Is There Really an Ideal Weight?
Ask four formulas for your ideal weight and you will get four different answers. That is not a flaw in the arithmetic - it reflects what these equations actually are. The Ideal Weight Calculator shows a range from height.
They were not built for this
The best known, Devine's formula, was published in 1974 and was designed for calculating drug doses, not for telling people what to weigh. It needed a quick rule of thumb for lean body mass so that medication could be scaled sensibly, and it was never validated as a health target.
Hamwi's came from 1964 and insurance actuarial tables. Robinson and Miller later adjusted the constants because Devine's figures looked too low for taller people. All four are simple linear rules of the form "a base weight, plus so much per inch over five feet", with constants chosen to fit a population of the time.
So when they disagree by five or six kilograms, that is four different judgement calls from four different decades, none of which knew anything about you.
What they cannot account for
Frame size. Two people of the same height can have genuinely different skeletal widths, and the formulas take no view on it. The Body Frame Size Calculator exists precisely because that variation is real.
Muscle. The formulas see only height, so a muscular athlete reads as overweight on every one of them. This is the same limitation BMI has, for the same reason.
Age, ethnicity and build. Healthy weight distributions differ across populations, and a formula fitted to mid-century American data does not travel perfectly.
What a range is actually useful for
As a rough bracket rather than a target. If your weight sits far outside any of the formulas for your height, that is worth a conversation with a doctor - not because the formula is authoritative, but because it is a signal worth checking.
What is more informative than any of them: waist measurement, which reflects where fat is carried rather than how much you weigh in total, and what your weight is doing over time rather than its absolute value. A stable weight and a reasonable waist measurement tell a doctor more than a formula from 1974.
And a point worth making plainly: a single target weight is not a health requirement, and treating one as a goal to reach at any cost is a well-trodden route to a difficult relationship with food. If weight is something you are worrying about, a GP or a registered dietitian is a much better place to take that than any calculator.
Common questions
Why do the four formulas disagree?
Different decades, different source populations, different purposes. There is no single correct one.
Is BMI better?
Not better, different - it is a ratio rather than a target, and it shares the same blindness to muscle. The BMI Calculator covers it.
Does frame size really change healthy weight?
Somewhat. It is a real source of variation and a modest one - a few kilograms, not twenty.
What should I use instead?
Waist measurement, how your weight is trending, and how you actually feel. Then ask a professional if something seems off.
Want to see what the formulas give for your height?