Ideal weight calculators: useful reference or outdated math?
The concept of an "ideal weight" has a complicated history. The earliest formulas — Hamwi in 1964, Devine in 1974, Robinson and Miller in 1983 — were developed not to set aesthetic targets but to support clinical drug dosing. Anaesthetists needed a reliable way to dose anaesthetics by weight; pharmacologists needed lean-mass approximations for medications that distribute primarily in lean tissue. Each formula encodes assumptions about what "normal" body composition looks like for a given height, and the four produce noticeably different answers for the same person.
The four classic formulas
The Devine formula is the most widely used in modern medicine. For men: 50 kg + 2.3 kg per inch over 5 feet. For women: 45.5 kg + 2.3 kg per inch over 5 feet. It produces moderate, clinically conservative estimates that have been validated in tens of thousands of drug-dosing studies.
The Robinson formula (1983) revises Devine downward slightly. For men: 52 kg + 1.9 kg per inch over 5 feet. For women: 49 kg + 1.7 kg per inch over 5 feet. It was developed using more modern population data and produces estimates around 2–3% lower than Devine for typical heights.
The Miller formula (1983) sits between Devine and Robinson. For men: 56.2 kg + 1.41 kg per inch over 5 feet. For women: 53.1 kg + 1.36 kg per inch over 5 feet. Miller has been used heavily in nutrition research, particularly in the context of dietary intake targets.
The Hamwi formula (1964) is the oldest and produces the highest values. For men: 48 kg + 2.7 kg per inch over 5 feet. For women: 45.5 kg + 2.2 kg per inch over 5 feet. Hamwi remains in clinical use, particularly in dietetics and diabetes management, but tends to slightly overestimate "ideal" weights by modern standards.
Where the formulas fall short
All four formulas share a fundamental limitation: they take only height and sex as inputs. They have no information about frame size, muscle mass, body fat percentage, age or ethnicity. A 1.85 m heavily muscled rugby player and a 1.85 m sedentary office worker get identical "ideal weights" — but the rugby player at "ideal" would be visibly underweight and probably underperforming, while the office worker at the same weight might be at a sensible target. This is the same critique that applies to BMI, and for the same reason: any formula that uses only height as a body input cannot account for body-composition variation.
The formulas also assume "normal" body composition based on mid-20th-century population averages. Modern populations carry more body fat for any given height, particularly in industrialised countries. This means that for many people today, hitting their formula-derived "ideal weight" would require losing meaningful amounts of fat — a reasonable health goal in many cases, but a much larger ask than the formulas casually imply.
The WHO healthy BMI range as a better reference
For most practical purposes, the WHO healthy BMI range (18.5–24.9) provides a more useful reference point than any single ideal-weight number. It produces a range rather than a single value, accommodating the natural variation in body frame, muscle mass and composition that single-number formulas can\'t. For a 1.70 m adult, the healthy BMI range translates to 53.5–72 kg — a 19 kg span that comfortably accommodates lean and athletic builds alike.
If you want a single number target, picking the middle of the healthy BMI range (BMI 22) gives you a sensible centre point: 63.6 kg for a 1.70 m adult, 75.5 kg for a 1.85 m adult. This matches the lowest-mortality BMI range in large epidemiological studies and provides natural buffer for weight fluctuations without pushing toward the underweight or overweight thresholds.
Frame size and the wrist test
The classic adjustment for body frame is the wrist circumference test. For a man over 1.65 m: wrist under 16.5 cm = small frame, 16.5–19 cm = medium, over 19 cm = large. For a woman over 1.65 m: wrist under 14 cm = small, 14–15 cm = medium, over 15 cm = large. Add roughly 5% to formula results for large frames, subtract 5% for small frames. This is a rough adjustment but captures real anatomical variation that pure height-based formulas miss.
How to use these numbers sensibly
Treat the four formula results as a range rather than four separate truths. If they cluster within 5 kg of each other (typical for average builds), the average is a reasonable target. If they spread wider, you\'re probably outside the populations they were calibrated for — likely shorter than 1.55 m or taller than 1.95 m. Cross-check against the WHO BMI healthy range, factor in your frame size and current muscle mass, and adjust expectations accordingly. For trained lifters, the formula numbers are usually too low; muscular athletes routinely sit 5–10 kg above their formula "ideal" at healthy body-fat percentages. For sedentary office workers, the formulas might land close to where they should be aesthetically — but reaching that weight by losing fat will produce far better health outcomes than reaching it by losing muscle.