Ideal Weight Explained: What the Formulas Say, What They Miss, and How to Use Them

The phrase ideal weight sounds like a precise target, but the formulas behind it are historical approximations, each built for a particular purpose and each with clear limits. The most widely known, the Devine formula, was developed in the 1970s for estimating drug dosages, not for defining health or attractiveness. Other formulas, such as Robinson and Miller, adjust the same basic idea slightly, while BMI-based ranges work backwards from a body mass index range to a weight range for a given height. The Ideal Weight Calculator brings these formulas together so you can compare them, but comparing them is useful only if you understand that they disagree with each other, sometimes by several kilograms, and that the disagreement itself is information: it shows how approximate any single ideal weight figure really is.

The Devine formula is the easiest to work by hand. For men, it starts at 50 kilograms for a height of 5 feet, or 152.4 centimetres, and adds 2.3 kilograms for each inch above 5 feet. For women, it starts at 45.5 kilograms for 5 feet and adds the same 2.3 kilograms per inch. Heights below 5 feet subtract the same amount per inch below. The formula is linear, which is both its appeal and its weakness: real relationships between height and healthy weight are not perfectly linear, and the formula takes no account of frame size, muscle mass or age. It was designed to give a reasonable dosing weight for adults of typical build, and it does that job adequately, but it was never intended as a personal health verdict.

Here is a worked Devine example, checked step by step. Take a man who is 5 feet 10 inches tall. Convert the height to inches: 5 feet is 60 inches, plus 10 inches equals 70 inches. The height above 5 feet is 70 minus 60, which equals 10 inches. Multiply by 2.3 kilograms per inch: 10 multiplied by 2.3 equals 23 kilograms. Add the base of 50 kilograms: 50 plus 23 equals 73 kilograms. The Devine ideal weight for this man is therefore 73 kilograms. For a woman of the same height, the same calculation with the 45.5 kilogram base gives 45.5 plus 23, which equals 68.5 kilograms. The Robinson formula, which uses a slightly different base and increment, would give a figure a few kilograms different, and that difference should not be treated as an error in either formula: it is a reminder that ideal weight is a range of reasonable estimates, not a single true value.

BMI-based ranges take a different route. Body mass index is weight in kilograms divided by height in metres squared. The commonly cited healthy BMI range is 18.5 to 24.9, although that range itself is a population guideline with known limitations. To turn it into a weight range, multiply each BMI boundary by height squared. For a person 1.78 metres tall, height squared is 1.78 multiplied by 1.78, which equals 3.1684. Multiplying by 18.5 gives about 58.6 kilograms, and multiplying by 24.9 gives about 78.9 kilograms, so the BMI healthy range for that height is roughly 58.6 to 78.9 kilograms. That is a span of more than 20 kilograms, which illustrates an important point: even within the BMI framework, healthy weight is a broad range, not a point, and two people of the same height can both be within range at very different weights.

The limits of all these formulas deserve emphasis, because they are routinely over-interpreted. Muscle weighs more than fat for the same volume, so a muscular person can exceed a formula ideal weight while having low body fat and good fitness. Bone density and frame size vary. Older adults may lose muscle while gaining fat at the same weight, so weight alone hides a change in composition. Athletes, pregnant people, children and adolescents should not be assessed with adult ideal weight formulas at all, because their growth, training or pregnancy changes the meaning of the numbers entirely. Ethnicity also affects the relationship between BMI and health risk, which is why some health authorities use different BMI thresholds for different populations. A calculator that ignores all of that cannot, by itself, tell you whether your weight is healthy for you.

So how should you use an ideal weight result? Treat it as a starting point for reflection, not as a target to hit at any cost. Compare the formulas the calculator shows: if they cluster around a similar range, that range is a reasonable reference. If they disagree widely, that disagreement is itself a signal to look at other measures, such as waist circumference, body composition, fitness, blood pressure, blood lipids, blood sugar, sleep and how you feel day to day. Look at trends over time rather than a single reading, because weight fluctuates with hydration, meals, clothing and time of day, and a single weigh-in can mislead by a kilogram or more. And set goals in terms of habits and health markers, such as regular activity, balanced eating and adequate sleep, rather than in terms of reaching a formula number by a deadline.

There is also a psychological dimension that responsible guidance cannot ignore. Ideal weight figures, presented without context, can fuel unhealthy restriction, disordered eating or a sense of failure when a body does not match a formula derived for drug dosing half a century ago. If thinking about weight causes distress, or if you are considering a large or rapid weight change, speak to a qualified health professional or a registered dietitian, who can assess your individual situation, including medical history and medications, rather than relying on a general calculator. The same advice applies in the other direction: a result within a formula range is not a guarantee of health, and a result outside it is not a diagnosis of illness. Health is assessed by professionals using multiple measures, not by a single equation.

In summary, ideal weight calculators are useful for education and for giving a rough sense of scale, provided their limits are kept in view. Learn the Devine calculation so you can check it, compare it with a BMI-based range so you see the breadth of reasonable values, and then put the numbers aside and look at the fuller picture of your health. The formulas are estimates, not medical advice, and they are no substitute for professional assessment. Used with that humility, they can inform a conversation with a health professional; used as a verdict, they mislead, and the difference between those two uses is entirely in how the reader treats the result.

Try it next: use the ideal-weight-calculator with your own figures, then compare with the BMI Calculator and the Body Fat Percentage Calculator. For background reading, see BMR Calculator Guide.