This adjusted body weight calculator computes AdjBW — the weight clinicians use for drug dosing and nutrition prescriptions in obesity — from ideal body weight (Devine formula) and actual weight, with the standard 0.4 correction factor.
AdjBW exists because dosing purely on actual weight overdoses many drugs in obesity (fat tissue doesn’t metabolize like lean mass), while dosing on ideal weight alone underdoses. The 0.4 factor credits 40% of the excess as pharmacologically active. Always confirm the dosing weight your specific drug’s protocol specifies — aminoglycosides, chemotherapy and enoxaparin each have their own conventions.
AdjBW = IBW + 0.4 × (Actual − IBW)
Three weights live in clinical charts: actual (the scale), ideal (Devine’s height-based estimate — detailed on our Ideal Body Weight Calculator), and adjusted — the weighted compromise used when a drug’s distribution volume tracks lean mass plus a fraction of adipose. The correction factor varies by drug class (0.4 is the common default; some references use 0.25). Part of our Health Calculators collection.
Worked Example (Verified)
- Man, 5’10” (70 in), actual 110 kg (defaults): IBW = 50 + 2.3 × 10 = 73 kg → AdjBW = 73 + 0.4 × 37 = 87.8 kg — the dosing weight that sits, sensibly, between the scale’s 110 and ideal’s 73
- A drug dosed at 5 mg/kg differs by 111 mg between actual-weight (550 mg) and AdjBW (439 mg) dosing — the clinical stakes of choosing the right weight, in one subtraction
- When actual < ideal (underweight patients), protocols generally dose on actual weight — the formula’s output below actual should not be used to inflate a dose
- Nutrition parallel: protein and calorie prescriptions in critical care similarly key to IBW or AdjBW rather than actual weight in obesity
Handle With Clinical Care
- This is an arithmetic reference, not a prescribing tool — verify dosing weight against the current drug monograph and local protocol.
- Devine IBW misbehaves at height extremes and says nothing about body composition; muscular patients can register “excess” that is not adipose.
- Pediatric, amputee and pregnancy contexts each modify or replace these formulas.
Related Tools
- Ideal Body Weight Calculator — Devine and other IBW formulas compared
- Lean Body Mass Calculator — the other dosing-relevant mass
- Health Calculators — the full collection
Frequently Asked Questions
How is adjusted body weight calculated?
AdjBW = IBW + correction factor × (actual weight − IBW), with the factor most often 0.4. First compute IBW (Devine: men 50 kg + 2.3 kg per inch over 5 feet; women 45.5 + 2.3 per inch). A 5’10” man at 110 kg: IBW 73, AdjBW 87.8 kg.
When is adjusted body weight used?
Mainly for medication dosing in patients with obesity — classically aminoglycoside antibiotics — and in some nutrition-support calculations. The idea: adipose tissue contributes partially to a drug’s distribution and clearance, so dosing weight should sit between ideal and actual. Each drug’s protocol specifies which weight it wants.
Why not just use actual or ideal weight?
Actual weight can overdose drugs that distribute poorly into fat (the excess isn’t metabolically equivalent tissue). Ideal weight can underdose by ignoring that larger bodies do have more lean mass, blood volume and clearance capacity than the “ideal” frame alone implies. AdjBW is the evidence-informed compromise.
What correction factor should I use?
0.4 is the most commonly cited (used for aminoglycosides in many references). Some drugs and institutions use 0.25–0.3, and some use lean body weight or actual weight outright. The calculator exposes the factor precisely because the correct answer is “whichever your protocol specifies.”
Is Devine IBW accurate?
It’s a 1974 convenience formula (derived for drug dosing, not health assessment) — linear in height, blind to frame and composition. For AdjBW purposes it remains the conventional input, which is a statement about protocols, not physiology. Don’t use IBW as a personal weight target without clinical context.
Is this adjusted body weight calculator free?
Yes — free, instant, no sign-up. A reference calculator; dosing decisions belong to prescribers with the full clinical picture.