Ideal weight is a reference range, not a single number
An “ideal weight” formula looks precise because it returns a number such as 68.4 kg. The mathematics is simple; the interpretation is not. The classic formulas on this page were created as height-based reference equations, largely for clinical dosing, and they do not measure body fat, muscle mass, frame size, or overall health.
The useful way to read the calculator is therefore as a comparison: four established formulas give four slightly different reference weights, while the BMI band gives a separate height-based range. When those results overlap, you have a useful zone of reference rather than a magic target.
Why the calculator gives four answers
All four classic equations share the same architecture. They start from a base weight at exactly 5 ft (60 in), then add a fixed amount for every inch above 5 ft. What changes from one formula to another is the starting point and the per-inch increment.
| Formula | Male | Female | What changes |
|---|---|---|---|
| Hamwi (1964) | 48 + 2.7 × inches | 45.5 + 2.2 × inches | Largest height slope among the four. |
| Devine (1974) | 50 + 2.3 × inches | 45.5 + 2.3 × inches | Common clinical reference for dosing estimates. |
| Robinson (1983) | 52 + 1.9 × inches | 49 + 1.7 × inches | A later modification with a smaller slope. |
| Miller (1983) | 56.2 + 1.41 × inches | 53.1 + 1.36 × inches | Smallest height slope among these four. |
Step-by-step: how one formula becomes a number
Suppose a man is 5 ft 10 in tall. The first five feet establish the base. The remaining height is 10 inches. Devine then adds 2.3 kg for each of those ten inches to the 50 kg male base.
Only the height above 5 ft enters the increment.
The result is a reference estimate, not a biological optimum.
This same structure is used by the other three equations; only their coefficients change. Calculator.net describes the same 5-ft baseline and per-inch structure for these formulas.
The healthy BMI band answers a different question
Unlike IBW formulas, BMI does not try to produce one “ideal” weight. It asks which weights correspond to a particular BMI interval at a given height. For adults, the commonly used reference band is BMI 18.5–24.9; the calculator uses that band to produce a lower and upper weight at your height.
The band is a screening reference, not a complete assessment of health or body composition.
Upper weight = 24.9 × height(m)²
What should you do when the four formulas disagree?
Nothing is “wrong” simply because the four values are different. They were created from different equations and assumptions. A taller person can see the gap widen because each formula applies a different amount for every additional inch. The comparison is valuable precisely because it exposes that uncertainty instead of hiding it behind one number.
For example, if Devine says 73 kg while another equation says 76 kg, that does not mean one result is correct and the other is a failed calculation. It means the historical models encode different reference points. Treat the cluster as context.
Why height alone cannot define a healthy weight
The calculator does not know your body composition. Two people can have the same height and very different amounts of muscle, bone, fat, and lean tissue. The classic equations also do not capture activity level, fat distribution, or many individual health factors. Calculator.net explicitly cautions that IBW is imperfect and should not be treated as a strict value a person must reach.
That limitation matters especially for muscular athletes and for people at the extremes of height or body composition. BMI has similar limitations because it is based on height and weight rather than a direct measurement of body fat.
Use the calculator as a map, not a verdict
- Start with the range: compare the BMI-based band with the four formula estimates.
- Look for agreement: a cluster of values is more informative than one isolated figure.
- Consider context: muscle mass, age, activity, and health history can change what a sensible target means.
- For medication dosing: never substitute a website estimate for the dosing protocol used by a clinician or pharmacist.
Common questions
Devine is widely cited and commonly used as a clinical reference, particularly in medication-dosing contexts. That does not make it a universal definition of a healthy weight.
That is part of the historical design of the equations: each uses a base weight at 60 inches and then adds a fixed amount per inch above that baseline.
No. The classic IBW equations are intended for adults. Children and adolescents require age- and sex-specific growth references rather than an adult fixed-height formula.
Because these equations and BMI do not directly measure muscle mass or body composition. A muscular person can weigh more than a height-based reference without that alone indicating poor health.