target weight based solely on height and sex — originally developed not for aesthetics, but for medication dosing. Knowing your IBW helps pharmacists calculate doses for antibiotics, anaesthetics, and chemotherapy agents that distribute into lean tissue rather than fat.
No single IBW formula is universally "correct." Each was derived from a different population sample and era. The clinical consensus is to use the Devine formula for drug dosing, and to treat the range across all four formulas as a realistic healthy weight window.
The Four IBW Formulas Compared
| Formula | Year | Men (base + per inch over 5 ft) | Women (base + per inch) | Originally Developed For |
|---|---|---|---|---|
| Devine | 1974 | 50 + 2.3 kg | 45.5 + 2.3 kg | Gentamicin dosing |
| Robinson | 1983 | 52 + 1.9 kg | 49 + 1.7 kg | General clinical use |
| Miller | 1983 | 56.2 + 1.41 kg | 53.1 + 1.36 kg | Tidal volume (mechanical ventilation) |
| Hamwi | 1964 | 48 + 2.7 kg | 45.5 + 2.2 kg | Diabetes nutrition counselling |
Formula Details
Devine Formula (1974) — Pharmacokinetic Gold Standard
The most cited IBW formula worldwide. Used for aminoglycoside antibiotics, adjusted body weight, creatinine clearance (Cockcroft-Gault), and mechanical ventilator tidal volume settings.
Robinson Formula (1983)
Slightly higher base weight than Devine; derived from Metropolitan Life Insurance actuarial data. Often used in nutrition assessments.
Miller Formula (1983)
Highest base weight of the four; produces the most generous IBW estimates. Most appropriate for individuals with larger skeletal frames.
Hamwi Formula (1964) — Frame-Adjusted
The only formula that explicitly adjusts for skeletal frame size. Small frames subtract 10%; large frames add 10%. Widely used in clinical dietetics and diabetes education.
IBW vs. WHO Healthy Weight Range
IBW formulas produce a single target number derived from height and sex — they do not account for body composition, age, ethnicity, or muscle mass. The WHO healthy weight range (BMI 18.5–24.9) is broader and accounts for natural variation in healthy body weights.
For practical use: the IBW formulas' consensus range and the WHO range typically overlap significantly. The overlap zone is the most defensible healthy weight target.
| Height | Devine IBW (M/F) | WHO Healthy Range (M/F) |
|---|---|---|
| 160 cm | 54.5 / 50.0 kg | 47.4 – 63.8 kg |
| 170 cm | 66.3 / 61.8 kg | 53.5 – 72.0 kg |
| 180 cm | 78.0 / 73.5 kg | 59.9 – 80.6 kg |
Clinical Limitations of IBW
- Not validated for obesity: All four formulas were derived from average-weight populations. Using IBW directly for very obese patients can underestimate clinical needs — use Adjusted Body Weight in those cases.
- Ignores age: An 70-year-old and a 25-year-old of the same height get the same IBW, despite profoundly different body composition norms.
- Not a body fat measure: A person exactly at their IBW may still have poor body composition (high fat, low muscle) or excellent composition (low fat, high muscle).