An ideal weight calculator hands back one tidy number, the kind that looks like settled science. The doubt starts the moment you open a second calculator, on another health site or app, and the result comes back different.
There are at least four ideal weight formulas in use today. For the same height, the same age, and the same sex, they disagree by as much as 8 kg, depending on build. That gap is not rounding. It is how each formula was designed decades ago, in hospitals, for different jobs.
Here you will see what those formulas are, why they fight each other, what ideal weight misses about your body, and how to use the calculator without falling for the single number trap.
Four formulas, four weights for the same person
An ideal weight calculator may run Devine, Robinson, Miller, or Hamwi, and not one of the four returns the same number for the same body. The formula choice shifts the result by several kilos, even when you type in identical data.
Take a man of 6 ft 1 in (1.85 m). The inputs stay the same, only the formula changes. The result it calls "ideal" looks like this:
| Formula | Year | Ideal weight (man, 6 ft 1 in) |
|---|---|---|
| Hamwi | 1964 | 82.6 kg (182 lb) |
| Devine | 1974 | 79.5 kg (175 lb) |
| Robinson | 1983 | 76.4 kg (168 lb) |
| Miller | 1983 | 74.3 kg (164 lb) |
Between the lowest and the highest value, 8.3 kg of spread, close to 18 lb. For a woman of average height, the gap narrows to 2 or 3 kg, but it never closes. The taller the person, the wider the disagreement, because each formula adds a different weight per inch above 5 ft (1.52 m).
Most calculators never tell you which formula runs under the hood. US and international apps often default to Robinson or an average of the four. You rarely get to see the engine, because that detail almost never shows on screen.
Why each ideal weight calculator formula disagrees
The four ideal weight formulas were built in hospitals to set medication doses, not to define a healthy weight. The original context explains why they split apart.
The Devine formula appeared in 1974, in a note on calculating antibiotic doses for adults. The doctor needed a fast estimate of standard body mass to adjust a gentamicin dose. There was no cosmetic goal behind that number.
Robinson, Miller, and Hamwi came later, each calibrated on a different population sample. Hamwi looked at people living with diabetes. Robinson and Miller revised Devine to correct what they saw as overestimation in shorter people. A review indexed on PubMed compares the four and shows they all tend to slip in very tall or very short adults.
📊 Worth keeping: none of the four classic ideal weight formulas was designed with the average adult of 2026 in mind. All of them came out of hospital studies from the 1960s to the 1980s, on narrow samples.
So the result of an ideal weight calculator works as a general reference, not a clinical target. For anyone thinking about losing or gaining weight, the number points a direction and nothing more.
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What an ideal weight calculator cannot see
An ideal weight calculator treats a kilo of fat the same as a kilo of muscle, and that blind spot changes how you read the whole result. To the formula, the scale number is all that counts, whether it comes from muscle, fat, or retained water.
Two adults can share the exact same calculated ideal weight and live in completely different bodies. An athlete at 5 ft 9 in and 165 lb carries high lean mass and low body fat. A sedentary adult of the same height and weight can hold twice the body fat and half the muscle. The calculator cannot tell them apart.
The other blind spot is distribution. Fat packed around the abdomen carries more metabolic risk than fat spread across hips and legs. A tape measure at the navel reads what the calculator ignores. As a general reference, Harvard's Nutrition Source flags waist values above 40 inches (102 cm) in men and 35 inches (88 cm) in women.
⚠️ Careful: hitting the ideal weight band on a calculator does not mean you are healthy. People with little muscle can sit inside the "ideal" and still carry a high body fat percentage, a pattern sometimes called "skinny fat".
The healthy range beats the single number
Healthy weight is not a point, it is a range, and the people who use the range decide better than the ones chasing a fixed screen number. The range grows out of a BMI between 18.5 and 24.9, called normal by the World Health Organization for adults aged 20 to 59.
Applied to height, that band opens a wide window. For 5 ft 7 in (1.70 m), it runs from 117 lb (53.5 kg) at BMI 18.5 to 158 lb (72 kg) at BMI 24.9. For 5 ft 11 in (1.80 m), it runs from 132 lb to 178 lb. That is close to 20 kg of margin inside the zone considered healthy, room enough to hold very different builds.
The exact point inside that band depends on muscle mass, age, and health history. To see where the band comes from, it helps to read what BMI is and how it is calculated. If you want the tool itself, an adult BMI calculator shows the band applied to your case, and the BMI calculator for women covers the female read.
How to use an ideal weight calculator without fooling yourself
Use an ideal weight calculator as a starting point, cross it with a tape measure, and prefer the range to the single number. Three simple rules keep the result from turning into a source of frustration.
The first rule is to run the math on at least two calculators from different sites. If the results diverge by more than 3 kg, write down the interval and work with it, not with one value. That interval is more honest than any tidy number.
The second rule is to pair the calculator with a tape measure. Measure your waist at the navel, without holding your breath, breathing out normally. That single reading tells a story the raw weight cannot. From there, knowing how many calories you should eat and how to hold a calorie deficit turns the number into a plan.
The third rule is to treat the result as a clue, not a scale target. ContaCal is the app that reads the food on your plate from a photo and works out calories and macros for you. Whoever chases an exact 137 lb when the healthy range runs from 128 to 156 almost always ends up a hostage to the scale. Weight answers to what you eat and how much you move, and both shift with habit, not willpower.