Adjusted Body Weight Calculator

This number is not a target, and it is not about you

Most people who land on a page called "adjusted body weight" are hoping it is a kinder version of ideal body weight. Something that takes account of the fact that you are a real person rather than a formula, and adjusts your target accordingly.

It is not that. It is not a target at all.

Adjusted body weight is a pharmacy tool. It exists so that clinicians can work out how much of a drug to put into a patient who is carrying a lot of body fat. It is not a weight you should aim for, it is not a healthier version of ideal weight, and it has no opinion whatsoever about what you should look like. If you are here to find out what to weigh, our Healthy Weight Calculator is the page you actually want.

If you are still reading, good, because the reason this thing exists is a genuinely elegant piece of problem-solving and it tells you something real about how bodies work.

The problem it was invented to solve

Start with the two obvious ways to dose a drug by weight, and watch both of them fail.

Dose on actual weight. Take a patient's real weight and multiply. This works beautifully for most people and becomes dangerous in someone with a lot of body fat, because many drugs barely distribute into fat tissue at all. They spread through lean tissue and body water. So if you dose 140 kg of patient as though all 140 kg were going to absorb the drug, when much of it will not, you have concentrated the dose into a smaller effective space. For a drug with a narrow margin between working and harming, that is how you damage someone's kidneys. As our Ideal Body Weight Calculator explains, this exact problem is why ideal body weight was invented in the first place, in a 1974 case study about the antibiotic gentamicin.

So dose on ideal weight instead. Use the height-based figure and ignore the excess entirely. Safer, and now you have the opposite problem: you have underdosed them. Because the assumption behind that move, that fat tissue takes up none of the drug, is not true either. Fat is living tissue. It has blood flowing through it. It does take up some of the drug, just not as much per kilogram as muscle does. Ignore it completely and your patient does not get enough, their infection does not clear, and you have failed them in a quieter way.

So: actual weight overshoots, ideal weight undershoots, and the truth is somewhere in between. Adjusted body weight is that in-between number, and the formula is a very direct expression of exactly that thought:

Adjusted body weight = Ideal body weight + 0.4 × (Actual weight − Ideal body weight)

Read it in plain English. Start at your ideal weight. Look at how much you are carrying above it. Count 40 percent of that excess, and ignore the other 60 percent. The answer is the weight to dose on.

Why 0.4, and not some other fraction

That 0.4 is doing all the work, so it deserves an explanation.

It is a correction factor for how drug-accessible fat tissue actually is. When you gain weight, what you gain is not pure fat. Adipose tissue comes with blood vessels to supply it and some extra lean tissue to carry it around, and the fat itself is perfused with blood rather than being an inert lump. So a kilogram of gained weight does take up drug. It simply takes up substantially less than a kilogram of muscle would.

The measured answer to "how much less" lands, for a lot of drugs, at somewhere around 40 percent. So the formula counts each excess kilogram as roughly four tenths of a kilogram for dosing purposes. It is an empirical fudge factor, not a law of nature, and that is exactly what it was designed to be.

Which is why it is worth saying clearly: 0.4 is not universal. Different drugs distribute differently, and the clinical literature uses other factors for other agents. Some drugs use 0.3. Some are dosed on actual weight even in obese patients. Some use lean body weight instead. A hospital pharmacist chooses the correction for the specific drug in front of them, and this calculator, which does not know what drug you are asking about, uses the most common general-purpose value.

Running it through

Take a man of 175 cm weighing 110 kg.

First his ideal body weight. 175 cm is about 68.9 inches, so 8.9 inches over five feet:

IBW = 52 + 1.9 × 8.9
IBW ≈ 68.9 kg

Now the adjustment. He is carrying about 41.1 kg above that figure:

AdjBW = 68.9 + 0.4 × (110 − 68.9)
AdjBW = 68.9 + 0.4 × 41.1
AdjBW ≈ 85.3 kg

Three numbers for the same man: 110 kg, 68.9 kg and 85.3 kg. All correct, all useful, all for different jobs. Dose him on 110 and you may harm him. Dose him on 68.9 and you may not treat him. Dose him on 85.3 and you have accounted for the fact that his extra tissue is real, is perfused, and takes up some of the drug.

Notice too what the arithmetic does at the edges. If your actual weight equals your ideal weight, the bracket is zero and adjusted weight simply equals ideal weight, which is correct: there is no excess to account for. And if you weigh less than your ideal figure, the bracket goes negative and the adjusted weight drops below ideal. That is the formula being used outside its intended range. Adjusted body weight is for people above their ideal weight, generally well above it, and it has nothing sensible to say about anyone else.

Three weights, three jobs

It is worth laying the family out, because the confusion between these is constant and this tool sits in the middle of it.

Only one of those four, lean body mass, is a real measurement of your body rather than an estimate from a formula, and our Lean Body Mass Calculator handles it. The other three are all arithmetic performed on your height.

Adjusted body weight also shows up outside pharmacy. Dietitians use it to set calorie and protein targets for patients with obesity, on the same logic: feeding someone based on 110 kg of body when a good deal of that body is metabolically quiet tissue tends to overshoot what they actually need.

One honest note about our version

A point of transparency, since somebody checking our arithmetic against a hospital reference will notice.

This calculator builds its ideal body weight from the Robinson formula. Much of the clinical literature builds adjusted body weight on the Devine formula instead, since Devine is the equation the dosing world grew up with. The two disagree, generally by a couple of kilograms, and after the 0.4 adjustment the difference shrinks further. So the number here will be in the right neighbourhood but may not match a pharmacist's figure exactly.

The result box now names which formula it used, so the number does not travel without that context. If you need this figure for anything that matters clinically, get it from the clinician who is prescribing, not from a web page. That is not us being coy. It is that the correction factor depends on the drug, and a page that does not know the drug cannot give you a dosing weight you should act on.

Questions people ask

Is adjusted body weight what I should weigh?

No. It is a dosing figure, not a goal. It is a number invented so that clinicians can medicate people safely, and it says nothing about what a healthy weight for you would be.

When would a clinician use this instead of my real weight?

Typically when a patient is well above their ideal weight, often around 20 to 30 percent above it or more, and the drug in question distributes poorly into fat tissue.

What if I weigh less than my ideal body weight?

Then this formula is not for you and will return a figure below your ideal weight, which is meaningless. Adjusted body weight only applies above the ideal figure.

Is the 0.4 factor always right?

No. It is the most common general-purpose value, but the correct factor depends on the specific drug, and clinical practice uses others. This is one reason a real dose should come from a pharmacist rather than a calculator.

References

Where this comes from. The ideal body weight equations underneath adjusted body weight originate with Devine's 1974 gentamicin case study and Robinson's 1983 revision, whose title states its purpose as drug dosage calculation. Pai and Paloucek traced the origins of the whole family of equations. Green and Duffull review how body weight should be described for dosing obese patients, which is the question adjusted body weight exists to answer.

1. Devine BJ. Case number 25: gentamicin therapy. Drug Intelligence and Clinical Pharmacy. 1974;8:650-655.
2. 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.
3. 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
4. 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