How ideal weight calculators actually work, the difference between the major formulas, and why body type (somatotype) and frame size change what "ideal" really means for you.
Several formulas exist for estimating "ideal" body weight, all developed decades ago primarily for medical dosing calculations rather than as a personal fitness target. The most common include the Devine, Robinson, and Hamwi formulas — each uses height and sex as the main inputs, with slightly different base weights and adjustments per centimetre of height above a reference point.
These formulas produce a single number, but it's important to understand what that number actually represents: a population-level statistical estimate, not a precise individual target. Real healthy weight varies meaningfully around these figures based on muscle mass, frame size, and individual body composition.
Get an estimate using multiple recognised formulas, side by side.
Open Ideal Weight Calculator →Two people of identical height can have genuinely different healthy weights due to differences in bone structure and frame size. Someone with a naturally larger frame (broader shoulders, larger bone structure) will carry healthy weight at the higher end of any formula's range, while someone with a smaller frame may be perfectly healthy at the lower end — at the exact same height.
This is one of the clearest illustrations of why ideal weight calculators give a useful starting estimate rather than a definitive target — the same way BMI gives a starting point rather than a complete diagnosis.
Body typing (somatotype theory) categorises natural body tendencies into three broad types. Most people are a blend rather than a pure single type, but the framework is a useful way to understand why identical training and diet can produce different results for different people:
| Type | Natural Tendency | Common Characteristics |
|---|---|---|
| Ectomorph | Naturally lean | Narrow frame, fast metabolism, finds it harder to gain weight/muscle |
| Mesomorph | Naturally muscular | Athletic build, gains muscle relatively easily, moderate frame |
| Endomorph | Naturally higher body fat | Wider frame, gains weight more easily, often strong baseline strength |
Get an estimate of your natural body type tendency to better understand your training and nutrition response.
Open Body Type Calculator →The most useful way to apply ideal weight and body type information is as context rather than as a rigid target:
The formulas behind ideal weight calculators — Devine, Robinson, Miller, Hamwi — were not developed to tell individuals what to weigh. Most originated in clinical settings for a narrow purpose: estimating appropriate medication dosages, where a rough weight reference was needed and body composition was not the concern.
Their later adoption as personal weight targets was not what they were designed for. This matters, because it explains why they disagree with each other. The same person can enter identical height and sex into four different formulas and receive four different answers spanning several kilograms.
None of these formulas incorporates body composition, bone density, muscle mass, age, ethnicity, or health status. They are height-and-sex lookup tables with different constants.
The three body types — ectomorph, mesomorph, endomorph — were proposed in the 1940s as a way of describing physique. They remain a reasonably intuitive vocabulary for talking about the fact that people are built differently.
What they are not is a scientifically validated system for prescribing diet or training. The original classification has limited empirical support as a predictor of how an individual responds to nutrition or exercise, and most people do not fall cleanly into one category.
The genuinely useful observation buried in the concept is unremarkable but true: people differ, and a plan that works for one person may not suit another. That does not require a typology to justify it.
See your GP if you are concerned about your weight, if your weight has changed substantially without an obvious explanation, or if you want guidance grounded in your actual health rather than a formula.
If thinking about weight, food, or body shape is causing you distress, occupying a great deal of your attention, or affecting how you eat, that is worth talking to someone about. Your GP can help, and in Australia the Butterfly Foundation National Helpline offers free and confidential support on 1800 33 4673.
The Hamwi formula was published in 1964 for estimating dietary needs in diabetes management. Devine appeared in 1974 to guide medication dosing. Robinson and Miller followed in the 1980s, each refitting the constants against different reference populations.
They differ because they were fitted to different groups of people, for different clinical purposes, in different decades. None was validated as a personal weight target, and none of the authors proposed it as one. The spread between their outputs for the same person is not a flaw in any single formula — it is an honest reflection of how little a height-and-sex equation can determine.
When a calculator presents several formulas side by side, the useful information is the width of the range, not the average of it. A spread of several kilograms tells you the concept itself is imprecise.
Frame size adjustments, usually estimated from wrist circumference or elbow breadth, attempt to correct for skeletal differences between people of the same height. The adjustment is real but modest, and the measurement itself is imprecise.
It is worth understanding what frame size cannot do. It does not account for muscle mass, which varies far more between individuals than skeletal frame does. Someone who has trained with resistance for years may sit well above every adjusted figure while carrying little excess fat. Frame size narrows the error slightly; it does not make the formula personal.
Ideal weight formulas are clinical estimation tools that acquired a second life they were never designed for. They disagree with one another, ignore body composition entirely, and describe nothing about your health.
Somatotypes offer a rough vocabulary for the fact that bodies differ, and little more than that. Neither should be used to set a personal target.
This page provides general information only and is not medical advice. If you have questions about your weight or health, speak with your GP or an accredited practising dietitian, who can consider your circumstances as a whole.
BMI is a ratio of weight to height. It cannot see body composition, which is why it is a population screening tool rather than an individual diagnosis.