What Is Ideal Weight? The 4 Medical Formulas That Actually Answer It

Why There Are 4 Different Answers to the Same Question

The concept of "ideal body weight" (IBW) did not begin with fitness culture or diet apps. It started in hospital wards and intensive care units, where clinicians needed quick bedside estimates to calculate medication doses, ventilator settings, and nutritional requirements for critically ill patients.

The problem was practical: you cannot dose certain drugs by total body weight because obese patients would receive dangerously high amounts, and underweight patients would receive too little. Doctors needed a standardised "target weight" based on height alone — a number that represented a healthy, typical body regardless of the patient's actual size.

Four different research groups published four different formulas across a 20-year period. None of them was validated against a large population study. All were derived from small samples or professional clinical judgment. This is why the numbers differ, and why no single formula has ever been officially adopted as the global standard.

The 4 Formulas — Origins and Exact Calculations

1. The Hamwi Formula (1964)

G.J. Hamwi introduced the first widely used bedside IBW formula in 1964, published in Therapy of Obesity with Anorexiants. It was designed as a quick-reference rule of thumb for physicians prescribing caloric intake targets — not a precise clinical measurement.

Formula:

• Men: 48.0 kg + 2.7 kg for each inch over 5 feet
• Women: 45.4 kg + 2.27 kg for each inch over 5 feet

Worked example — 5'10" man (10 inches over 5 feet):
IBW = 48.0 + (2.7 × 10) = 48.0 + 27.0 = 75.0 kg (165.3 lb)

Worked example — 5'4" woman (4 inches over 5 feet):
IBW = 45.4 + (2.27 × 4) = 45.4 + 9.1 = 54.5 kg (120.2 lb)

Hamwi tends to produce the highest results for men compared to the other three formulas, which is why it is most often used when estimating caloric needs for male patients in clinical nutrition settings. Its larger per-inch increment (2.7 kg) means the formula diverges most sharply from others at greater heights.

2. The Devine Formula (1974)

B.J. Devine published this formula in 1974 in the journal Drug Intelligence and Clinical Pharmacy while working on dosing calculations for the antibiotic gentamicin. It became the most widely adopted IBW formula in pharmacy and clinical medicine — partly because it appeared in a widely read journal, and partly because it produces conservative, easy-to-work-with numbers.

The Devine formula is the one most likely to be used when a pharmacist calculates a medication dose by "ideal body weight" today.

Formula:

• Men: 50.0 kg + 2.3 kg for each inch over 5 feet
• Women: 45.5 kg + 2.3 kg for each inch over 5 feet

Worked example — 5'10" man:
IBW = 50.0 + (2.3 × 10) = 50.0 + 23.0 = 73.0 kg (160.9 lb)

Worked example — 5'4" woman:
IBW = 45.5 + (2.3 × 4) = 45.5 + 9.2 = 54.7 kg (120.6 lb)

One notable feature: Devine uses the same per-inch increment (2.3 kg) for both sexes above 5 feet, so the male/female difference comes entirely from the base weight (50 vs 45.5 kg). Critics argue this ignores meaningful differences in body composition between male and female frames at the same height.

3. The Robinson Formula (1983)

J.D. Robinson published this formula in 1983 in the American Journal of Hospital Pharmacy as a refinement of Devine. Robinson argued that Devine overestimated IBW for shorter individuals and underestimated it for taller ones, adjusting the coefficients accordingly.

Formula:

• Men: 52.0 kg + 1.9 kg for each inch over 5 feet
• Women: 49.0 kg + 1.7 kg for each inch over 5 feet

Worked example — 5'10" man:
IBW = 52.0 + (1.9 × 10) = 52.0 + 19.0 = 71.0 kg (156.5 lb)

Worked example — 5'4" woman:
IBW = 49.0 + (1.7 × 4) = 49.0 + 6.8 = 55.8 kg (123.0 lb)

Robinson produces the most conservative estimates for taller men while giving higher baseline weights for women (49 kg starting point vs Hamwi/Devine at 45 kg). Many dietitians prefer Robinson for female patients because the higher baseline is closer to what healthy, average-framed women naturally weigh at 5 feet.

4. The Miller Formula (1983)

D.R. Miller published an alternative formula in the same year as Robinson, also in the American Journal of Hospital Pharmacy. Where Robinson adjusted per-inch increments, Miller raised the baseline weight for both sexes and used smaller per-inch adjustments — producing results that more closely reflect what healthy, active adults of average height actually weigh in modern populations.

Formula:

• Men: 56.2 kg + 1.41 kg for each inch over 5 feet
• Women: 53.1 kg + 1.36 kg for each inch over 5 feet

Worked example — 5'10" man:
IBW = 56.2 + (1.41 × 10) = 56.2 + 14.1 = 70.3 kg (154.9 lb)

Worked example — 5'4" woman:
IBW = 53.1 + (1.36 × 4) = 53.1 + 5.44 = 58.5 kg (129.0 lb)

Miller consistently produces the highest estimates for women and the lowest per-inch increments of all four formulas. Dietitians working with women who have larger, athletic frames often find Miller's results the most realistic starting point for weight goals.

Full Height-by-Height Comparison Table

The tables below show IBW in kilograms (lb in parentheses) for all four formulas at common heights, rounded to one decimal place. The final column shows the average of all four.

Men

Women

The average of all four formulas (rightmost column) is the most balanced estimate and the value our Ideal Weight Calculator highlights by default.

Ideal Weight vs BMI — What Is the Difference?

BMI and IBW are related but answer different questions:

• BMI tells you where your current weight sits relative to healthy ranges for your height. It is a diagnostic classification tool (underweight / normal / overweight / obese).
• IBW gives you a target weight — a single number (or formula-derived range) that represents a healthy body at your height, regardless of where you are now.

An IBW result that falls within a BMI of 18.5–24.9 is considered healthy by the WHO. In practice, all four formulas produce results that correspond to a BMI of roughly 20–22 for most adults — comfortably in the middle of the healthy range.

The key limitation they share: neither BMI nor IBW accounts for body composition. A 5'10" man at 185 lb with 10% body fat is clinically very different from a 5'10" man at 185 lb with 32% body fat — yet both get the same BMI and the same IBW. For a more complete picture, use our BMI Calculator alongside a Body Fat Calculator.

When Ideal Body Weight Actually Matters

Outside of clinical settings, IBW is most useful as a directional reference, not a rigid goal. Here are the four contexts where it has real practical value:

1. Medication and Drug Dosing

Pharmacists use IBW (typically Devine) to calculate dosing for renally-cleared drugs — antibiotics like gentamicin and vancomycin, heparin, and some chemotherapy agents. For these drugs, dosing by actual body weight in an obese patient would cause toxicity; IBW provides the safer reference point. This is the reason IBW was invented in the first place.

2. Mechanical Ventilation Settings

In ICUs, ventilator tidal volume is set per kilogram of IBW, not actual body weight. The landmark ARDSNet trial established that lower tidal volumes (6 mL/kg IBW) reduced mortality in acute respiratory distress syndrome patients — making IBW a literal life-or-death calculation in critical care.

3. Weight Loss Goal-Setting

When someone needs to lose a significant amount of weight, IBW provides a clinically-grounded long-term target. Fitness coaches often use the average of the four formulas — or add 10% for larger or more muscular frames — to set realistic goals that do not require extreme leanness.

4. Clinical Nutrition and Protein Targets

Dietitians use IBW as the denominator for protein requirement calculations (e.g. 1.2–2.0 g protein per kg IBW for athletes). This prevents overfeeding protein to patients or athletes who are significantly above typical body composition. Use our BMR Calculator alongside your IBW to estimate total daily energy needs at your target weight.

Limitations Every Formula Shares

All four formulas were derived from small samples of primarily Western adults in the mid-20th century. They share these well-documented limitations:

• No muscle mass adjustment. A 5'10" elite athlete at 185 lb with 8% body fat may be in perfect health — yet every IBW formula would flag them as above their "ideal" weight.
• No age adjustment. Research suggests optimal BMI for adults over 65 may be 25–35, considerably above what IBW formulas target for the same height.
• No ethnicity adjustment. The WHO has noted that South Asian, East Asian, and Pacific Islander populations have different body composition at the same BMI, suggesting IBW thresholds may need recalibration for these groups.
• No frame-size adjustment. Hamwi originally recommended adjusting ±10% for small or large skeletal frames, but this is rarely done in practice today.
• Unreliable below 5 feet. Most formulas were not validated for adults shorter than 5 feet and may produce results that do not align with healthy weights at those heights.

The bottom line: use IBW as a starting point for a conversation with a healthcare provider, not a verdict. A weight 5–10 lb above your IBW, when you are fit and active, is not a health problem.

Frequently Asked Questions

Summary: The 4 Formulas at a Glance

If your weight falls within the range spanned by the four formulas for your height, you are in healthy territory by every clinical standard in use today. If you are significantly above or below that range, it is worth a conversation with your physician — not because the number is an absolute truth, but because it flags a meaningful deviation from population baselines.