Adjusted Body Weight Calculator
Compute Adjusted Body Weight (ABW) — the value clinicians use for dosing many medications when actual weight is markedly above ideal.
What is Adjusted Body Weight?
Adjusted Body Weight is a clinical concept used when a patient's actual weight greatly exceeds their ideal body weight (IBW). Many drugs do not distribute evenly into adipose tissue, so dosing on actual weight risks toxicity while dosing on IBW under-treats. ABW splits the difference by adding 40% of the excess weight to the IBW.
Common uses include aminoglycoside antibiotics, certain anticoagulants (e.g. low-molecular-weight heparins) and some chemotherapy regimens. Always defer to your clinician's protocol.
How is it calculated?
The standard formula is: ABW = IBW + 0.4 × (Actual Weight − IBW). We compute IBW with the Devine formula: men 50 kg + 2.3 kg per inch over 5 ft; women 45.5 kg + 2.3 kg per inch over 5 ft. The factor 0.4 reflects the proportion of fat-free mass in adipose tissue.
Frequently Asked Questions
- When is ABW used instead of actual weight?
- Typically when actual weight exceeds IBW by more than 20–30%.
- Is the correction factor always 0.4?
- 0.4 is most common, but some drugs use 0.3 or 0.5. Follow your specific dosing reference.
- Does ABW replace clinical judgement?
- No. It is one input into dosing decisions made by qualified prescribers.