Ideal Weight Formulas: Why They Disagree (And Why It's OK)

A Number Built for Doctors, Misread by Everyone Else

"Ideal weight" sounds like a personal fitness goal, a specific number on the scale to strive toward. The formulas that generate ideal weight values were created in the 1970s and 1980s for a very different purpose: clinical drug dosing. Pharmacologists needed a way to estimate the weight of lean body mass so they could calculate correct doses of medications that distribute through lean tissue rather than fat. The resulting formulas give a clinical reference weight, not a personal target that defines health or attractiveness.

Three formulas dominate modern clinical practice: Devine (1974), Robinson (1983), and Miller (1983). Each produces a slightly different number. For a 5 ft 9 in man, Devine gives 69.9 kg (154 lbs), Robinson gives 74.4 kg (164 lbs), and Miller gives 71.7 kg (158 lbs), a 10-lb spread from the same height. None of these formulas is "correct" in an absolute sense; each reflects the dataset and methodology of its original study.

This guide explains all three formulas, shows their calculations step by step, compares them to BMI-based weight ranges, and explains the specific clinical contexts where ideal weight formulas matter.

The Three Main Ideal Weight Formulas

Devine Formula (1974), the oldest and most widely referenced in clinical settings. Originally derived for predicting creatinine clearance in renal pharmacology.

Men: IBW (kg) = 50 + 2.3 × (height in inches − 60)
Women: IBW (kg) = 45.5 + 2.3 × (height in inches − 60)

Robinson Formula (1983), derived from a study of 250 healthy male patients. Tends to produce slightly higher values for tall individuals than Devine.

Men: IBW (kg) = 52 + 1.9 × (height in inches − 60)
Women: IBW (kg) = 49 + 1.7 × (height in inches − 60)

Miller Formula (1983), derived similarly to Robinson but yields intermediate values between Devine and Robinson for most heights.

Men: IBW (kg) = 56.2 + 1.41 × (height in inches − 60)
Women: IBW (kg) = 53.1 + 1.36 × (height in inches − 60)

All three formulas use height in inches above 5 feet (60 inches) as the primary input. None includes age, frame size, ethnicity, or body composition. For individuals under 5 feet, the formulas require separate adjustments (typically 2-3% per inch below 5 feet).

How the Math Works: A Step-by-Step Example

Take a 5 ft 9 in (69 inches) woman. Height above 60 inches: 69 − 60 = 9 inches.

Devine: 45.5 + (2.3 × 9) = 45.5 + 20.7 = 66.2 kg (146 lbs).

Robinson: 49 + (1.7 × 9) = 49 + 15.3 = 64.3 kg (142 lbs).

Miller: 53.1 + (1.36 × 9) = 53.1 + 12.2 = 65.3 kg (144 lbs).

The spread: 142-146 lbs for a 5'9" woman. Compare this to the BMI "normal weight" range for the same height: BMI 18.5-24.9 corresponds to 125-168 lbs. The ideal weight formulas cluster near the middle of the BMI normal range, they target a moderate lean body mass estimate rather than any particular BMI category.

For the same height man: Devine gives 74.7 kg (165 lbs), Robinson gives 69.1 kg (152 lbs), Miller gives 71.1 kg (157 lbs). The male spread covers 13 lbs at the same height, more variation than for women across the same three formulas.

Common Misconceptions

• "Ideal weight" is my personal goal weight for health or appearance. These formulas estimate clinical reference weights for drug dosing calculations. They were never validated as health targets. Many healthy adults weigh significantly above or below their ideal weight formula output with optimal metabolic markers. Use body fat percentage and waist circumference for health-related body composition goals.
• The formula accounts for muscle mass. All three formulas use height as the sole input (plus a sex constant). They assume an average body composition for a given height. A muscular 5'9" man at 185 lbs with 12% body fat falls outside the "ideal" range but is clinically healthier than the formula implies.
• Ideal weight and healthy BMI range mean the same thing. The ideal weight formulas produce a single point estimate. The BMI normal weight range (18.5-24.9) produces a 43-lb range for a 5'9" person. The ideal weight formulas sit within the lower-to-middle portion of that range, they are more conservative weight targets than the full BMI range suggests.