Ideal Weight Formulas Explained
Four formulas, four answers, none of them personal. Here is what they measure and how to use a range instead.
Four formulas and a BMI midpoint, all defensible, spanning 5 kg at the same height. That spread is the argument for using a range.
Why the formulas disagree
The classic ideal weight equations were fitted to different populations for different purposes, mostly in the mid-twentieth century. Devine was written for drug dosing, Hamwi for quick clinical estimates, and Robinson and Miller as later refinements of the same idea. They are regressions on height, not models of health.
For a 178 cm man the four formulas return roughly 70 to 75 kg; for a 165 cm woman, about 56 to 60 kg. The spread of 4 to 5 kg is the honest signal in the data: any single number is a convention rather than a finding.
The four classic formulas
Each formula starts from a base weight at 5 feet and adds a fixed amount per additional inch of height. The coefficients differ, which is the entire reason the answers differ.
| Formula | Year | Men | Women |
|---|---|---|---|
| Devine | 1974 | 50 kg + 2.3 kg per inch over 5 ft | 45.5 kg + 2.3 kg per inch |
| Hamwi | 1964 | 48 kg + 2.7 kg per inch | 45.5 kg + 2.2 kg per inch |
| Robinson | 1983 | 52 kg + 1.9 kg per inch | 49 kg + 1.7 kg per inch |
| Miller | 1983 | 56.2 kg + 1.41 kg per inch | 53.1 kg + 1.36 kg per inch |
What the formulas were designed for
None of them was designed to set a goal weight. Devine comes from a paper about drug dosing, where a rough estimate of lean body mass is enough to adjust a dose. Hamwi was a bedside rule of thumb for clinicians. Robinson and Miller refined the same clinical use.
That context explains their biggest limitation. They were built from height and sex alone, in populations that were less muscular and heavier than today, and they assumed an average frame. Using them as a personal target mistakes a dosing convenience for a health recommendation.
The healthy range is wider than a formula
A body mass index between 18.5 and 24.9 is the standard healthy range for adults, and it translates into a band of roughly 12 to 15 kg for a given height. That band, not a single figure, is the defensible answer to how much you should weigh.
| Height | Healthy range (BMI 18.5-24.9) | Men, average of the four formulas |
|---|---|---|
| 160 cm | 47 to 64 kg | about 58 kg |
| 170 cm | 53 to 72 kg | about 66 kg |
| 178 cm | 59 to 79 kg | about 72 kg |
| 185 cm | 63 to 85 kg | about 78 kg |
Frame size, muscle and the limits of height
Height explains a lot of the variation in body weight, and it does not explain all of it. Two people of the same height can differ by 10 kg at the same body fat because of skeletal frame, limb length and muscle mass, and a formula that sees only height has no way to know which of them it is describing.
This is why a muscular person can sit above every formula and be lean, while a sedentary person can sit inside the range with a high body fat percentage. Weight without composition is an incomplete measurement, and no ideal-weight formula fixes that.
Turning a range into a target
A workable target comes from three sources. The healthy BMI band sets the outer limits. Your own history sets the middle, because a weight you held comfortably for years is more achievable than one you have never been at as an adult. And a waist measurement under half your height checks that the weight is distributed in a way that carries the least risk.
Where the three disagree, act on the waist. A weight inside the BMI band with a waist above half your height is a sign that the composition, not the scale, is the problem, and further weight loss may not be the right first move.
Why a single target weight often fails
A single number invites an all-or-nothing reading: above it is failure, below it is success. That framing ignores normal fluctuation of 1 to 2 kg from water and food, and it ignores that the useful question is what you can maintain rather than what you can reach.
A range of 3 to 4 kg with a waist ceiling is easier to hold, survives a holiday, and still captures the health benefit. Most of the benefit of weight loss comes from the first 5% to 10% of body weight, which is often less than the distance to a formula weight.
When a target weight is harmful
For anyone with a history of disordered eating, setting a numeric goal can be counterproductive and needs clinical support rather than a calculator. The same applies to athletes in weight-class sports, where the target interacts with performance and hydration in ways that need a coach and a dietitian.
- A history of disordered eating: no numeric target without support
- Pregnancy and breastfeeding: BMI targets do not apply
- Under 18: adult formulas do not apply
- Rapid unexplained weight change: see a clinician first
Reading the calculator output
The ideal weight calculator shows all four formulas side by side, plus the healthy BMI band for your height. The point of showing them together is that the spread is the honest part of the answer: any single figure would imply a precision the underlying data does not support.
Two practical readings follow. If your weight sits inside the BMI band and your waist is under half your height, the formulas are not telling you anything you need to act on. If your weight is outside the band, the distance to the nearest formula is a worse guide than the distance to the top of the healthy range.
Frequently confused with BMI
The two tools answer different questions. BMI describes where a weight sits relative to a population distribution for a height, and the ideal weight formulas predict a weight from height and sex. Both use height, neither uses body composition, and neither describes an individual.
If you want a single measurement that carries information about risk for you, the waist-to-height ratio does more work than either. It costs one tape measurement and it moves in the right direction when fat is lost from the trunk, which is the fat that matters most.
Frequently asked questions
What is my ideal weight?
There is no single ideal weight. A healthy band from BMI 18.5 to 24.9, a weight you have maintained comfortably before, and a waist under half your height together describe a range that is more useful than any formula output.
Why do ideal weight formulas give different answers?
They were fitted to different populations for clinical purposes such as drug dosing, and they use height and sex alone. The 4 to 5 kg spread between them reflects the limits of the method rather than a disagreement about health.
Is ideal weight the same as healthy weight?
No. Ideal weight formulas produce a clinical convenience figure. Healthy weight is a range derived from population risk data, which is why the BMI band is the better reference.
Does ideal weight change with age?
The formulas include no age term, so they return the same number at 25 and 65. In practice a slightly higher weight is often appropriate with age, and muscle mass matters more than the number on the scale.
What if I am above the healthy range but fit?
Then the waist measurement is the number to watch. Muscular people routinely sit above the BMI range with a waist well under half their height, which is a very different risk profile from the same weight carried as fat.
Should I aim for the lowest healthy BMI?
No. There is no evidence that the bottom of the range is better than the middle. A weight you can hold while eating and training well is a better target than the lowest number the chart allows.
Sources
- Devine BJ, Gentamicin therapy
- Pai MP, Paloucek FP, The origin of the ideal body weight equations
- WHO: Body mass index
- NICE CG189: Obesity identification and classification
Formulas and thresholds are taken from the sources above. They are a starting point for adults, not a diagnosis.
Last reviewed September 12, 2026. How we calculate · Editorial policy