Adjusted Body Weight Calculator Formula

Understand the math behind the adjusted body weight calculator. Each variable explained with a worked example.

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Formulas Used

Ideal Body Weight

ideal_weight = ibw

Adjusted Body Weight

adjusted_weight = ibw + 0.4 * (actual_weight_kg - ibw)

Percent Over IBW

pct_over_ibw = (actual_weight_kg - ibw) / ibw * 100

Variables

Variable	Description	Default
`height_inches`	Height(inches)	70
`actual_weight_kg`	Actual Body Weight(kg)	110
`gender_base`	Gender	50
`ibw`	Derived value`= gender_base + 2.3 * max(height_inches - 60, 0)`	calculated

How It Works

Dosing Weight Calculations

Ideal Body Weight (Devine Formula)

Male: IBW = 50 + 2.3 x (height in inches - 60) Female: IBW = 45.5 + 2.3 x (height in inches - 60)

Adjusted Body Weight

AdjBW = IBW + 0.4 x (Actual Weight - IBW)

Adjusted body weight is used for drug dosing in obese patients (>20% above IBW) because many drugs do not distribute fully into adipose tissue.

Worked Example

A 70-inch tall male weighing 110 kg.

height_inches = 70actual_weight_kg = 110gender_base = 50

01IBW = 50 + 2.3 x (70 - 60) = 73 kg
02AdjBW = 73 + 0.4 x (110 - 73) = 73 + 14.8 = 87.8 kg
03Percent over IBW = (110 - 73) / 73 x 100 = 51%

Frequently Asked Questions

When should adjusted body weight be used?

Adjusted body weight is typically used when actual weight exceeds ideal body weight by more than 20-30%. Many antibiotics and chemotherapy agents use AdjBW for dosing.

What is ideal body weight?

IBW is a height-based weight estimate originally developed for drug dosing. It does not account for muscle mass or body composition.

Why not just use actual weight for all drugs?

Some drugs do not distribute well into fat tissue. Using actual weight in obese patients could lead to overdosing. Other drugs distribute well into fat and use actual weight.

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