What Doctors Mean By Ideal Body Weight (4 Formulas Compared)

If you've ever had a medication dose calculated in a hospital, it was probably based on your ideal body weight (IBW) rather than your actual weight. IBW is a clinical tool, and it predicts drug distribution through the body more accurately than actual weight for many drugs — particularly those that don't distribute well into fat tissue.

But IBW has a curious problem: there are four major formulas, all still in common use, and they frequently disagree. For a 175cm woman, the Hamwi formula gives 61.4kg, the Devine gives 63.6kg, the Robinson gives 58.8kg, and the Miller gives 58.4kg. A 4–5kg range on a number meant to define the ideal is significant. Here's where each formula came from, how they differ, and what the right interpretation is.

A History Driven by Drug Dosing

The IBW formulas were not developed from health outcome research. They were developed for pharmacology. The original aim was to find a body weight that predicted the volume of distribution of water-soluble drugs — drugs that do not dissolve into adipose (fat) tissue, meaning a 150kg person who carries 50kg of excess fat should not receive a proportionally larger dose of such a drug than a lean 100kg person.

Dr. B.J. Devine published what became the most widely adopted formula in 1974, in a paper addressing drug dosing in obese patients. Devine's formula was itself adapted from earlier work by Dr. G.J. Hamwi, an endocrinologist who proposed his version in 1964 as a rough clinical rule of thumb — not as a result of any systematic study. Hamwi never published his formula in a peer-reviewed paper; it circulated informally before being formalised.

The Robinson, Miller, and other formulas emerged in the 1980s as researchers attempted to validate or improve on Devine and Hamwi using actual body composition data. None of the four formulas were originally derived from or validated against health outcomes such as mortality, cardiovascular disease risk, or longevity.

The Four Formulas

All four formulas share the same basic structure: a base weight for a reference height (typically 5 feet / 152.4cm), plus an increment per additional inch (or 2.54cm) of height. They differ in the base weights and increments used, and in how they were calibrated.

Hamwi (1964):
Men: 48.0 kg + 2.7 kg per cm above 152.4cm
Women: 45.5 kg + 2.2 kg per cm above 152.4cm
The original clinical rule. Developed informally for drug dosing guidance.

Devine (1974):
Men: 50.0 kg + 2.3 kg per cm above 152.4cm
Women: 45.5 kg + 2.3 kg per cm above 152.4cm
The most widely used formula in clinical practice. Cited in thousands of drug dosing guidelines.

Robinson (1983):
Men: 52.0 kg + 1.9 kg per cm above 152.4cm
Women: 49.0 kg + 1.7 kg per cm above 152.4cm
Attempts to account for frame size differences. Gives lower estimates at taller heights.

Miller (1983):
Men: 56.2 kg + 1.41 kg per cm above 152.4cm
Women: 53.1 kg + 1.36 kg per cm above 152.4cm
Uses the smallest increment per centimetre, producing the most conservative (lowest) estimates for tall individuals.

Why They Disagree — and Which Is More Accurate

The four formulas converge at shorter heights and diverge significantly at taller heights, because the per-centimetre increments differ. At 152cm (5'0"), the formulas give similar values. At 190cm (6'3"), they can differ by 8–10kg.

This divergence at taller heights is a known limitation of all four formulas. Because each uses a fixed linear increment per centimetre of height, they cannot capture the non-linear relationship between height and optimal body mass. In practice, all four tend to underestimate clinically appropriate weight in taller individuals — which is why pharmacists and critical care physicians often apply frame-size corrections (±10%, per Metropolitan Life 1983 tables) or use adjusted body weight formulas for drug dosing in these populations.

A more height-responsive approach is to simply target a healthy BMI range (20–25 kg/m²) and calculate the corresponding weight for a given height. For example, at 190cm, a BMI of 22 gives ~79kg — compared to the 75–82kg range the four formulas produce. This BMI-based method inherently scales with height² rather than linearly, making it more physiologically appropriate.

The Critical Limitation: IBW ≠ "Optimal" Weight

Perhaps the most important point about IBW is what it was never designed to do. None of the four formulas were derived from studies of health outcomes. They do not tell you the weight at which you will live longest, have the best metabolic health, or feel best. They are pharmacokinetic tools repurposed — somewhat awkwardly — as health benchmarks.

For health-outcome guidance, the research more commonly points to BMI in the 20–25 range, or better yet, to waist circumference and body fat percentage as superior predictors of metabolic and cardiovascular risk. A person at 90% of their Devine IBW may be significantly overfat (if they have low muscle mass), while a muscular athlete may exceed their IBW considerably without any health consequence.

IBW remains genuinely useful in clinical pharmacy, critical care medicine, and for rough orientation when more precise body composition data isn't available. As a personal health target, it should be interpreted loosely — a general reference zone, not a precise prescription.

Sources

1. Hamwi, G.J. (1964). Therapy: changing dietary concepts. In T.S. Danowski (Ed.), Diabetes Mellitus: Diagnosis and Treatment (Vol. 1, pp. 73–78). New York: American Diabetes Association.
2. Devine, B.J. (1974). Gentamicin therapy. Drug Intelligence and Clinical Pharmacy, 8, 650–655.
3. Robinson, J.D. et al. (1983). Determination of ideal body weight. Drug Intelligence and Clinical Pharmacy, 17(5), 340–345.
4. Miller, D.R. et al. (1983). Ideal body weight for dosing aminoglycosides — development and assessment of a simplified method. Drug Intelligence and Clinical Pharmacy, 17, 508.
5. Metropolitan Life Insurance Company (1983). Metropolitan height and weight tables. Statistical Bulletin, 64(1), 2–9.