There is no such thing as one ideal body weight, but there are several formulas that will happily give you a number anyway. BMI, the Hamwi formula and the Mifflin-St Jeor equation get used interchangeably, and they should not be. Two of them estimate a weight. The third does something else entirely.
BMI: a population tool doing a personal job
BMI is your weight in kilograms divided by the square of your height in metres. That is the whole calculation. It was built to describe groups of people, and for that it works well enough.
The trouble starts when you apply it to one person. It knows nothing about muscle, bone, or where your fat sits. A lean rugby player and a sedentary man of the same height and weight get the same score, and only one of them has a problem. It misses the opposite case too: the person whose BMI sits comfortably in the normal band while carrying very little muscle and a fair amount of abdominal fat.
BMI is a screening tool, not a verdict. We go through where it breaks down in more detail in our piece on BMI accuracy and its limits.
The Hamwi formula
Dr G. J. Hamwi published his method in the 1960s and it survives because you can do it in your head.
For men, start at 106 pounds for the first five feet of height, then add 6 pounds for every inch above that. For women, start at 100 pounds and add 5 pounds per inch. So a man of 5 foot 10 lands at 166 pounds, and a woman of 5 foot 4 lands at 120.
It was made for quick clinical estimates, not for athletes or for anyone at the extremes of height, and it has no idea what you are made of. But it is fast, and as a rough anchor it holds up better than its age suggests.
Mifflin-St Jeor is not an ideal weight formula
This one gets miscategorised constantly. Mifflin-St Jeor does not tell you what you should weigh. It estimates how many calories you burn at rest from your current weight, height, age and sex, and you multiply that by an activity factor to get your daily total.
That makes it a different kind of tool. Instead of naming a target on the scale, it tells you what to eat to move toward one. If you want a number you can act on this week rather than a number to aspire to, this is it, and the TDEE calculator on the homepage will run it for you.
So which one should you use
For a quick check across a whole population, BMI. For a rough personal target with no equipment, Hamwi. For deciding what to actually eat, Mifflin-St Jeor.
None of the three measures body composition, and that is the gap that matters. Two people at the same ideal weight can carry very different amounts of muscle, and the one with more of it will be healthier, stronger and hungrier. If composition is what you care about, measure it directly instead of inferring it from a height chart. Our guide to measuring body fat covers the practical options, and waist-to-height ratio works better than BMI as a one-number screen for most people.
Picking a target you can actually use
Start with what you are trying to do. Losing fat, holding steady and building muscle are different jobs and they do not share a target weight. Someone adding muscle may watch the scale go up while every other marker improves.
Then be honest about your build and your training history. A lifter with ten years behind them will sit above all of these formulas at a lower body fat than the chart assumes. Someone who has never trained will sit closer to the estimate.
The number you get is a starting point for a conversation, not a finish line. Pick one, treat it as a direction of travel, and pay more attention to your waist measurement, your training, and how you feel at the weight you end up at.