Medical & Clinical
Adjusted Body Weight Calculator
Some medications should be dosed on your actual weight; some shouldn't. Adjusted body weight is the clinical compromise.
Adjusted body weight (AjBW) is a dosing compromise for obesity: ideal body weight + 0.4 × (actual − ideal). Clinicians use it for drugs that distribute poorly into fat, to avoid over-dosing very heavy patients. Interpretation belongs to your clinician — use these numbers to ask better questions.
AjBW = IBW + 0.4×(actual − IBW).
Used when actual > 130% of IBW.
Avoids over-dosing in obesity.
Calculate Your Adjusted Body Weight
Adjusted Body Weight
What Is Adjusted Body Weight?
Fat tissue takes up only a fraction of the water and blood flow that lean tissue does, so drugs that distribute into lean compartments can be over-dosed by actual weight in obesity.
Adjusted body weight splits the difference: it counts all of the ideal weight but only 40% of the excess, a pragmatic correction used for aminoglycosides and selected other medications.
How It Is Calculated
AjBW = IBW + 0.4 × (actual − IBW)
IBW (Devine) = 50 + 2.3 kg per inch over 5 ft (men) · 45.5 + 2.3 (women)
Example: Example: 110 kg man, 170 cm → IBW 66 kg → AjBW ≈ 83.6 kg
When to adjust
| Actual vs IBW | Dosing weight |
|---|---|
| ≤ 130% of IBW | Use actual weight |
| 130–200% of IBW | Consider AjBW (drug-dependent) |
| ≥ 200% of IBW | Follow drug-specific guidance |
Limitations
- Adjustment applies to specific drugs, not universally, many modern agents dose on actual weight.
- This is educational; prescribing decisions belong to clinicians and pharmacists.
- The 40% factor is a convention, refined by newer population models.
Sources & Review
References used for this calculator’s formulas and thresholds:
Adjusted Body Weight FAQ
What is adjusted body weight used for?
Dosing selected medications, mostly ones that distribute into lean tissue, when actual weight exceeds about 130% of ideal.
How is adjusted body weight calculated?
Ideal weight + 40% of the excess: AjBW = IBW + 0.4 × (actual − IBW).
Should I dose my own medication with this?
No, dosing decisions are clinical. Use this to understand why your prescriber's dose may differ from what you expected.
A note before you begin: this tool offers educational estimates only. It cannot replace advice from your doctor or another qualified professional.