Ideal Body Weight (IBW) Calculator — Devine Formula
Ideal Body Weight (IBW) estimates the optimal weight for an individual based on height and sex. It is commonly used for drug dosing, ventilator settings, and nutritional assessment.
About
Ideal Body Weight (IBW) is a weight estimate based on height and sex, developed by Dr. Bernard Devine in 1974 for the purpose of dosing gentamicin in renal impairment. The Devine formula was derived from a 1959 Metropolitan Life Insurance Company height-weight table and was never originally intended for widespread clinical use. Despite this, IBW has become a cornerstone of modern clinical pharmacology and critical care. IBW is most commonly used in clinical practice for: (1) Calculating drug doses, particularly for anesthesia agents such as propofol and neuromuscular blockers, renally cleared medications such as aminoglycosides and vancomycin, and certain chemotherapy agents, (2) Setting ventilator tidal volumes in ARDS (6–8 mL/kg IBW reduces ventilator-induced lung injury per ARDS Network protocols), and (3) Nutritional assessment to calculate caloric and protein requirements. The adjusted body weight (ABW = IBW + 0.4 × (actual weight − IBW)) is used for dosing in obese patients when actual weight exceeds IBW by more than 20%. The correction factor of 0.4 accounts for the fact that approximately 40% of excess weight is metabolically active lean tissue. Alternative IBW formulas include the Robinson, Miller, and Hammond methods, but the Devine formula remains the most widely used. The evidence level is Grade B, supported by pharmacological validation studies.
Formula
Male: IBW = 50 kg + 2.3 kg × (Height(in) - 60) | Female: IBW = 45.5 kg + 2.3 kg × (Height(in) - 60) | Adjusted BW = IBW + 0.4 × (Actual - IBW)
The Devine formula calculates IBW starting from a baseline of 50 kg for men and 45.5 kg for women at a height of 60 inches (152.4 cm), which corresponds to 5 feet. For every additional inch (2.54 cm) above 5 feet, 2.3 kg is added. For men: IBW = 50 + 2.3 × (height in inches − 60). For women: IBW = 45.5 + 2.3 × (height in inches − 60). For example, a man who is 175 cm (68.9 inches) tall has IBW = 50 + 2.3 × (68.9 − 60) = 50 + 2.3 × 8.9 = 50 + 20.5 = 70.5 kg. The adjusted body weight (ABW) is calculated only when actual body weight exceeds IBW by more than 20%, using the formula: ABW = IBW + 0.4 × (actual weight − IBW). The factor 0.4 represents the proportion of excess weight that is metabolically active lean tissue. For patients who are below IBW, using actual body weight for drug dosing is generally appropriate. To interpret the result, IBW serves as a reference: tidal volume in ARDS is set at 6–8 mL/kg IBW; for vancomycin loading doses, 15–25 mg/kg IBW is used; and for aminoglycosides, 7 mg/kg IBW for gentamicin/tobramycin. ABW is preferred over actual weight for dosing when actual weight exceeds IBW by >20%, as using actual weight would overestimate the volume of distribution for hydrophilic drugs.
Score Interpretation
IBW is essential across multiple clinical domains with guideline-level support. In anesthesia, the American Society of Anesthesiologists (ASA) recommends IBW-based dosing for propofol induction (1.5–2.5 mg/kg IBW) and succinylcholine (1 mg/kg IBW) to prevent dose-related complications. In critical care, the ARDS Network protocol (published in NEJM 2000) established lung-protective ventilation using tidal volumes of 6 mL/kg predicted body weight (a concept closely related to IBW), reducing mortality by 22% compared to traditional ventilation. The Surviving Sepsis Campaign guidelines continue to endorse this approach. In clinical pharmacology, the Infectious Diseases Society of America (IDSA) guidelines for vancomycin therapeutic drug monitoring recommend loading doses based on actual body weight but maintenance doses based on IBW or ABW. Aminoglycoside dosing requires IBW/ABW calculation to achieve therapeutic peak and trough levels while minimizing nephrotoxicity and ototoxicity. In nutritional assessment, the Academy of Nutrition and Dietetics recommends using IBW to calculate resting energy expenditure via the Mifflin-St Jeor equation when actual weight is >120% of IBW. IBW also guides appropriate tidal volume settings in laparoscopic surgery where high airway pressures may cause barotrauma. Despite its widespread use, clinicians must recognize that IBW is a mathematical construct — it does not necessarily represent a healthy or achievable weight target for all patients. The trend toward personalized medicine has led to increased adoption of therapeutic drug monitoring as a complement to IBW-based dosing, particularly for narrow-therapeutic-index drugs such as aminoglycosides and vancomycin.
Normal Range — 0+
IBW is an estimate — actual healthy weight may vary within ±10% of calculated IBW.
Management: Use IBW for drug dosing and ventilator tidal volume calculations.
Reference Ranges
| Population | Normal Range |
|---|---|
| Adults — Male | 50 kg + 2.3 kg/inch over 5 ft |
| Adults — Female | 45.5 kg + 2.3 kg/inch over 5 ft |
Dr. Omar Farouk
Dr. Omar Farouk is a board-certified internist specializing in general medicine and clinical pharmacology.
View medical review board & editorial policy →Example Calculation
A 62-year-old man with a history of hypertension and type 2 diabetes is admitted to the intensive care unit with severe community-acquired pneumonia progressing to acute respiratory distress syndrome (ARDS). He requires mechanical ventilation. His height is 178 cm (70.1 inches) and his actual body weight is 120 kg (BMI 37.9, Class II obesity). The critical care team must determine his IBW for lung-protective ventilation settings and antibiotic dosing. Step 1: Calculate IBW using the Devine formula: IBW = 50 + 2.3 × (70.1 − 60) = 50 + 2.3 × 10.1 = 50 + 23.2 = 73.2 kg. Step 2: Since his actual weight (120 kg) exceeds IBW by (120 − 73.2) / 73.2 = 64%, which is well above 20%, adjusted body weight should be used for certain drug dosing: ABW = 73.2 + 0.4 × (120 − 73.2) = 73.2 + 0.4 × 46.8 = 73.2 + 18.7 = 91.9 kg. Step 3: ARDS ventilation settings — tidal volume at 6 mL/kg IBW: 6 × 73.2 = 439 mL (use 440 mL). The ARDS Network protocol confirms that IBW-based tidal volumes reduce mortality. Step 4: Vancomycin loading dose — 25 mg/kg actual body weight: 25 × 120 = 3000 mg (max single dose 3000 mg). Maintenance dose — 15 mg/kg ABW: 15 × 91.9 = 1379 mg every 12 hours with therapeutic drug monitoring. Step 5: Gentamicin (if indicated) — 7 mg/kg IBW: 7 × 73.2 = 512 mg, adjusted to 500 mg with extended-interval dosing. This case illustrates the critical distinction between using IBW, ABW, and actual body weight depending on the clinical context. Using actual weight for all calculations would significantly overdose this patient, particularly for hydrophilic drugs with a low volume of distribution.
Related Conditions
Related Medications
Common Mistakes
Using IBW for all drug dosing regardless of body habitus
For obese patients, use adjusted body weight for drugs that distribute into lean tissue. For lipophilic drugs, actual body weight may be more appropriate.
Assuming IBW represents a healthy target weight
IBW is a clinical calculation tool, not a health outcome target. Actual healthy weight depends on body composition, muscle mass, and individual factors.
Applying Devine formula to pediatric patients
The Devine formula was developed for adults. Use pediatric-specific IBW formulas (e.g., Traub-Johnson or McLaren methods) for children.
Using actual body weight for all drug dosing in obesity
Hydrophilic drugs (aminoglycosides, neuromuscular blockers) should be dosed on IBW or ABW. Lipophilic drugs (propofol, benzodiazepines) may require actual body weight. Check each drug's pharmacokinetics.
Confusing IBW with lean body weight formulas
IBW (Devine) and lean body weight (James/Boer/Hume) are different calculations. LBW formulas are more complex and generally yield higher values than IBW at tall heights. Use the formula specific to the protocol being followed.
Frequently Asked Questions
When should adjusted body weight be used instead of IBW?
Is the Devine formula still recommended?
What is the difference between IBW and lean body weight?
Should I use IBW for ventilator settings in all patients?
Does IBW apply to all ethnicities equally?
What is the adjusted body weight formula and when is it used?
References
- Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650-655.
- Pai MP, Paloucek FP. The origin of the "ideal" body weight equations. Ann Pharmacother. 2000;34(9):1066-1069. PubMed
- ARDS Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301-1308. PubMed
- Rybak MJ, Le J, Lodise TP, et al. Therapeutic monitoring of vancomycin for serious MRSA infections: revised consensus guidelines. Am J Health Syst Pharm. 2020;77(11):835-864. PubMed
- Ingrande J, Lemmens HJ. Dose adjustment of anaesthetics in the morbidly obese. Br J Anaesth. 2010;105 Suppl 1:i16-i23. PubMed
- Wurtz R, Itokazu G, Rodvold K. Antimicrobial dosing in obese patients. Clin Infect Dis. 1997;25(1):112-118. PubMed
- Green B, Duffull SB. What is the best size descriptor to use for pharmacokinetic studies in the obese? Br J Clin Pharmacol. 2004;58(2):119-133. PubMed