Ideal and adjusted body weight calculator

Ideal body weight by the Devine formula is 50 kg for men or 45.5 kg for women, plus 2.3 kg for every inch of height above 60 inches. Adjusted body weight adds 40 percent of the excess over ideal, and choosing the wrong one of the three weights is what shifts a dose band.

When to use the ideal body weight calculator

Use ideal body weight for drugs that distribute mainly into lean tissue, and adjusted body weight in obesity for drugs that distribute partly into fat. Aminoglycosides are the classic example where the choice of weight materially changes the dose.

The ideal body weight formula

Devine IBW = 50 kg for males or 45.5 kg for females, plus 2.3 kg for every inch of height above 60 inches. Adjusted body weight = IBW + 0.4 × (actual weight − IBW).

Worked example

A male patient is 170 cm and 110 kg. 170 cm is 66.9 inches, so 6.9 inches above 60. IBW = 50 + (2.3 × 6.9) = 65.9 kg. Adjusted = 65.9 + 0.4 × (110 − 65.9) = 83.5 kg.

The mistake that costs marks

Using actual body weight for a lipophobic drug in an obese patient overdoses them, and using ideal body weight for a lipophilic drug underdoses them. The question is always which compartment the drug actually enters, not which number is easiest to find.

Study aid, not a clinical tool

This calculator exists so you can learn the method and check your working for an examination. It is not medical advice and not clinical decision support, and it must not be used to make a decision about a real patient. Verify any result against your local formulary, the product literature and a qualified pharmacist. Our medical disclaimer sets out the full position.

Calculators you will usually need alongside this

Most questions that reach for one of these reach for another a step later, so these are the three that most often pair with it.

• Creatinine clearance calculator (Cockcroft-Gault) — Creatinine clearance
• Body surface area calculator (Mosteller and Du Bois) — Body surface area
• mg/kg dose calculator — Weight-based dose

Keep going

Calculations are only half of what examinations test. The other half is knowing why a drug does what it does, which is what decides whether a number you calculated is even the right number to be calculating. Start with Aminoglycoside monitoring. The drug class pages cover the mechanism behind the number, or browse the rest of the free calculators and try the practice questions.