Adjusted Body Weight Calculator

Ideal, actual and adjusted weight for dosing.

Calculate adjusted body weight for drug dosing in obesity, alongside ideal body weight by the Devine formula and your actual weight.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Units

Sex

Height

Actual body weight

Correction factor

Set by the drug's distribution into fat. Follow the specific agent's guidance.

This is arithmetic, not a prescribing decision. Dosing also depends on renal and hepatic function, the agent, the indication and monitoring.

Adjusted body weight

70.3 kg

Ideal body weight (Devine)70.5 kg

Actual body weight70.0 kg

Percent over ideal-1%

Actual weight is at or below ideal, so adjusted body weight is not normally used, dose on actual weight unless the protocol says otherwise.

How the Adjusted Body Weight Calculator works

Some drugs distribute into fat tissue and some do not, so neither actual weight nor ideal weight is the right dosing basis for every medication in obesity. Adjusted body weight splits the difference, adding a fraction of the excess weight to the ideal figure.

Also known as: AdjBW calculator · adjusted body weight dosing · obesity adjusted weight · adjusted body weight calculator · adjusted body weight calculation

Why an adjusted weight exists

Some drugs distribute through lean tissue and very little into fat. Dosing those by total body weight in a patient with substantial excess weight risks overdose; dosing by ideal body weight risks underdose, because adipose tissue does receive some drug.

Adjusted body weight splits the difference. It takes ideal body weight and adds a proportion of the excess, conventionally 40%, giving a figure between the two.

The formula is ideal body weight plus 0.4 times the difference between actual and ideal body weight. The 0.4 factor is a convention derived from the observation that lean tissue accounts for roughly that proportion of weight gained in obesity.

The ideal body weight it starts from

The usual basis is the Devine formula from 1974: for men, 50 kilograms plus 2.3 kilograms for each inch over five feet; for women, 45.5 kilograms plus 2.3 kilograms per inch.

Devine was published as a dosing tool rather than as a health target, which is an important distinction that its name obscures. It was never intended to describe a weight anyone should aim for.

Robinson, Miller and Hamwi formulas exist and give slightly different figures. The choice matters less than consistency, and most clinical protocols specify which to use.

Which drugs use which weight

Aminoglycoside antibiotics such as gentamicin conventionally use adjusted body weight in obesity, because they distribute mainly into lean tissue.

Some drugs are dosed on total body weight, some on ideal, some on lean body mass, and some on body surface area. Which applies depends on the drug's distribution characteristics and on the local protocol.

This is why a general-purpose adjusted weight figure is a starting point rather than an answer. The correct weight descriptor for any particular drug is specified in its own guidance, and using the wrong one is a recognised source of dosing error.

The correction factor

The 0.4 factor is the most common and it is not universal. Some protocols use 0.3 and some 0.5, depending on the drug and the evidence available for it.

The variation reflects genuine uncertainty about how much of a given drug distributes into adipose tissue, which differs between agents and is not always well characterised.

Where a protocol specifies a factor, that factor should be used. Where none is specified, the calculation should not be improvised, because the consequences of getting a dose wrong in either direction are real.

The boundaries of this tool

This calculator performs the arithmetic and nothing more. It does not know which weight descriptor a particular drug requires, what the local protocol says, or anything about the patient beyond the numbers entered.

Adjusted body weight is a clinical concept used by prescribers and pharmacists working from validated references and checking each other's arithmetic. It is not a health metric and it has no meaning outside dosing.

Anyone whose treatment involves a weight-based dose should raise questions about it with their prescriber or pharmacist rather than with a website. This page exists to check a calculation, not to inform a decision.

It is also worth knowing that the concept applies only above ideal body weight. Where actual weight is below ideal, adjusted body weight is not used and actual body weight is generally the correct descriptor, since there is no excess adipose tissue to account for. Applying the formula regardless produces a figure above the patient's actual weight, which is exactly the wrong direction, and it is a mistake that arithmetic alone will not flag.

Where to go next

The Adjusted Body Weight question rarely arrives on its own. These are the ones that usually come with it:

Ideal Weight Calculator — Target weight from four clinical formulas plus BMI range.
Lean Body Mass Calculator — Everything you weigh that is not fat.
BMI Calculator — Body mass index with category and healthy weight range.
Calorie & BMR Calculator — Daily calorie needs for losing, holding, or gaining weight.

Sources

Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.

Frequently asked questions

How is adjusted body weight calculated?

AdjBW = ideal body weight + 0.4 × (actual body weight − ideal body weight). The 0.4 factor is the most common, though 0.3 and 0.5 appear in some protocols depending on the drug's distribution into adipose tissue.

When is adjusted body weight used instead of actual weight?

For drugs that distribute only partially into fat, aminoglycosides are the classic example. Hydrophilic drugs are often dosed on ideal body weight, and highly lipophilic ones on actual weight. Always follow the specific drug's guidance rather than a general rule.

How is ideal body weight calculated here?

By the Devine formula: 50 kg + 2.3 kg per inch over 5 feet for men, and 45.5 kg + 2.3 kg per inch over 5 feet for women. It was devised in 1974 for exactly this purpose, drug dosing, rather than as a health target.

Is this calculator a substitute for clinical judgement?

No. Dosing depends on renal and hepatic function, the specific agent, the indication and monitoring, none of which a weight formula captures. This gives the arithmetic; the prescribing decision belongs to a qualified clinician.