Ideal Body Weight Calculator
Estimate ideal body weight in kg from height using the Devine formula, returning a single reference weight for a given sex. Use it as a baseline for medication dosing or clinical assessment, not as a personal weight target.
Last updated: May 2026
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About this calculator
The formula is the Devine (1974) equation: men IBW = 50 + 2.3 × (height_in_inches − 60); women IBW = 45.5 + 2.3 × (height_in_inches − 60). Height enters in cm and is converted to inches by dividing by 2.54. The formula adds 2.3 kg per inch of height above 5 feet (60 inches), with the male/female base differing by 4.5 kg. It was originally derived to estimate aminoglycoside dosing in adult patients and is now used widely in clinical pharmacy for hydrophilic drugs (whose volume of distribution scales with lean body mass), in ICU fluid resuscitation, and in ventilator tidal-volume settings (typically 6–8 mL/kg IBW for ARDS lung-protective ventilation). Edge cases: the formula gives a negative IBW for adults under 5 feet tall — it was not designed for such heights and should not be extrapolated. For pediatric patients, use Broselow tape or age/length-based charts instead. For obese patients, IBW underestimates the actual lean body mass; pharmacists sometimes use Adjusted Body Weight = IBW + 0.4 × (actual − IBW) for dosing certain drugs. Alternative formulas (Robinson, Miller, Hamwi) give slightly different values but agree within 2–5 kg for typical adult heights. IBW is a clinical reference weight, not a health target — the WHO normal BMI range (18.5–24.9 kg/m²) often spans well below and above the Devine IBW.
How to use
Example 1 — adult male, average height. Height 175 cm, gender male. Step 1: height in inches = 175 / 2.54 ≈ 68.90. Step 2: inches above 60 = 68.90 − 60 = 8.90. Step 3: IBW = 50 + 2.3 × 8.90 ≈ 50 + 20.47 ≈ 70.5 kg. Verify: a 175-cm man at IBW 70.5 kg has BMI ≈ 23.0, well inside the WHO normal range (18.5–24.9) ✓. Example 2 — adult female, taller. Height 168 cm, gender female. Step 1: height in inches = 168 / 2.54 ≈ 66.14. Step 2: inches above 60 = 6.14. Step 3: IBW = 45.5 + 2.3 × 6.14 ≈ 45.5 + 14.12 ≈ 59.6 kg. Verify: a 168-cm woman at IBW 59.6 kg has BMI ≈ 21.1, also within the WHO normal range ✓. The 11 kg gap between same-height males and females reflects the formula's empirical adjustment for typical lean-body-mass differences and is consistent across other IBW formulas (Hamwi, Robinson). For clinical dosing in this patient, drugs like vancomycin or aminoglycosides would be dosed using actual weight or IBW depending on the agent's distribution properties.
Frequently asked questions
What is the difference between ideal body weight (IBW), lean body weight (LBW), and adjusted body weight?
Ideal body weight is a height-based reference value computed from formulas like Devine, useful for drug dosing and ventilator settings — it is not a personal target weight. Lean body weight (also called fat-free mass) is the actual body mass minus fat, typically measured by DEXA, bioimpedance, or estimated from formulas like Janmahasatian; LBW correlates with drug distribution for hydrophilic drugs. Adjusted body weight (AdjBW) is used in obese patients where actual weight overestimates lean tissue: AdjBW = IBW + 0.4 × (actual weight − IBW). The 0.4 correction factor reflects that fat tissue contributes some, but not all, to drug distribution volume for many drugs. Different drugs use different weight metrics: aminoglycosides use AdjBW; vancomycin uses actual body weight; lipophilic drugs like propofol use total body weight; tidal volume for ventilation uses IBW. Always check the dosing reference for which weight to use; using the wrong one can produce sub-therapeutic or toxic doses, especially in patients at the extremes of body composition.
Why does the male/female difference of 4.5 kg exist in the formula?
The 4.5 kg difference (50 vs 45.5 kg base) and the same 2.3 kg/inch slope reflect average lean-body-mass differences observed in the 1960s–70s populations the formula was derived from. Adult men carry roughly 15% more skeletal muscle than women of the same height and age, mostly due to androgen-driven muscle accretion; the formula captures this empirically rather than from first principles. Newer formulas (e.g., Janmahasatian for LBW) use different functional forms but agree on the male-female gap. The gendered formula is a clinical convention that doesn't apply well to transgender patients, people on long-term hormone therapy, or those at the extremes of muscularity (elite athletes, sarcopenic elderly). For these populations, body-composition measurement (DEXA, bioimpedance) gives a better estimate than any height-and-sex formula. In practice, clinicians use IBW for screening and dosing starting points, then adjust based on observed response, drug levels, or biomarkers.
Can IBW give negative or unreasonable values?
Yes — the Devine formula returns negative weights for adults under 5 feet (152 cm) tall and very low weights for adults between 150 and 155 cm. The formula was developed from a sample of adults with normal heights and is not validated below ~5 feet. For very short adults, clinicians typically use a floor value (e.g., IBW = 50 kg for males, 45.5 kg for females regardless of height), use a different formula (Hamwi: 48 + 2.7 per inch for men), or use actual body weight. For pediatric patients the formula does not apply — use length-based methods like Broselow tape or BMI-for-age percentiles. The formula also extrapolates uncomfortably high for very tall patients (e.g., 2.2 m gives IBW ~115 kg for males) — at these heights muscle and bone mass don't scale linearly with height. Always sanity-check the result; if it seems clinically unreasonable, switch to an alternative formula or use measured body composition.
What are the common mistakes when using IBW in clinical practice?
The biggest mistake is using IBW as a weight-loss target — IBW is a reference for drug dosing and ventilation, not a healthy weight goal; WHO BMI ranges are far more appropriate for that purpose. The second is using IBW when actual body weight should be used (e.g., calculating drug doses for lipophilic agents like propofol or amiodarone, which distribute into fat tissue) — use the drug's specific dosing reference. The third is forgetting to use adjusted body weight (AdjBW) for obese patients with hydrophilic drugs, leading to under-dosing if using only IBW. People also apply the formula to children or adolescents, where it gives meaningless results; pediatric dosing uses weight-based mg/kg or BSA. Unit confusion — entering height in inches without converting to cm, or vice versa — produces wildly wrong IBW (the formula here expects cm and converts internally). Finally, using a sex-binary formula for transgender or non-binary patients should be done thoughtfully; some clinicians use the patient's assigned sex at birth, some their current hormone profile, depending on the clinical context.
When should I not use this calculator?
Do not use it for pediatric dosing — children's body composition differs from adults, and pediatric dosing uses weight-per-kg or BSA-based formulas with length-and-age references like Broselow tape. It is not appropriate for adults under 5 feet (152 cm) tall, where the formula extrapolates and may give negative values; use Hamwi (48 + 2.7 per inch men; 45.5 + 2.2 per inch women) or empirical clinical judgment. Do not use it as a personal weight goal — IBW is a clinical-dosing reference, not a health target; use BMI ranges, body-fat percentage, or waist-to-height ratio for health assessment. It is unsuitable for amputees, patients with significant edema or ascites, those with limb-length asymmetries, or extreme muscularity (bodybuilders) where height alone is a poor predictor of expected weight. For drug dosing in obese patients, use Adjusted Body Weight (AdjBW = IBW + 0.4 × (actual − IBW)) instead of raw IBW for hydrophilic drugs. Always consult the drug-specific dosing reference rather than defaulting to IBW for everything.