Adjusted Body Weight (AjBW) Calculator

What is Adjusted Body Weight?

Adjusted Body Weight (AjBW) is a calculated weight used mainly for medication dosing and nutrition planning in adults with obesity, combining ideal body weight with 40% of excess body weight.

It is not a weight goal. Think of it as a dosing weight: a more accurate number for clinical calculations than either actual or ideal weight, because fat tissue is less metabolically active than lean tissue.

Actual vs Ideal vs Adjusted body weight

Actual body weight is your scale weight, ideal body weight is a healthy reference, and adjusted body weight is the figure used for drug dosing and nutrition.

When should you use Adjusted Body Weight?

Use Adjusted Body Weight when actual weight would overestimate dosing or nutrition, mainly in overweight and obese adults, and not for normal-weight adults, athletes, pregnancy, or children.

Adjusted Body Weight formula

The Adjusted Body Weight formula is AjBW = IBW + 0.4 x (actual weight - IBW), where IBW is ideal body weight from the Devine formula.

Worked example

For a 5 ft 10 in man weighing 95 kg, ideal body weight is 73 kg and adjusted body weight is 81.8 kg.

Common clinical uses

Adjusted Body Weight is used for medication dosing, clinical nutrition, and calorie estimation, especially in hospital and critical care settings.

• Medication dosing
• Clinical nutrition
• Calorie estimation
• Hospital calculations
• Renal dosing
• Chemotherapy
• Critical care

Who should use this calculator?

This calculator is built for healthcare professionals, dietitians, pharmacists, students, and patients who need an adjusted body weight for dosing or nutrition.

• Healthcare professionals
• Dietitians
• Pharmacists
• Medical students
• Nurses
• Patients with obesity
• Clinical researchers

When not to use AjBW

Do not use Adjusted Body Weight for elite athletes, pregnancy, fluid retention, or children, where the 40% assumption does not hold.

• Elite athletes with high muscle mass
• Pregnancy
• Fluid retention or edema
• Children

For lean or very muscular people, lean body mass or fat-free mass is a better basis. Always confirm clinical dosing with a qualified professional.

Adjusted Body Weight in Clinical Dosing

Adjusted body weight shows up in clinical dosing because, for some weight-based medications, using a person's full actual weight can overshoot the dose, so AjBW sits as a middle ground between actual and ideal body weight.

The reason it exists is pharmacokinetic. A number of drugs do not distribute evenly into fatty tissue, so in someone carrying excess weight, dosing on total body weight can push the amount higher than the body actually clears. AjBW nudges the dosing weight part of the way back toward ideal, which is why it turns up in certain hospital and pharmacy calculations. Which drugs, and exactly how, is a clinical decision that sits with the prescriber and current guidelines.

It is worth being clear about one thing, because people often assume it works this way. GLP-1 medications are generally not dosed by body weight at all. Semaglutide and tirzepatide follow a fixed titration schedule set out in their approved prescribing information, stepping up on a set timeline rather than being calculated from your weight. You can see what that standard ladder looks like on the semaglutide dose calculator. So adjusted body weight does not set a GLP-1 dose. That comes from the approved schedule and your prescriber's judgement, not a weight-based formula.

Where adjusted body weight can still matter for someone on a GLP-1 is elsewhere in their care. A clinician might consider AjBW or body composition when making other medication decisions for the same patient, for drugs that genuinely are dosed by weight. Those are separate calls from the GLP-1 itself.

None of this is dosing advice. Any medication dose, GLP-1 or otherwise, should come from your prescriber and the approved prescribing information for that drug.

Frequently asked questions

Clinical references and evidence

• Devine BJ. Gentamicin therapy. Drug Intelligence and Clinical Pharmacy. 1974 (origin of the ideal body weight formula).
• Pai MP. Drug dosing based on weight and body surface area. Pharmacotherapy. 2012 (review of adjusted body weight in dosing).
• Bauer LA. Applied Clinical Pharmacokinetics (adjusted body weight in aminoglycoside and vancomycin dosing).
• ASPEN clinical guidelines on nutrition support, which use adjusted body weight for energy needs in obesity.

See also our Ideal Body Weight calculator and BMI calculator.