Body Weight Calculators

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Body weight is the total mass of the human body in kilograms or pounds, reflecting water (~60%), protein (~15%), fat (~15–30%), and minerals (~5%). It is used to calculate BMI, drug doses (mg/kg), nutritional requirements, and fitness metrics. Ideal body weight (IBW) formulas (Devine) estimate the weight appropriate for a given height and are used primarily for drug dosing. Adjusted body weight (ABW = IBW + 0.4 × (actual − IBW)) is used for obese patients. Body weight fluctuates 0.5–2 kg daily due to hydration, food, and hormonal changes — weekly averages are more meaningful for tracking trends.

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Ideal Body Weight (IBW) — Devine Formula

Men: IBW (kg) = 50 + 2.3 × (height in inches − 60). Women: IBW (kg) = 45.5 + 2.3 × (height in inches − 60). Example: man 5'10" (70 in): IBW = 50 + 2.3×10 = 73 kg.

Adjusted Body Weight

ABW = IBW + 0.4 × (actual weight − IBW). Used for drug dosing in obese patients: assumes 40% of excess weight is lean tissue.

BMI Classification

BMI = weight(kg)/height(m)². < 18.5 underweight; 18.5–24.9 normal; 25–29.9 overweight; ≥ 30 obese. Limitation: doesn't distinguish fat from muscle.

Weight and Calories

1 lb fat ≈ 3,500 kcal. 500 kcal/day deficit → ~1 lb/week loss. 1 kg fat ≈ 7,700 kcal.

Glossary

Ideal Body Weight (IBW)
Devine formula: men = 50 + 2.3×(height in inches − 60); women = 45.5 + 2.3×(height in inches − 60); used for drug dosing in lean patients; not a patient weight goal.
Adjusted Body Weight (ABW)
IBW + 0.4 × (actual weight − IBW); used for drug dosing in obese patients; assumes 40% of excess weight above IBW is lean tissue.
BMI
Weight (kg) / height (m)²; WHO categories: < 18.5 underweight; 18.5–24.9 normal; 25–29.9 overweight; ≥ 30 obese; does not distinguish fat from muscle mass.

Frequently Asked Questions

Ideal body weight (IBW) is a reference weight for a given height used primarily for drug dosing. Devine formula: Men: IBW = 50 + 2.3 × (height in inches − 60). Women: IBW = 45.5 + 2.3 × (height in inches − 60). Example: woman 5'6" (66 in): IBW = 45.5 + 2.3×6 = 59.3 kg. If actual weight < IBW: use actual weight for dosing. IBW should not be used as a weight goal for patients — it does not represent optimal health weight.

Adjusted body weight (ABW) = IBW + 0.4 × (actual weight − IBW). The 0.4 factor reflects that ~40% of excess weight above IBW is lean tissue (not just fat), so some drug distribution into that tissue must be accounted for. Used in obese patients for aminoglycosides (some protocols), vancomycin initial loading dose, and caloric calculations in ICU. Not universal: some drugs dose on actual weight in obesity (daptomycin); others on IBW. Always verify which weight to use from pharmacokinetic guidelines.

Weight-based dosing (mg/kg) accounts for larger patients having larger volumes of distribution: Actual body weight: for drugs distributing into fat (lipophilic drugs: propofol, some sedatives). Ideal body weight: for drugs distributing poorly into fat (aminoglycosides, vancomycin). Adjusted body weight: compromise for obese patients for certain drugs. Fixed dosing: when weight effects are negligible or the drug has flat dose-response. Clinical risk: dosing renally eliminated drugs on actual weight in an obese patient can overdose because kidneys don't scale with fat mass.

Daily body weight fluctuates 0.5–2 kg due to: Water balance — sweat, urination, fluid intake, and sodium intake (which affects water retention); GI contents — a full meal adds 0.5–1 kg temporarily; Glycogen — each gram stores 3 g water; high-carbohydrate meals expand glycogen + associated water; Hormonal variation — cortisol, aldosterone, and sex hormones affect fluid retention. Best tracking practice: weigh at the same time daily (morning, post-urination, pre-eating); track weekly averages to see meaningful trends of 0.5–1 kg/week.