Ideal Body Weight Formulas Explained (Devine, Robinson, Hamwi)

Where These Formulas Actually Came From

This context changes how much weight you should give the output, so it goes first.

The Hamwi formula arrived in 1964 as a quick bedside method for setting dietary targets for people with diabetes. The Devine formula followed in 1974, developed for estimating drug dosages, since some medications are dosed against lean mass rather than total body weight. Robinson and Miller both published revisions in 1983, adjusting the constants to fit population data more closely.

None of these were built as personal health targets. They were clinical shortcuts, designed to be calculable in seconds without a computer, and calibrated against the populations their authors had in front of them decades ago. That they migrated into fitness culture as statements about how much a person ought to weigh is an accident of history rather than a scientific development.

The Four Formulas

All four share the same shape: a base weight for someone 5 feet tall, plus a fixed amount per additional inch. All produce kilograms.

Notice the structural weakness. Every formula breaks down below 5 feet, where the term in brackets goes negative, and the differing per-inch increments mean the four answers diverge more the taller you get. Hamwi adds 2.7 kg per inch for men while Miller adds 1.41, so by 6’4″ they are describing noticeably different people.

To convert your height: multiply centimetres by 0.3937 to get inches, or divide by 2.54.

What They Produce for the Same Person

Man, 5’8″ (173 cm)

That is 68 inches, so 8 inches above the 60-inch base.

Woman, 5’5″ (165 cm)

65 inches, so 5 above the base.

The spread is 2.4 kg for the man and 3.4 kg for the woman. Four methods, all published in peer-reviewed clinical literature, disagreeing by several kilograms about the same body. That disagreement is the most useful thing these formulas tell you, because it demonstrates that no single ideal number exists to be found.

How This Compares to a Healthy BMI Range

Put the woman above through BMI instead. At 1.65 m, the healthy BMI band of 18.5 to 24.9 corresponds to a weight range of roughly 50 to 68 kg.

The four formulas produced 56.5 to 59.9 kg, which sits comfortably inside that band, toward the lower middle. So the formulas are not wrong. They are just answering a different question, offering a point estimate where BMI offers a range.

An 18 kg span sounds imprecise next to a tidy 57.0 kg. It is also considerably more honest, because two people of identical height can be healthy at genuinely different weights depending on frame size, muscle mass, and where they carry fat. Our guide to healthy BMI ranges covers how to read that band and where it needs qualifying.

What the Formulas Cannot Account For

Every one of these formulas takes exactly two inputs: height and sex. Everything below is invisible to them.

• Muscle mass, which is the big one. A trained 5’8″ man at 78 kg may be leaner and healthier than an untrained one at 68 kg, yet the formula flags him as 10 kg over.
• Frame size. Wrist and elbow breadth vary substantially between people of the same height, and older clinical practice adjusted IBW by roughly 10% up or down for large or small frames.
• Age, which shifts body composition considerably across adulthood.
• Ethnicity and ancestry, given the formulas were derived largely from mid-twentieth-century Western populations.
• Anyone under 5 feet tall, where the maths simply fails.
• Pregnancy, amputation, and many medical conditions, all of which require different approaches entirely.

Body composition is the gap that matters most, and it is fillable. The body fat calculator tells you what proportion of your weight is actually fat, which is the question the ideal weight formulas were never able to ask.

Where They Genuinely Matter

Having spent several sections questioning these formulas as personal targets, it is worth saying clearly that they remain genuinely useful in clinical settings.

Certain drugs distribute through lean tissue rather than fat, so dosing by actual body weight would overdose a person carrying significant fat mass. Devine’s formula exists precisely for this and is still used. Mechanical ventilation is another case: lung volume scales with height rather than weight, so protective ventilation protocols set tidal volume against predicted body weight, and using actual weight could injure a patient’s lungs.

In both situations the formula is not making a claim about how anyone should look. It is estimating a physiological quantity that correlates with height. That is a reasonable job for a two-variable equation, and a very different one from prescribing a personal goal weight.

A Better Way to Set a Target

If you want a number to aim at, build it from a range rather than a formula.

Start with the healthy BMI band for your height, which gives you the outer boundaries. Narrow it with a body fat measurement, since that tells you whether weight at the upper end is muscle or fat. Add a waist measurement, keeping it under half your height, because central fat carries most of the metabolic risk that any of this is ultimately about.

Then weigh your own history against all of it. A weight you maintained comfortably for years without restriction is far better evidence of what suits your body than a 1974 equation calibrated on someone else’s population.

And if a single target number tends to become something you fixate on, skip it. Waist measurement and how your clothes fit track the same underlying changes without handing you a figure to fight with.

Frequently Asked Questions

Which ideal weight formula is most accurate?

None is meaningfully more accurate than the others, because there is no gold-standard ideal weight to be accurate against. Devine is the most widely used, largely through clinical convention rather than superiority. Robinson and Miller were designed to fit population data slightly better. Treat the spread between all four as the honest answer.

Why do the formulas give different answers?

They were derived at different times, from different populations, using different base weights and per-inch increments. Hamwi adds 2.7 kg per inch for men where Miller adds 1.41, so the results diverge progressively with height. The disagreement is a feature of the underlying uncertainty, not an error in any one formula.

Should I actually aim for my ideal body weight?

Aim for a healthy range instead. The formulas produce a single figure that implies a precision they do not possess, and they cannot see muscle mass, frame size, or fat distribution. A weight anywhere inside your healthy BMI band, with a waist under half your height, is a more sensible goal.

Do these formulas work for very short or very tall people?

Poorly at both extremes. Below 5 feet the calculation goes negative and produces nonsense, and above roughly 6’2″ the four formulas diverge enough that picking one becomes arbitrary. Height-based ranges from BMI hold up better outside the middle of the distribution.

Is ideal weight different for men and women?

Yes, and every formula includes a sex adjustment. Men are assigned a higher base weight at the same height, reflecting greater average lean mass and bone density. Devine, for instance, starts men at 50 kg and women at 45.5 kg for a 5-foot frame.

How do I adjust for frame size?

Traditional clinical practice adjusted the result by about 10% in either direction, subtracting for a small frame and adding for a large one, with frame estimated from wrist circumference or elbow breadth. It is a rough correction, but it does acknowledge a real source of variation the base formulas ignore.

What should I use instead if I am building muscle?

Track body fat percentage and waist measurement rather than scale weight. Someone gaining muscle while losing fat can hold a completely steady weight for months, which makes every weight-based formula useless for measuring their progress and body composition measures ideal.

The Bottom Line

The Devine, Robinson, Miller, and Hamwi formulas each take your height and sex and return a single number, and for the same person those numbers can sit three kilograms apart. That spread is worth more than any individual result, because it shows there is no precise ideal weight waiting to be calculated. Use the formulas as a rough anchor, then set your actual target from your healthy BMI range, your body fat percentage, and a waist measurement under half your height. Compare all four at once with the ideal weight calculator, and treat the disagreement as information rather than a problem.

Sources and Further Reading

• Devine BJ. “Gentamicin therapy.” Drug Intelligence and Clinical Pharmacy, 1974 – origin of the Devine formula for drug dosing.
• Hamwi GJ. “Therapy: changing dietary concepts.” American Diabetes Association, 1964.
• Robinson JD, et al. “Determination of ideal body weight for drug dosage calculations.” American Journal of Hospital Pharmacy, 1983.
• Pai MP, Paloucek FP. “The origin of the ideal body weight equations.” Annals of Pharmacotherapy, 2000.