Ideal Weight vs Healthy Weight Range — What's the Difference?
If you have ever used an ideal weight calculator and found a single number, then looked up your healthy BMI range and found a 15–20 kg spread, you have run into the core confusion here.
Are these measuring the same thing? Not exactly. And understanding the difference matters if you want to set a weight goal that is realistic, grounded in evidence, and useful in practice.
What "Ideal Weight" Actually Means
The term "ideal weight" is a bit misleading. It implies a precise scientific target — the one correct number for your body.
In reality, the formulas used to calculate ideal weight (Hamwi, Devine, Robinson, Miller) were developed in the 1960s–80s for a specific clinical purpose: estimating medication doses. Drug dosing often needs to account for lean body mass rather than total weight, and these formulas provided a quick estimate.
They were never designed to tell individuals what they should weigh. They use only two inputs — height and sex — and produce a single number because a single number is what clinicians needed for a dosing formula, not because any single weight is inherently right for every person of that height.
The Ideal Weight Calculator shows results from all four formulas side by side, which is useful precisely because the spread between them illustrates how much these estimates vary even with the same inputs.
What "Healthy Weight Range" Means
A healthy weight range is defined by BMI (Body Mass Index). The World Health Organization defines healthy as a BMI between 18.5 and 24.9.
Because BMI is calculated from height and weight, you can convert those boundaries back to a weight range for any given height. For a 5'9" (175 cm) person, that range is roughly 58–79 kg — a span of about 21 kg.
That wide range reflects a real truth: there is no single healthy weight, only a zone within which most people are at lower risk for weight-related health problems. Two people of the same height can weigh 15 kg apart, both be healthy, and both fall within a normal BMI.
How the Two Numbers Compare
For most heights, the "ideal weight" from the common formulas falls somewhere in the lower-middle portion of the healthy BMI range.
For example, for a 5'7" (170 cm) woman:
- Healthy BMI weight range: roughly 53–72 kg
- Devine ideal weight: 58.2 kg
- Robinson ideal weight: 60.4 kg
- Miller ideal weight: 62.8 kg
- Hamwi ideal weight: 56.8 kg
The formula results cluster around 57–63 kg, all within the healthy range but toward the lower end. A woman in this height who weighs 70 kg is within the healthy BMI range but above what the formulas call "ideal."
That gap is not a problem with the woman's weight. It is a limitation of the formulas.
Why Ideal Weight Formulas Miss Important Factors
Muscle mass
Muscle is denser than fat. A person with significant muscle mass will weigh more than someone of the same height with average muscle. The formulas do not account for this at all.
An athlete who is lean, fit, and at low body fat percentage might weigh 10–15 kg more than the formula's "ideal" — and be in far better health than someone at exactly the formula number with little muscle and high body fat.
This is sometimes called normal weight obesity: falling within the "healthy" BMI range while having excess body fat and low muscle mass. The number looks fine; the body composition does not.
Frame size
People naturally have different bone structures. A large-framed person — broader shoulders, wider hips, heavier bones — will weigh more than a small-framed person of the same height even at the same level of leanness. The formulas ignore this entirely.
A rough way to estimate frame size: wrap your thumb and middle finger around your wrist. If they overlap, you are small-framed. If they just touch, medium. If there is a gap, large-framed. A large-framed person should probably target the upper half of their healthy BMI range, not the lower end.
Age
Older adults naturally carry somewhat more weight relative to height without adverse health effects. Some research suggests that for people over 65, the lowest mortality risk is associated with a BMI in the 25–27 range — technically "overweight" by WHO standards. For this group, targeting the "ideal weight" formula output may actually be too low.
Ethnicity
The healthy BMI thresholds and ideal weight formulas were developed on predominantly white Western populations. Research has shown that people of Asian descent, for example, have higher health risks at lower BMI values — some health organizations recommend lower thresholds (23 for overweight rather than 25) for Asian populations. For other groups, the standard ranges may not be well-calibrated either.
Which Number Should You Actually Use?
Neither number on its own tells you much. What they give you is a starting point for a more complete picture.
A reasonable way to think about it:
1. Healthy BMI range gives you the outer boundaries. If you are within this range and have no metabolic red flags, you are probably at a reasonable weight for your height.
2. Ideal weight formulas give you a mid-range reference point. If you fall well above the formula results and above your healthy BMI range, that is a useful signal that weight loss may benefit your health.
3. Body fat percentage tells you more than either. Knowing whether your weight is mostly lean mass or mostly fat is far more informative than a number on a scale. A body fat percentage in a healthy range — roughly 15–20% for men, 20–25% for women as a broad guide — is a stronger indicator of health than hitting a specific weight target.
The Ideal Weight Calculator shows the formula estimates alongside a BMI-based healthy range, which gives you both data points together. From there, tools like the BMI Calculator and Body Fat Calculator can round out the picture if you want to go deeper.
Setting a Realistic Weight Goal
For most people, a practical weight goal is somewhere within the healthy BMI range — not necessarily the formula's single number.
If you have more muscle than average, you might target the upper portion of the healthy range. If you are naturally small-framed and lightly built, the lower portion might be more appropriate. If you are over 60, staying at the mid-to-upper end of the healthy range is supported by research on longevity outcomes.
The ideal weight formulas are a useful reference. But treating one specific number as the goal — especially one from a formula designed for drug dosing rather than body weight targets — sets a more rigid standard than the evidence supports. A range, adjusted for your own body composition and health context, is a more honest and more useful goal.
