Adjusted Body Weight Calculator

Adjusted body weight for drug dosing in obesity — with the range the adjustment factor actually spans, and a clear answer when the formula does not apply to you.

Reference table

Actual weight	Adjusted Body Weight	Which Weight to Use	Ideal Body Weight (Devine)
20	20.0 kg	Use actual weight — you are at or below ideal, so the adjustment has no meaning	70.5 kg
25	25.0 kg	Use actual weight — you are at or below ideal, so the adjustment has no meaning	70.5 kg
30	30.0 kg	Use actual weight — you are at or below ideal, so the adjustment has no meaning	70.5 kg
40	40.0 kg	Use actual weight — you are at or below ideal, so the adjustment has no meaning	70.5 kg
50	50.0 kg	Use actual weight — you are at or below ideal, so the adjustment has no meaning	70.5 kg
60	60.0 kg	Use actual weight — you are at or below ideal, so the adjustment has no meaning	70.5 kg
70	70.0 kg	Use actual weight — you are at or below ideal, so the adjustment has no meaning	70.5 kg
75	72.3 kg	Barely above ideal — actual and adjusted are close enough that most protocols use actual	70.5 kg
80	74.3 kg	Barely above ideal — actual and adjusted are close enough that most protocols use actual	70.5 kg
90	78.3 kg	Adjusted weight applies	70.5 kg
100	82.3 kg	Adjusted weight applies	70.5 kg
150	102.3 kg	Adjusted weight applies	70.5 kg
200	122.3 kg	Adjusted weight applies	70.5 kg

📊 Which Weight for Which Drug

Aminoglycosides (gentamicin, tobramycin)Adjusted
Vancomycin (loading)Actual
Unfractionated heparinActual, often capped
Enoxaparin (treatment)Actual, protocol-dependent
Ideal-weight drugs (some anaesthetics)Ideal
Anything elseWhatever the protocol says

Before You Use This Number

The 0.4 factor is a convention, not a constant. Different drugs and different guidelines use 0.3, 0.4 or 0.5, and this page shows the whole span so you can see how much rides on that choice.
If actual weight is at or below ideal, the formula produces a number without clinical meaning. Use actual weight instead — this page says so rather than handing you a figure.
Ideal body weight here is Devine, because that is what the dosing guidance is written against. If you want Devine, Robinson, Miller and Hamwi compared, that is a different calculator.
Which weight a drug needs is a property of the drug, not the patient. This tool gives you the number; the protocol decides whether it is the right one.

Why There Are Three Weights

A drug distributes through the body according to its own chemistry. Something that stays largely in lean tissue and water needs a dose scaled to lean mass; something that partitions readily into fat needs more than that. Adipose tissue is not inert — it takes up some drug — but it takes up far less per kilogram than muscle does. Dosing a lean-distributing drug on total body weight in someone with obesity therefore overshoots, sometimes dangerously; dosing it on ideal body weight undershoots, because the extra tissue does absorb some. Adjusted body weight splits the difference: take ideal body weight and add back a fraction of the excess. The convention is to add back four tenths, which is where the familiar formula comes from — ideal plus 0.4 times the difference between actual and ideal.

The 0.4 Is a Convention, and It Matters

The adjustment factor was never derived from a single definitive study. Values between 0.3 and 0.5 appear across the literature and across institutional protocols, chosen for particular drug classes on the basis of pharmacokinetic modelling that varies in quality. For someone 50 kilograms above their ideal weight, the difference between a factor of 0.3 and one of 0.5 is 10 kilograms of dosing weight — more than enough to matter for a narrow-therapeutic-index drug. That is why this page shows the full 0.3-to-0.5 span alongside the standard figure rather than presenting 0.4 as though it were a physical constant. If your protocol specifies a factor, use theirs; if it does not, knowing how wide the uncertainty is tells you how much confidence the number deserves.

When This Calculation Does Not Apply

If actual weight is at or below ideal body weight, there is no excess to adjust. Use actual weight; the formula would return something between the two that means nothing.
Adjusted weight is generally reserved for patients meaningfully above ideal — commonly taken as 120 to 130 percent of ideal or a BMI over 30.
Some drugs are dosed on total body weight regardless of size, and some on ideal weight regardless. The drug decides, not the patient.
Devine's formula returns nothing sensible below 152 centimetres of height, because it was built as a base weight plus an allowance per inch over five feet.
Children are outside its scope entirely. Paediatric dosing uses weight bands or body surface area.
Pregnancy changes distribution volumes substantially and is not covered by any of these formulas.

How It Is Used in Practice

Aminoglycoside dosing is the classic case: gentamicin and tobramycin distribute into lean tissue and extracellular water, and adjusted weight is standard for them.
Vancomycin loading doses generally use total body weight, because its distribution follows total mass more closely.
Many institutions cap heparin dosing at a fixed weight rather than using an adjustment, which is a different approach to the same problem.
Creatinine clearance by Cockcroft-Gault is another place adjusted weight appears, and the choice of weight there changes the estimate substantially.
Local protocols vary and take precedence over anything general. A number from a web page is a starting point for a conversation, not an instruction.
Any real dose comes from the prescriber and the pharmacy, with the protocol, the indication and organ function all in play.

Worked Examples

Including the case where it does not apply

Male, 120 kg, 1.75 m

Height = 68.9 in, so 8.9 in over five feet
Devine IBW = 50 + 2.3 × 8.9 = 70.5 kg
ABW = 70.5 + 0.4 × (120 − 70.5) = 70.5 + 19.8

90.3 kg — and 85.3 to 95.2 across factors 0.3 to 0.5

Male, 62 kg, 1.75 m

Devine IBW = 70.5 kg
Actual weight is below ideal, so there is no excess
The formula would return 67.1 kg, between the two, meaning nothing

Use actual weight — 62 kg

Frequently Asked Questions

What is adjusted body weight used for?

Dosing drugs in patients with obesity when the drug distributes mainly into lean tissue. Total body weight would overshoot because fat takes up far less drug per kilogram; ideal body weight would undershoot because it takes up some. Adjusted weight adds a fraction of the excess back to ideal weight, and aminoglycosides are the classic example.

What is the adjusted body weight formula?

Adjusted body weight equals ideal body weight plus 0.4 times the difference between actual and ideal weight. Ideal body weight here is the Devine formula: 50 kilograms for men or 45.5 for women, plus 2.3 kilograms for each inch above five feet.

Why 0.4 and not some other number?

Because it is the most common convention, not because it is settled. Values from 0.3 to 0.5 appear across drug classes and institutional protocols. For someone 50 kilograms above ideal, that span is 10 kilograms of dosing weight, which is why this page shows the whole range next to the standard figure.

What if my weight is below my ideal body weight?

Then the calculation does not apply. There is no excess to adjust, and the formula would hand you a number between actual and ideal that has no clinical meaning. Use actual weight. This page tells you that instead of quietly returning a figure.

How is this different from ideal body weight?

Ideal body weight is a height-based reference figure and takes no account of what you actually weigh. Adjusted body weight starts from it and adds back part of the difference between it and your real weight, so it moves as you do. If you want the four ideal-weight formulas compared against each other, that is a separate calculator.

Can I use this to work out my dose?

No. This gives you a weight, not a dose. Whether a drug uses total, ideal or adjusted weight is a property of the drug, and the dose itself comes from the protocol, the prescriber and the pharmacy, with organ function and indication all involved. Treat this as a reference figure.