Adjusted Body Weight Calculator

Adjusted Body Weight (ABW) is a dosing weight, not a description of a person. It sits between Ideal Body Weight (IBW) and Total Body Weight (TBW), and it exists because neither of those alone gives the right dose in a patient carrying a lot of excess weight.

The formula:

ABW = IBW + CF × (TBW − IBW)

CF is the correction factor, and 0.4 is the usual value. In words: take the ideal weight, then add 40% of the excess.

Why 40%, and not all of it

Adipose tissue is not inert. It is roughly 30% lean tissue and water by mass, and it carries its own blood supply. So when someone is 40 kg over their ideal weight, that 40 kg does contribute to the volume a drug distributes into, just far less per kilogram than muscle does. Dose on total weight and you overshoot, sometimes badly. Dose on ideal weight and you underdose a patient who genuinely has more body to fill. The correction factor is the empirical middle, fitted from pharmacokinetic studies rather than derived from first principles, which is why different drugs and different references use 0.3, 0.4 or 0.5.

Which weight goes with which drug

This is the part that matters at the bedside, and getting it wrong in either direction causes harm. A rough guide for common anesthetic and critical care agents:

The one that catches people out is the ventilator. Tidal volume is set from predicted body weight, which comes from height and sex alone and never from the scale. A 160 cm patient weighing 130 kg still gets the tidal volume of a 160 cm patient. Setting 6 to 8 mL/kg on actual weight there is a classic and genuinely dangerous error.

Ideal Body Weight, if you need to work it out first

The Devine formulas, which is what most protocols mean by IBW:

• Male IBW (kg) = 50 + 2.3 × (height in inches − 60)
• Female IBW (kg) = 45.5 + 2.3 × (height in inches − 60)

Worked example

Female patient, 5 ft 4 in (64 inches), 100 kg.

• IBW = 45.5 + 2.3 × (64 − 60) = 45.5 + 9.2 = 54.7 kg
• Excess over IBW = 100 − 54.7 = 45.3 kg, which is 83% above ideal
• ABW = 54.7 + 0.4 × 45.3 = 54.7 + 18.1 = 72.8 kg

A gentamicin dose calculated on 100 kg would be roughly 37% higher than one calculated on 72.8 kg, which for an aminoglycoside is the difference between a therapeutic peak and a nephrotoxic one.

When ABW does not apply

If the patient weighs less than their ideal weight, the formula returns a number below IBW, and that number is meaningless for dosing. Use actual body weight instead. ABW is also not usually applied below roughly 120 to 130% of IBW, because the correction is doing almost nothing at that point and simply dosing on actual weight is close enough.

Abbreviations used above: ABW (Adjusted Body Weight), IBW (Ideal Body Weight), TBW (Total Body Weight), CF (Correction Factor).