The most widely used ideal body weight formula is the Devine formula, published in 1974, which estimates a “healthy” weight based solely on a person’s height and sex. For men, it starts at about 50 kg for someone five feet tall and adds roughly 2.3 kg per inch above that. For women, it starts at about 45.5 kg at five feet and adds the same 2.3 kg per inch. Despite being the go-to formula in hospitals, pharmacies, and online calculators, the Devine equation was never designed as a universal health benchmark, and its accuracy breaks down in ways that matter for real medical decisions.
How the Devine Formula Became the Standard
In 1974, a pharmacist named B.J. Devine published the formula as a quick reference for calculating drug doses, specifically for the antibiotic gentamicin. It was meant to give clinicians a rough proxy for lean body mass so they could figure out how much drug to give a patient without over- or under-dosing. The formula was not built from large population studies or anthropometric measurements. It was derived for a narrow pharmacological purpose, yet it spread rapidly through clinical practice and eventually became the default ideal body weight equation used in nutrition, respiratory medicine, and general health screening.
That backstory matters because it explains a fundamental mismatch: a formula created to approximate drug-dosing weight in adults ended up being treated as a definition of what people should weigh. The Devine formula is still the most commonly cited method for calculating ideal body weight across various medical applications, even though researchers have documented substantial variability when it is compared against other approaches.1PubMed Central. Variability in ideal body weight formulae
Other Formulas You Will Encounter
The Devine equation is not the only game in town. Several other formulas compete for clinical use, and each gives slightly different results for the same person. The most common alternatives include:
- Robinson formula (1983): Generally considered the best equation for estimating desirable weight in men. It uses a similar structure to Devine but with different coefficients that produce somewhat lower values for taller individuals.
- Hamwi formula (1964): Predates Devine and uses a round-number approach (men start at 106 pounds for 5 feet, adding 6 pounds per inch; women start at 100 pounds, adding 5 per inch). It is easy to memorize but similarly rough.
- Broca’s Index: An even older method from the 19th century that simply subtracts 100 from height in centimeters to get ideal weight in kilograms. Broca’s Index does not account for body frame or sex-based differences, and it tends to overestimate ideal weight compared to more modern tools.2International Journal of Innovative Research in Medical Science. Sensitive and Reliable Indicator between Broca’s Index and Metropolitan Life Insurance Height – Weight Charts in First Year Medical Student
When researchers compared these formulas head-to-head, they found that most predicted ideal weights falling within a BMI range of 20 to 25, which is broadly reasonable. But the formulas diverged in a telling pattern: at shorter heights, they clustered near a BMI of 20, and at taller heights, they drifted toward a BMI of 25. Robinson’s formula came closest to reasonable values for men, but none of the standard formulas predicted weights corresponding to a BMI of 22 for women.3PubMed. Comparison of ideal body weight equations and published height-weight tables with body mass index tables for healthy adults in the United States
The Fundamental Problem With All Linear Formulas
Every classic ideal body weight formula shares the same structural flaw: they assume weight increases as a straight line alongside height. Add an inch, add a fixed number of kilograms. But human bodies do not scale that way. Weight increases not just with height but with volume, meaning body width, trunk length, and musculature all contribute. A person who is six foot three does not just have longer legs than someone who is five foot two; they have a proportionally larger ribcage, broader shoulders, and more tissue in every dimension.4PubMed Central. Ideal body weight: a commentary – Section: Criticism of the Predictive Formulas: Ideal Body Weight and Body Mass Index
This linearity problem explains why the formulas underestimate ideal weight for very tall people and overestimate it for very short people. BMI tries to address this by squaring height in its denominator, but even BMI has been criticized for not fully solving the issue at the extremes of stature. A more recent approach developed using national survey data created a universal equation that aligns directly with BMI values for both men and women, avoiding the over- and underestimation that has plagued older ideal body weight formulas. That equation achieved an empirical accuracy within about half a percent to less than one percent across the height spectrum.5PubMed Central. Universal equation for estimating ideal body weight and body weight at any BMI
Beyond the math, the formulas also ignore body frame, genetics, and existing health conditions. Two people of the same height and sex can have very different skeletal structures, and a formula that spits out a single number cannot capture that variation. Researchers have argued that the real intent behind ideal body weight formulas was always to approximate lean body mass, but height and weight alone are blunt instruments for that job.6PubMed Central. Ideal body weight: a commentary – Section: Criticism of the Predictive Formulas: Ideal Body Weight and Body Mass Index
Where Ideal Body Weight Still Matters in Medicine
Even with all their limitations, ideal body weight formulas remain clinically important in specific settings where using a patient’s actual body weight could cause harm. Two areas stand out: ventilator settings and drug dosing.
Ventilator Settings
When someone is on a mechanical ventilator, the volume of air pushed into their lungs with each breath needs to match the size of their lungs, not the size of their body overall. Lung capacity tracks with height and sex rather than total body weight. A person who is five foot six with healthy lungs has roughly the same lung volume whether they weigh 140 pounds or 280 pounds. If you set the ventilator based on actual weight in an obese patient, you risk blowing too much air into lungs that are no bigger than those of a lighter person at the same height. This can cause serious injury.
Lung-protective ventilation strategies use predicted body weight (essentially a synonym for ideal body weight in this context) to calibrate tidal volumes, typically aiming for about 6 to 8 milliliters per kilogram of predicted weight.7PubMed Central. Tidal volume in acute respiratory distress syndrome: how best to select it In pediatric patients, the distinction between actual and ideal body weight becomes even more consequential. One study of children with acute respiratory distress syndrome found that when tidal volume was recalculated using ideal body weight, the proportion of children receiving volumes above 10 mL/kg was substantially higher than it appeared when calculated from actual weight. Higher tidal volumes were associated with worse outcomes in overweight children and in those with severe disease.8PubMed Central. Association Between Tidal Volumes Adjusted for Ideal Body Weight and Outcomes in Pediatric Acute Respiratory Distress Syndrome
Drug Dosing
Many medications distribute through the body based on lean tissue rather than total weight. For relatively water-soluble drugs, the volume of distribution correlates strongly with lean body mass, with correlation coefficients reported as high as 0.9. Using lean body mass to predict loading doses for these drugs gives a much more accurate way to hit target blood concentrations than using total body weight.9PubMed. Lean body mass as a predictor of drug dosage. Implications for drug therapy
This is why the Devine formula persists despite its flaws. When a pharmacist or intensivist needs a quick estimate of lean mass at the bedside, a formula that takes five seconds to calculate is enormously practical, even if it is imperfect. The alternative, measuring body composition directly, is time-consuming and often impossible in an emergency.
Adjusted Body Weight for Larger Patients
For patients who are significantly overweight or obese, neither actual body weight nor ideal body weight gives the right answer for dosing certain drugs. Actual weight overestimates how much drug is needed, because fat tissue does not absorb most medications the way lean tissue does. But ideal body weight underestimates it, because obese patients do carry some additional lean mass along with the extra fat. The compromise is adjusted body weight, which splits the difference by adding a fraction of the excess weight (typically 25 to 40 percent of the difference between actual and ideal weight) back onto the ideal weight number.
This approach has real clinical consequences. In a study of the antifungal drug voriconazole in overweight and obese patients, using adjusted body weight to calculate doses produced significantly better results than using total body weight. About 65 percent of patients dosed by adjusted weight hit the target drug level, compared to about 47 percent of those dosed by total weight.10PubMed Central. Evaluation of Total Body Weight versus Adjusted Body Weight Voriconazole Dosing in Obese Patients That kind of gap in therapeutic accuracy shows why getting the weight basis right is not just academic bookkeeping.
How Ideal Body Weight Compares to BMI
You might wonder why clinicians use ideal body weight formulas at all when BMI is so widely available. The two concepts overlap but serve different purposes. BMI is a continuous scale that categorizes people into ranges (underweight, normal, overweight, obese). Ideal body weight gives you a single target number. In practice, the ideal body weight predicted by most formulas lands somewhere in the BMI 20 to 25 range, but the correspondence is imperfect, especially for women and for people at the extremes of height, as noted earlier.
Some researchers have suggested that using BMI ranges directly might be more practical than relying on ideal body weight formulas, since the formulas do not consistently match a midpoint BMI of 22 for women.11PubMed. Comparison of ideal body weight equations and published height-weight tables with body mass index tables for healthy adults in the United States A study comparing different ways to define ideal body weight found that calculating it as a BMI of 21 multiplied by height in meters squared produced the best agreement with standard BMI categories, with very high consistency between the two systems.12Korean Journal of Family Medicine. Comparative consistency between obesity determination standards using Body Mass Index and Ideal body weight
That finding hints at a practical workaround: rather than memorizing the Devine or Robinson formulas, you can get a reasonable ideal weight estimate by simply calculating what you would weigh at a BMI of 21 to 22. For many purposes outside of specific drug-dosing or ventilator protocols, that approach may be good enough.
Racial and Ethnic Variations the Formulas Miss
The standard ideal body weight formulas were developed and validated primarily on white Western populations, and they do not account for documented differences in body frame and composition across racial and ethnic groups. Research has found significant variation in frame sizes between Caucasians and African Americans, and frame size has a stronger correlation with weight than height alone does. Meanwhile, the World Health Organization has observed that some Asian populations carry higher health risks from conditions like type 2 diabetes and cardiovascular disease at BMI values between 22 and 25, a range that most formulas would classify as perfectly healthy.13PubMed Central. Ideal body weight: a commentary – Section: Criticism of the Predictive Formulas: Ideal Body Weight and Body Mass Index
This means that a formula-generated ideal weight might be meaningfully wrong for a person of East Asian, South Asian, or African descent, not because the math is broken but because the population assumptions baked into the formula do not apply. Some countries and clinical guidelines have started using lower BMI cutoffs for Asian populations specifically because of this mismatch. If you are using an ideal body weight calculator online, there is almost certainly no adjustment for ethnicity built in.
Ideal Body Weight in Children
Estimating ideal body weight in children is even more complicated than in adults, because children are growing and their proportions change dramatically with age. Adult formulas like Devine simply do not work for pediatric patients. Instead, clinicians use methods based on growth charts, such as the McLaren method, which plots a child’s weight against their height percentile on standard reference charts.
When researchers compared seven different methods for calculating ideal body weight in children, they found that most produced statistically different results from the McLaren reference standard. One method that performed relatively well in younger children (the Moore method, with a mean difference of only about 0.07 kg) diverged more in older children. A BMI-based method showed good agreement in a separate cohort, with a mean difference of about 0.17 kg. Across the board, discrepancies became more pronounced at very high or very low height percentiles and in older age groups.14PubMed Central. Evaluation of Different Methods Used to Calculate Ideal Body Weight in the Pediatric Population
For children with obesity who need medication dosing, the lack of agreement among ideal body weight calculations is a genuine safety concern. A study evaluating five different methods for selecting drug doses in children with obesity found limited agreement between them, meaning the same child could receive meaningfully different drug doses depending on which formula a clinician chose.15PubMed Central. Assessing the Agreement of 5 Ideal Body Weight Calculations for Selecting Medication Dosages for Children With Obesity Until a more standardized pediatric method emerges, clinicians often rely on institutional protocols, which vary from hospital to hospital.
The Role of Ideal Body Weight in Bariatric Surgery Outcomes
If you have looked into weight-loss surgery, you have probably come across “percent excess weight loss,” which is the main way bariatric surgery results get reported. This metric takes the difference between a patient’s starting weight and their ideal body weight, then reports how much of that excess has been lost after surgery. The catch is that the reported success rate of a surgery can vary significantly depending on which ideal body weight definition is used in the denominator.16Surgery for Obesity and Related Diseases. Reported excess weight loss after bariatric surgery could vary significantly depending on calculation method: a plea for standardization
Using the Devine formula as your ideal weight gives a different excess weight number than using a BMI of 25 as your target, which gives a different number than using a BMI of 22. The surgical outcome itself has not changed at all; only the yardstick has. Researchers have called for standardization in how these results are reported, but as of now, when you see a claim that a particular procedure achieves “70 percent excess weight loss,” it is worth asking: 70 percent of excess weight defined by what? The answer matters more than the headline number.
Waist-to-Height Ratio as a Different Approach Entirely
Ideal body weight formulas and BMI both try to infer health risk from height and weight. But a growing body of research suggests that where you carry your weight matters more than how much you weigh. Waist-to-height ratio, which is exactly what it sounds like (measure your waist, divide by your height), has emerged as a simple alternative that captures abdominal fat distribution without needing any formula at all.
A meta-analysis found that waist-to-height ratio had a stronger association than BMI with diabetes and metabolic syndrome in cross-sectional studies. In prospective studies, it also appeared superior to BMI for predicting cardiovascular disease, cardiovascular death, and death from all causes.17PubMed Central. Predicting cardiometabolic risk: waist-to-height ratio or BMI. A meta-analysis In children and adolescents, waist-to-height ratio performed similarly to both BMI and waist circumference for identifying those at elevated cardiometabolic risk.18PubMed Central. Waist-to-height ratio as a screening tool for obesity and cardiometabolic risk
The practical appeal is obvious: you need a tape measure and nothing else. A general boundary of keeping your waist circumference below half your height has been proposed as a simple screening threshold. Unlike ideal body weight formulas, this approach does not hand you a single target number to chase. Instead, it gives you a ratio that reflects the kind of fat distribution most closely linked to disease risk. For someone trying to gauge their own health rather than calculate a drug dose or ventilator setting, it may be a more useful tool than any ideal body weight formula ever was.
The Psychology of a Single “Ideal” Number
There is a reason ideal body weight calculators are among the most popular health tools online: people want a definitive number to aim for. But the science behind these formulas does not support that level of certainty. A formula that was created to dose antibiotics in the 1970s, validated on a narrow population, and known to underperform at height extremes and across ethnic groups is a shaky foundation for a personal weight goal.
Research has documented that physical attributes like weight and height, when scrutinized against societal standards, contribute to a range of mental health effects including lower self-esteem and disordered eating. The existence of a single, formula-derived “ideal” weight can amplify that pressure by framing any deviation as being medically suboptimal, even when the formula itself is crude and the person is perfectly healthy. For someone who is naturally broad-shouldered, heavily muscled, or simply built differently from the population the formula was calibrated on, the number can be more misleading than helpful.
If you do use an ideal body weight formula, treat it as one rough input among many rather than a verdict. Your doctor, a body composition measurement, your waist-to-height ratio, and your actual metabolic health markers will all tell you more about where you stand than a formula that has not fundamentally changed since a pharmacist scribbled it down to dose gentamicin fifty years ago.