What is adjusted body weight?

Adjusted body weight is a dosing weight for obese patients that sits between ideal and actual weight. It is calculated as ideal body weight plus 0.4 times the gap between actual and ideal weight, recognising that some drug distributes into excess adipose tissue but not all of it.

Why use 0.4 as the correction factor?

The 0.4 factor reflects that hydrophilic drugs partition partially into fat. About 40 percent of the excess weight over ideal contributes to the drug's volume of distribution. Some references use slightly different factors for specific drugs, but 0.4 is the common default.

When should adjusted body weight be used?

It is generally applied when actual weight is more than about 20 to 30 percent above ideal weight. Typical uses include aminoglycosides, unfractionated heparin, and low-molecular-weight heparin. For patients near or below ideal weight the adjustment is not needed.

What happens if actual weight is below ideal?

Then there is no excess weight to adjust for, so the adjustment is not applied and ideal body weight (or actual weight, if even lower) is used. This tool detects that case and reports that ideal weight should be the dosing basis.

Which ideal-weight formula does this use?

It uses the Devine formula to compute ideal body weight: 50 kg for men or 45.5 kg for women, plus 2.3 kg per inch of height over 60 inches. The adjusted weight is then layered on top of that figure.

What is the Adjusted Body Weight Calculator?

Calculate adjusted body weight as IBW plus 0.4 times the difference between actual weight and IBW, the standard dosing weight for obese patients. Used by pharmacists for aminoglycosides, heparin, and low-molecular-weight heparin. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Adjusted Body Weight Calculator

Name: Adjusted Body Weight Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Adjusted body weight bridges the gap between ideal and actual weight when dosing drugs in obese patients. Ideal body weight alone can under-dose because part of a hydrophilic drug does distribute into excess tissue; actual body weight can over-dose because the drug does not fully distribute into all adipose tissue. The adjustment captures a defensible middle figure.

How it works

The calculation runs in two steps: first the Devine ideal body weight (IBW), then the ABW adjustment:

IBW (male)   = 50.0 + 2.3 × (height in inches − 60)
IBW (female) = 45.5 + 2.3 × (height in inches − 60)
ABW          = IBW + 0.4 × (actual weight − IBW)

The 0.4 correction factor assigns 40 percent of the excess weight above ideal to the drug’s effective volume of distribution. This figure comes from pharmacokinetic observations on hydrophilic drugs (aminoglycosides, heparin) in obese patients, where roughly 40 percent of excess adipose tissue participates in drug distribution.

When actual weight is at or below ideal weight, there is no excess to adjust for. In that case, the lower of actual weight and IBW is used as the dosing weight.

Worked example

Male patient, height 170 cm (66.9 inches), actual weight 110 kg.

Height above 60 inches: 66.9 − 60 = 6.9 inches

IBW = 50.0 + 2.3 × 6.9 = 65.9 kg

ABW = 65.9 + 0.4 × (110 − 65.9) = 65.9 + 17.6 = 83.5 kg

For an aminoglycoside dose, 83.5 kg would be used rather than the full 110 kg. Using 110 kg would over-estimate the volume of distribution for the drug, risking toxicity.

When to apply adjusted body weight

The adjustment is generally indicated when actual weight exceeds ideal weight by more than 20 to 30 percent. Below that threshold the difference between IBW and ABW is small and may not change the practical dose.

Common drugs dosed on ABW in clinical practice include:

Aminoglycosides (gentamicin, tobramycin, amikacin)
Unfractionated heparin — some protocols use actual weight for loading, ABW for maintenance
Low-molecular-weight heparins (enoxaparin, dalteparin) — many protocols recommend ABW for treatment doses in obesity

Not all drugs use the same dosing weight. Some agents use actual body weight regardless of obesity (for example, some chemotherapy protocols). Always verify the specific dosing guidance for the drug in question and use clinical judgement or pharmacy review for any dose that requires adjustment.