Ideal Body Weight Calculator

The most famous one was Case Number 25

The phrase "ideal body weight" carries an enormous amount of authority. It sounds like a fact about you, established by science, sitting there waiting to be looked up. Type your height into this calculator and four separate formulas will hand you a number, each named after a doctor, each precise to the kilogram.

So it is worth knowing where the most famous of those four actually came from.

In 1974, a pharmacist named B. J. Devine published a piece in a journal called Drug Intelligence and Clinical Pharmacy. It was not a study. It was not a survey of healthy weights, or an analysis of who lives longest, or research into what human beings ought to weigh. It was Case Number 25 in a series of clinical pharmacy case studies, and its title was "Gentamicin Therapy".

Gentamicin is an antibiotic. It is effective and it is dangerous: too little does not clear the infection, too much damages your kidneys and your hearing. Devine needed a rule of thumb for how much to give a patient, and he needed it because of a specific problem we will come to in a moment. So he wrote one down. For men, allow 110 pounds for the first five feet of height and 5 pounds for each additional inch. For women, 100 pounds and 5 pounds an inch.

That is it. That is the origin of the number that millions of people now measure their bodies against. A dosing heuristic, in an antibiotic case study, half a century ago.

When two researchers reviewed all of this in 2000, they titled their paper "The origin of the 'ideal' body weight equations", and they put the word ideal in quotation marks. They had earned them.

Why a pharmacist needed to invent your ideal weight

The obvious question is why an antibiotic problem produced a weight formula at all. The answer is genuinely interesting, and it is the key to understanding what these numbers are for.

Most drugs are dosed by body weight, because a bigger person has more blood and more tissue to distribute the drug through. Simple enough. But it stops being simple when the patient is carrying a lot of body fat, because fat is not like the rest of you. Many drugs, gentamicin among them, do not distribute into fat tissue very well at all. They spread through lean tissue and water.

So imagine a 140 kg patient. Dose them on their actual weight and you are dosing as though all 140 kg were going to take up the drug, when perhaps 60 kg of it will not. You have just given them a serious overdose of something that damages kidneys. The arithmetic that keeps a 70 kg patient alive can poison a 140 kg one.

What Devine needed was a number that answered "how much of this patient is the part that matters for the drug?" And a decent proxy for that is: what would this patient weigh if they were not carrying the excess? Hence a formula built from height and sex alone, deliberately ignoring what the person actually weighs.

Read that back, because it is the whole point. The formula ignores your actual weight not because your weight does not matter, but because for dosing purposes it is the wrong variable. It was never asking what you should weigh. It was asking how big your lean frame probably is, so a pharmacist could work out how much drug to put in it.

Two of the other three formulas are equally frank about their purpose once you read their titles. Robinson's 1983 paper is called "Determination of ideal body weight for drug dosage calculations". Miller's, from the same year, is "Determining ideal body weight (and mass)", published in the American Journal of Hospital Pharmacy. Not a nutrition journal. Not a public health journal. A hospital pharmacy journal. The odd one out is Hamwi, whose figures came from a 1964 American Diabetes Association text on dietary therapy, so at least that one was thinking about food.

Three of the four numbers on your screen were built by pharmacists, for pharmacists, to stop people being poisoned. None of them was designed or validated to tell a healthy adult what to weigh.

Four formulas, four different answers

Which explains something you would otherwise find puzzling: they disagree with each other.

All four share the same shape. Start with a base weight for someone five feet tall, then add a fixed amount for every inch above that. They differ in both numbers:

Take a man of 180 cm, which is about 70.9 inches, so 10.9 inches over five feet:

Nearly six kilograms between the highest and the lowest, which is why this tool reports a range of 72 to 77 kg rather than pretending to a single answer. Same man, same height, four respectable formulas, six kilos of disagreement.

Notice the pattern in the coefficients. Hamwi adds 2.7 kg an inch and Miller adds 1.41, almost exactly half. So the taller you are, the more they diverge, and at 6 ft 6 in the gap becomes frankly silly. That is not a flaw in anyone's arithmetic. It is four different people making four different guesses about how weight scales with height, from four different sets of patients, decades apart, for purposes that had nothing to do with your bathroom scale.

The tool also shows you the healthy BMI range for your height, from 18.5 to 25, and for our 180 cm man that runs from about 60 kg to 81 kg. Look at how wide that is compared with the formulas' six-kilo spread. The formulas argue over kilos inside a range that is twenty kilos across, and the range is the thing with the actual evidence behind it.

Where these equations still save lives

None of this makes the formulas junk. Quite the opposite, and it is worth being fair to them, because in the job they were built for they are excellent.

Ideal body weight is still how hospitals dose drugs with narrow margins: aminoglycoside antibiotics, chemotherapy, some anaesthetics. It is used to set nutrition targets for critically ill patients, because feeding an obese patient on their actual weight overshoots what their body can use. It is used as the reference point for bariatric surgery eligibility and outcomes.

And there is one use that is genuinely remarkable. When a patient is on a ventilator, the volume of air pushed into their lungs each breath is set per kilogram of predicted body weight, which is calculated with these very equations. The reason is that lung size tracks your height, not your waistline; a heavier person does not have bigger lungs. The landmark trial on this found that setting tidal volumes at 6 mL per kilogram of predicted body weight, rather than by actual weight, substantially reduced deaths in intensive care.

So Devine's rule of thumb from a 1974 antibiotic case study is, right now, helping to keep people alive in intensive care units. It is a serious piece of clinical equipment. It just is not a verdict on your body.

So what do you do with the number?

Here is our honest advice, and it is slightly awkward advice to give underneath our own calculator.

Treat the four figures as trivia, and read the BMI range instead. The range is the number built from data about actual human health outcomes. The four formulas are pharmacology heuristics that escaped the hospital and got adopted as lifestyle targets, largely because "ideal body weight" is a phrase that sounds like it means something.

If your weight sits inside the healthy BMI range for your height, you are fine. Do not go on a diet to chase Miller's number because it happens to be two kilos below where you are. That is not a health decision, it is a decision to obey a 1983 hospital pharmacy paper that was not talking to you.

And none of these can see the thing that most often makes them wrong, which is what you are made of. A muscular person will sail past every one of these figures while being in excellent health, because the formulas only know your height. Our Lean Body Mass Calculator asks better questions, and our Healthy Weight Calculator gives you the range without the false precision.

Questions people ask

Which formula is the best?

Devine is the most medically validated, in the sense that it is the one the dosing literature actually uses. That makes it the best formula for calculating a drug dose. It does not make it the best answer to what you should weigh, because no formula here answers that.

Why do they give different answers?

Because they were derived from different groups of patients at different times, and each made its own assumption about how weight should scale with height. Hamwi adds nearly twice as much per inch as Miller does.

Why does it not ask my current weight?

Because it is not comparing you to anything. These formulas take height and sex only, by design, since they were built to estimate the size of a person's lean frame for dosing rather than to assess an individual.

I am muscular and the numbers say I am overweight. Are they wrong?

For you, yes. Every formula here is blind to body composition. If you carry significant muscle, these figures will underestimate a healthy weight for you, and so will BMI.

What if I am under five feet tall?

The formulas are built to add weight above a five-foot baseline and they become unreliable below it. The convention in dosing is to subtract per inch below five feet, but the equations were not designed for that and you should treat the output with real caution.

References

Where this story comes from. Devine's original rule appeared in a 1974 gentamicin therapy case study in Drug Intelligence and Clinical Pharmacy. Robinson and Miller both published their revisions in the American Journal of Hospital Pharmacy in 1983, and Robinson's title states its purpose plainly as drug dosage calculation. Hamwi's figures come from a 1964 American Diabetes Association text. Pai and Paloucek traced the whole lineage in the Annals of Pharmacotherapy in 2000. Shah and colleagues compared the equations against BMI tables for healthy US adults.

1. Pai MP, Paloucek FP. The origin of the "ideal" body weight equations. Annals of Pharmacotherapy. 2000;34(9):1066-1069. https://doi.org/10.1345/aph.19381
2. Devine BJ. Case number 25: gentamicin therapy. Drug Intelligence and Clinical Pharmacy. 1974;8:650-655.
3. Robinson JD, Lupkiewicz SM, Palenik L, Lopez LM, Ariet M. Determination of ideal body weight for drug dosage calculations. American Journal of Hospital Pharmacy. 1983;40(6):1016-1019.
4. Miller DR, Carlson JD, Lloyd BJ, Day BJ. Determining ideal body weight (and mass). American Journal of Hospital Pharmacy. 1983;40(9):1622-1625.
5. Hamwi GJ. Therapy: changing dietary concepts. New York: American Diabetes Association; 1964.
6. Shah B, Sucher K, Hollenbeck CB. Comparison of ideal body weight equations and published height-weight tables with body mass index tables for healthy adults in the United States. Nutrition in Clinical Practice. 2006;21(3):312-319. https://doi.org/10.1177/0115426506021003312