Type "what's my ideal weight" into any search bar and you'll get a calculator. Type your height, get a number. Done. Except — try a different calculator and you'll get a different number. Try four and you'll get four. So which one is right?

The answer most people don't want to hear: none of them are. Every "ideal weight" formula was built for a specific clinical purpose, none of which is "tell you the perfect number to weigh." Understanding what they actually do is the difference between chasing a number and finding a weight you can live at — comfortably, healthily, sustainably.

Where ideal weight formulas came from (and why they disagree)

The four formulas you'll see on most calculators are Hamwi (1964), Devine (1974), Robinson (1983), and Miller (1983). All four were developed for medical drug dosing — anesthesiologists and pharmacists needed a quick way to estimate lean body mass so they could dose medications correctly. They were never designed to tell a healthy person what they should weigh.

Here's what each formula gives a 5'8" woman:

Five-pound spread between formulas, all calling themselves "ideal." That's because each one was tweaked using slightly different patient populations and slightly different goals. None of them factored in what the person actually wants out of their body.

Run your own numbers on the ideal weight calculator — but read on before treating any of them as gospel.

What "ideal" actually depends on

Your truly ideal weight — the one you should aim for and maintain — depends on a half-dozen things the formulas can't see:

1. Body composition, not body weight

Two people at the same height and weight can have radically different health risks if one is 30% body fat and the other is 18%. The "ideal weight" formulas assume an average body composition. If you carry more muscle than average, the formula will tell you you're 15 pounds overweight when your body fat percentage is actually low. Body composition is more useful than scale weight here — see our breakdown of body fat measurement methods.

2. Frame size

Skeletal frame size — small, medium, large — varies by 10–15% across the population. Large-framed people carry more bone and structural tissue. Their "ideal" weight at the same height can legitimately be 10+ pounds higher than someone with a small frame. The fastest proxy: measure your wrist circumference. For most adults, a wrist under 6.5" is small frame, 6.5–7.5" is medium, over 7.5" is large.

3. Where the weight sits

A 165-pound man with a flat stomach and most of his weight in shoulders and legs is in a different health category than a 165-pound man with the same height carrying that weight in his midsection. This is why waist-to-hip ratio often beats BMI as a risk metric.

4. Age

Some research suggests that a slightly higher BMI (24–27) is associated with the lowest mortality in adults over 65 — the so-called "obesity paradox." For older adults, being underweight carries more risk than being modestly overweight. The "ideal weight" you should target at 35 isn't necessarily the one to chase at 70.

5. Genetics and ethnicity

Standard BMI thresholds underestimate metabolic risk in South Asian, East Asian, and some Middle Eastern populations, where elevated diabetes risk shows up at lower BMIs. The WHO has issued lower thresholds for these groups (e.g., a "healthy" BMI top end of 23 instead of 24.9 for Asian adults). The ideal-weight formulas don't account for this at all.

6. What you can sustain

This is the one calculators ignore entirely, and it's arguably the most important. A weight you can only hold by white-knuckling your diet isn't your ideal weight — it's a temporary state. The weight you can comfortably maintain for years, eating like a normal person, sleeping enough, and not feeling food-obsessed, is your ideal weight, even if no formula points there.

A more useful definition

If all five of those are true at your current weight, you're already at your ideal weight, regardless of what any formula says. If two or three are off, the formulas can give you a rough target — but the target is always provisional. Adjust based on how the body responds.

How to use ideal weight numbers without getting trapped by them

1. Treat the number as a range, not a point. If three formulas put you between 140 and 150 lb, your reasonable target range is 140–150, not 145.0 exactly. Healthy people fluctuate within a 3–5 pound range every week.
2. Combine with body composition. If your weight is 10 pounds above the "ideal" but your body fat is 18%, you're not overweight in any meaningful sense. The formula is wrong about you.
3. Track changes, not snapshots. Your weight today vs. your weight three months ago tells you something useful. Your weight today vs. a 1974 anesthesiology formula tells you almost nothing.
4. Pay attention to how you weigh what you weigh. Same scale weight, more muscle and less fat over time, is a win — even though the scale didn't move.

The bottom line

Ideal weight calculators give you a single number derived from a 50-year-old drug-dosing formula and a guess about your height. They're a useful starting point for a conversation, not a destination. Your real ideal weight is the one that lets you live well in your body — at a body fat percentage that protects your health, with a fat distribution that doesn't raise your risks, and at a level you can hold without making your life smaller.

Use the ideal weight calculator for the rough range. Use body fat percentage and WHR to know whether the weight you're at is the right kind of weight. And use your own life — energy, sleep, mood, sustainability — to know whether you're actually there.