Ideal weight formulas are from the 1970s, and weren't built for health

Four formulas, one clinical origin

"Ideal body weight" calculators almost always offer a choice of four formulas — Devine, Robinson, Miller and Hamwi — and it's rarely explained that none of them were designed to define a healthy weight. All four were built by clinicians and pharmacists to solve a narrower problem: estimating a reference bodyweight for calculating drug doses in patients who were overweight or obese, where dosing by actual weight risks overdose for drugs that don't distribute into fat tissue. They were adopted far more broadly than that original purpose over the following decades, which is how a set of 1970s–80s pharmacy equations ended up powering "what should I weigh" calculators today.

Devine (1974): built for gentamicin dosing

Ben Devine published the first of the four formulas in 1974 in Drug Intelligence & Clinical Pharmacy, specifically to support dosing calculations for gentamicin, an antibiotic with a narrow safety margin where dosing by total bodyweight in an obese patient can cause toxicity. The formula is linear in height: 50 kg + 2.3 kg per inch over 5 feet for men, 45.5 kg + 2.3 kg per inch for women. It became the most widely used of the four in both clinical and general-fitness contexts, and it's the default on the ideal weight calculator on this site — not because it's more accurate for health purposes than the others, but because of how entrenched it became in pharmacy practice.

Hamwi, Robinson and Miller: three more clinical-era formulas

G.J. Hamwi published the earliest widely used version in 1964, as a quick bedside estimate for dietitians rather than a study — 106 lb + 6 lb per inch over 5 feet for men, 100 lb + 5 lb per inch for women. Robinson and colleagues published a revision in the American Journal of Hospital Pharmacy in 1983, explicitly framed as an update for drug-dosage calculations (52 kg + 1.9 kg per inch for men, 49 kg + 1.7 kg per inch for women). Miller's 1983 revision, published in the same pharmacy-journal tradition, adjusted the slope again (56.2 kg + 1.41 kg per inch for men, 53.1 kg + 1.36 kg per inch for women). A recent review of the variability across these formulas in clinical use confirms they were derived independently, on different reference populations, for the same narrow dosing purpose (PMC10621523).

Worked example: four formulas, one height, real disagreement

Take a man who is 180 cm (70.9 inches) tall — 10.9 inches over the 5-foot reference point each formula uses:

• Devine: 50 + 2.3 × 10.9 ≈ 75.0 kg
• Hamwi: (106 + 6 × 10.9) lb ≈ 171.2 lb ≈ 77.7 kg
• Robinson: 52 + 1.9 × 10.9 ≈ 72.7 kg
• Miller: 56.2 + 1.41 × 10.9 ≈ 71.5 kg

That's a 6.2 kg spread — from 71.5 to 77.7 kg — for the exact same height, produced entirely by which decades-old pharmacy formula happens to be selected. None of the four is "more correct" than the others for a fitness goal; they're four different regression lines fit to four different clinical reference groups, none of which had anything to do with defining a target physique.

Why BMI has the same structural blind spot

Body mass index — weight divided by height squared — was popularized decades before these formulas for population-level epidemiology, not individual assessment, and it shares their core limitation: it can't distinguish muscle from fat, or central (abdominal) fat from fat carried elsewhere. A heavily muscled person and a person carrying the same total weight as visceral fat around the waist can post an identical BMI while facing very different actual health profiles. This isn't a fringe criticism — it's the reason waist-based measures exist as an alternative screen in the first place.

Waist-to-height ratio: a better screen for the same purpose

Margaret Ashwell's research group has published the most direct comparisons. A 2012 systematic review and meta-analysis in Obesity Reviews compared waist-to-height ratio, waist circumference and BMI as predictors of cardiometabolic risk factors (diabetes, hypertension, cardiovascular disease) and found waist-to-height ratio the strongest discriminator of the three, with a mean area under the ROC curve of 0.70 versus 0.69 for waist circumference and 0.67 for BMI (PubMed). An earlier systematic review across fourteen countries and multiple ethnic groups found a waist-to-height ratio of 0.5 worked as a consistent boundary value across populations, unlike waist circumference, which needs different cutoffs for different ethnic groups (Cambridge Core). The practical version of the rule, popularized from this research, is simple: keep your waist circumference under half your height.

Worked example: where BMI and waist-to-height ratio disagree

Consider two people of different builds. Person A is 175 cm, 90 kg, with a visibly muscular frame and an 80 cm waist. Their BMI — 90 ÷ 1.75² ≈ 29.4 — falls in the "overweight" category, but their waist-to-height ratio — 80 ÷ 175 ≈ 0.46 — sits comfortably in the healthy range on the calculator on this site. Person B is 170 cm, 65 kg, with less visible muscle and more fat carried centrally, and a 100 cm waist — a "normal weight, big middle" body type sometimes described as thin-outside-fat-inside. Their BMI — 65 ÷ 1.70² ≈ 22.5 — reads as normal, while their waist-to-height ratio — 100 ÷ 170 ≈ 0.59 — flags increased risk. Same two metrics, opposite conclusions, and the waist-based one is tracking the thing (central fat) the cardiometabolic research actually links to risk.

Using the tools without over-trusting either one

The ideal weight calculator is useful as a rough historical reference point, not a target — pick Devine as the conventional default, but expect a several-kilogram spread if you compare formulas, and don't treat any of the four as accounting for your actual muscle mass or frame. The waist-to-height ratio calculator is a better general screening signal for central-fat-related risk, and it works the same way across different heights without needing population-specific cutoffs the way waist circumference alone does. Neither tool replaces an actual clinical assessment.

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This article explains the history and validation research behind common body-weight formulas. It is general information, not medical advice, and no single formula or ratio determines an individual's health status — talk to a doctor for a personalized assessment.