Ideal Weight Formulas Explained: Devine, Robinson, Miller, and Hamwi Compared

Ask four different doctors what your "ideal weight" should be and you might get four different numbers. That is not a sign that medicine cannot do basic math. It is because ideal body weight was never one formula. It is four separate formulas, built decades apart by different researchers solving different problems, and all four are still in use today. None of them were designed to tell you what you should weigh for aesthetic reasons. Most were built to solve a narrow clinical problem: how much medication to give a patient whose actual body weight might be misleading.

Understanding where these formulas came from, what they actually calculate, and why they disagree with each other and with BMI will help you use any ideal weight number correctly: as a rough reference point, not a verdict.

Why "Ideal Body Weight" Has Four Different Answers

The phrase "ideal body weight," often shortened to IBW, entered clinical use through life insurance tables in the early twentieth century. Insurers wanted a quick way to estimate mortality risk by height and frame size, so they built height-to-weight tables from policyholder data. Those tables were never designed for general health advice, but they became the seed for the formulas that followed.

The four formulas in wide use today, Devine, Robinson, Miller, and Hamwi, all share the same basic structure: a baseline weight for a height of five feet, plus a fixed amount added for every inch above that. What differs is the baseline number and the per-inch increment, and those small differences in the math can move the final estimate by ten pounds or more for the same height.

The Devine Formula: Built for Drug Dosing, Not Diet Advice

The Devine formula, published in 1974 by Dr. B.J. Devine, is the most widely used of the four, mostly because hospitals adopted it to calculate drug dosages. Many medications, especially anesthesia and chemotherapy drugs, need to be dosed against lean body mass rather than total body weight, because fat tissue does not metabolize most drugs the same way muscle and organ tissue do. Devine's formula gave clinicians a fast bedside estimate without needing a body composition scan.

The formula sets a baseline of 50 kg for men and 45.5 kg for women at five feet tall, then adds 2.3 kg for every inch over five feet. For a man who is 5'10", that works out to 50 + (2.3 x 10) = 73 kg, or roughly 161 pounds. The formula is simple by design. It was never meant to define a healthy weight range, only to approximate lean mass quickly enough to use during a procedure.

The Robinson and Miller Formulas: Two Attempts to Fix Devine

Devine's formula was widely adopted but not universally trusted, and two later research teams tried to refine it using updated population data. The Robinson formula, published in 1983, lowered the baseline slightly and adjusted the per-inch increment, producing numbers that ran a bit lower than Devine's for the same height. The Miller formula, published the same year by a different team, made a similar but distinct adjustment, landing in its own slightly different range.

Neither Robinson nor Miller fully replaced Devine in clinical practice. Instead, all three formulas continued to be cited side by side in medical literature, which is exactly why a single height can produce three different "ideal" numbers depending on which paper a calculator or textbook happens to cite. None of the three is more "correct" than the others. They are simply three separate regression models fit to three separate datasets, decades before anyone could measure body composition directly and cheaply.

The Hamwi Formula: The Quick Bedside Rule

The oldest of the four, the Hamwi formula, was published in 1964 by Dr. G.J. Hamwi as a rule of thumb for dietitians who needed a fast estimate without a calculator. It uses slightly different round numbers: 48 kg for men and 45.5 kg for women at five feet, with 2.7 kg added per inch for men and 2.2 kg per inch for women. Because the increments are rounder and the formula predates digital tools, it was designed to be done in your head or on paper during a clinic visit.

Hamwi numbers tend to run close to Devine's for most adult heights, but the gap widens at the extremes, particularly for very tall or very short individuals where a fixed per-inch increment stops scaling realistically. This is one of the core limitations shared by all four formulas: they were built and tested mostly on adults of average height, so accuracy drops off at the tails of the height distribution.

Why the Four Formulas Disagree With Each Other, and With BMI

For a 5'6" woman, Devine puts ideal weight around 58.5 kg, Robinson closer to 56.5 kg, Miller around 57.5 kg, and Hamwi near 56.8 kg. That spread of roughly four kilograms, about nine pounds, exists for one height using four formulas that all claim to measure the same thing. None of the differences come from new science about what is healthy. They come from which regression dataset each author used in the era before bioelectrical impedance scales or DEXA scans existed.

Body mass index, or BMI, measures something different entirely: a ratio of weight to the square of height, with no reference to frame size, sex-specific tissue distribution, or age. A person can sit in the "normal" BMI range while landing outside every ideal weight formula's range, or the reverse, especially if they carry more muscle mass than the original datasets accounted for. If you want to see how your numbers compare across different ranges, the BMI Calculator uses the standard weight-to-height-squared formula, which is useful context alongside an ideal weight estimate but is measuring a fundamentally different thing.

None of this means the formulas are useless. It means they are reference points pulled from old population averages, not individual diagnoses. A formula that disagrees with your actual healthy weight by ten pounds is not malfunctioning. It is doing exactly what a 1960s-to-1980s regression model was built to do: approximate a population average, not describe your specific body.

Pairing Ideal Weight With Body Fat Percentage for a Fuller Picture

The biggest blind spot shared by Devine, Robinson, Miller, and Hamwi is that none of them account for body composition. Two people at the same height and the same "ideal" weight can have completely different amounts of muscle and fat, and the formulas cannot tell the difference. This is the same blind spot BMI has, just expressed in pounds instead of a ratio.

Pairing an ideal weight estimate with a body fat percentage reading closes most of that gap. A higher-than-formula weight paired with a healthy body fat percentage usually means more muscle mass, not excess fat. A weight that matches the formula exactly but comes with a high body fat percentage can still indicate a composition problem the scale alone would miss. Checking both numbers together gives a far more honest picture than either one alone.

Turning a Formula Into a Realistic Weight Target

A Worked Example: Comparing All Four Formulas for One Height

Take a man who is 5'9". Devine sets his baseline at 50 kg plus 2.3 kg for each of the nine inches over five feet, landing at about 70.7 kg, or roughly 156 pounds. Hamwi uses 48 kg plus 2.7 kg per inch, landing at about 72.3 kg, or roughly 159 pounds. Robinson's lower baseline and increment put him closer to 68.5 kg, or about 151 pounds, while Miller lands near 69.8 kg, or about 154 pounds. That is a spread of roughly eight pounds for one fixed height, and every formula in that range is considered a textbook-correct answer. The spread alone is the clearest evidence that none of these numbers should be treated as a precise target.

If you want to use one of these formulas as a starting point rather than a strict target, treat the number as the center of a range, not a single digit to hit exactly. A practical approach is to calculate your estimate from two or three of the formulas, take the average, and then build a target range of plus or minus five percent around it. That range accounts for the disagreement between the formulas themselves and leaves room for your actual frame size and muscle mass.

From there, the number that actually matters day to day is not the ideal weight figure but the math behind reaching or maintaining it: how many calories you need to support your activity level, and how big a deficit or surplus makes sense for your timeline. The Calorie Calculator converts your target weight range into a daily calorie target based on your current activity level, which is a far more actionable number than a single ideal weight figure on its own.

It is also worth checking the result against common sense. If a formula's output is far outside what looks and feels sustainable for your frame, age, and activity level, that is a sign the formula has run into one of its known limitations, not a sign that you need to force your body toward an old regression model's average.

The Bottom Line

Devine, Robinson, Miller, and Hamwi are not competing truths about your body. They are four historical attempts to solve a narrow clinical problem, mostly drug dosing, using the population data available at the time each was published. They disagree with each other by several pounds for the same height, and they disagree with BMI for an entirely different reason: BMI measures a ratio, not an estimated lean weight. Used correctly, as a rough range rather than a precise verdict, and paired with body composition data and a realistic calorie target, these formulas can still be a useful starting point. Used as a strict number to hit on a scale, they will mislead you about as often as they help.