βοΈ Adjusted Body Weight Calculator
Ideal and adjusted body weight for medication dosing.
Adjusted body weight
For informational purposes only, not medical advice. Consult a healthcare professional.
Dosing a medication by total body weight works fine until the patient carries significant extra fat, which many drugs do not distribute into. Dose by actual weight and you overdose; dose by ideal weight and you may underdose drugs that do spread into adipose tissue. Adjusted body weight splits the difference.
This calculator computes ideal body weight with the Devine formula, then applies the standard 0.4 correction factor to estimate the dosing weight used for many hydrophilic drugs in overweight patients. It is an educational tool: the final dosing-weight choice always belongs to the prescriber and pharmacist.
How to use this calculator
Enter sex, height, and actual weight. The calculator handles the unit conversions and the conditional logic.
Select sex and enter height in feet and inches.
Enter actual body weight in pounds.
Read ideal body weight and the adjusted (dosing) weight, and confirm the choice with pharmacy for the specific drug.
The formulas
The Devine formula estimates the weight of a person of a given height at a lean body composition. The adjusted weight then adds back 40 percent of the excess, a compromise supported by pharmacokinetic studies for drugs that distribute partially into fat.
If actual weight is at or below ideal, no adjustment is made and actual weight is the dosing weight. The 0.4 factor is the most common, though some drugs use 0.3 or other corrections, which is why drug-specific references matter.
Worked example
A man, 5 feet 10 inches (70 inches), weighing 242 lb (109.8 kg). Ideal body weight is 50 + 2.3 x 10 = 73 kg. He exceeds ideal, so adjusted weight is 73 + 0.4 x (109.8 - 73) = 73 + 14.7 = 87.7 kg, about 193 lb. A weight-based drug dosed at 5 mg/kg would be 439 mg on adjusted weight versus 549 mg on actual weight, a meaningful safety difference.
Contrast a woman, 5 feet 4 inches (64 inches), 130 lb (59 kg). Her IBW is 45.5 + 2.3 x 4 = 54.7 kg. Actual weight is below ideal, so her dosing weight is simply her actual 59 kg. No adjustment needed.
Which weight for which drug
The honest answer is "it depends on the drug," and this is where the calculator stops and references begin. Hydrophilic drugs (aminoglycosides, many beta-lactams) often use adjusted weight. Lipophilic drugs may use total body weight. Narrow-therapeutic-index drugs like chemotherapy usually follow protocol-specific rules, and heparin and some ICU drugs have their own conventions.
For general weight context, our BMI calculator and ideal weight calculator cover the non-dosing side. But for any real prescription, the pharmacist's dosing reference for that specific drug outranks every calculator on the internet, including this one.
Limits of the Devine formula
Devine's formula dates to 1974 and was derived for drug dosing, not for defining anyone's "ideal" body. It ignores frame size, muscularity, age, and ethnicity. A muscular athlete will exceed their Devine IBW without excess fat, and the adjustment logic does not apply the same way.
Newer alternatives (like the Robinson or Miller formulas, or lean body mass equations) exist, but Devine remains the most cited in dosing references, which is why this calculator uses it. Consistency with the reference your pharmacist uses matters more than theoretical perfection.
Special populations and edge cases
Amputees need a correction before any of this math: subtract the missing limb's estimated body-weight percentage (roughly 16 percent for a below-knee leg, 7 percent for a forearm) from actual weight first, then compare to ideal. Spinal cord injury and muscular dystrophy lower the lean baseline, making Devine IBW an overestimate.
Pregnancy breaks the model entirely: use pre-pregnancy or early-pregnancy weight for dosing references, and follow obstetric dosing guidance rather than adjusted-weight math. Elderly patients with sarcopenia may sit below their Devine IBW while carrying excess fat, a combination where neither actual nor adjusted weight is quite right and pharmacist judgment matters most.
The deeper lesson: every weight descriptor is a proxy for the volume the drug actually distributes into, and proxies fail at the extremes. When the patient does not look like the population the formula was built on, say so and get pharmacy involved early.
Aminoglycosides: the classic use case
Gentamicin and tobramycin are why adjusted body weight exists in pharmacy practice. These drugs distribute into lean tissue and extracellular fluid but barely into fat, and they are nephrotoxic and ototoxic at high levels. Dosing them by total body weight in obesity reliably overshoots; dosing by ideal weight can undershoot in serious infection.
The standard approach uses adjusted body weight with the 0.4 correction for the mg/kg dose, then confirms with drug-level monitoring: a peak drawn after the infusion and a trough before the next dose. Extended-interval (once-daily) aminoglycoside protocols have their own nomograms built on the same weight logic.
Vancomycin, by contrast, is usually dosed on actual body weight with levels to guide, which shows why drug-specific references beat blanket rules. Same patient, same scale, different weight: the drug's distribution decides, not the diagnosis.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is adjusted body weight used for?
Estimating a safe dosing weight for medications in patients whose actual weight exceeds ideal body weight. It is commonly used for hydrophilic drugs that do not fully distribute into fat tissue.
How do you calculate adjusted body weight?
First find ideal body weight (Devine formula), then: adjusted = IBW + 0.4 x (actual - IBW). The 0.4 correction factor is the standard compromise between actual and ideal weight.
What is the Devine formula?
Men: 50 + 2.3 kg per inch over 5 feet. Women: 45.5 + 2.3 kg per inch over 5 feet. Published in 1974 for drug dosing, it remains the most cited ideal-weight formula in pharmacy references.
When is adjusted body weight not needed?
When actual weight is at or below ideal body weight, use actual weight. The adjustment only applies to the excess above ideal.
Is the 0.4 correction factor always right?
No. It is the most common default, but specific drugs may use 0.3, total body weight, or lean body mass. Always check the dosing reference for the particular drug.
Can adjusted body weight be used for children?
No. The Devine formula and the 0.4 correction were developed for adults. Pediatric dosing follows weight-based or body-surface-area nomograms instead.
Does adjusted body weight apply to obese patients for all drugs?
No. Lipophilic drugs, chemotherapy, and several ICU medications follow different conventions. The calculator gives the standard estimate; the drug's own dosing guidance decides.
Why not just dose by actual weight?
For hydrophilic drugs, dosing by total weight in obesity overshoots blood levels because the drug does not distribute into fat. Adjusted weight approximates the distributing mass better.