What is ideal body weight dosing?

For obese patients, some drugs should be dosed using ideal body weight (IBW) rather than actual body weight to avoid overdose. Others use adjusted body weight (ABW = IBW + 0.4 x (actual - IBW)). The choice depends on the specific drug. Use our Ideal Body Weight calculator and confirm the appropriate dosing weight with a pharmacist.

Which drugs use weight-based dosing?

Common drugs using weight-based dosing include aminoglycoside antibiotics (gentamicin, amikacin), vancomycin, heparin, enoxaparin, chemotherapy agents, some antivirals, most pediatric medications and several biologic agents. The appropriate dosing weight (actual, ideal or adjusted) varies by drug.

Weight-based dosage calculator

Q: How do you calculate a weight-based dose?

Dose (mg) = dose per kg (mg/kg) multiplied by patient weight in kg. For a 70kg adult prescribed gentamicin at 5mg/kg: dose = 5 x 70 = 350mg. Always cap the result at the published maximum dose for that drug.

Free

Calculate precise mg/kg doses for any medication and patient weight. Supports actual, ideal and adjusted body weight dosing, with automatic capping at the maximum published dose. Verify all results with a pharmacist before administration.

Dose (mg/kg)

Use mg/kg/dose for per-dose calculation. Use mg/kg/day and divide below for daily-divided dosing.

Dose unit

Dosing weight type

Actual patient weight

Doses per day (frequency)

Maximum single dose (optional)

Dose will be capped at this value if exceeded.

Maximum daily dose (optional)

Course duration (days, optional)

Available concentration (optional - for volume calculation)

mg per mL

Weight-based dosage result

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Weight-based dosing: actual vs ideal vs adjusted body weight

The formula is simple: dose (mg) = dose per kg x patient weight. The hard part is knowing which weight to use.

Actual body weight (ABW)

Most drugs use the patient's actual weight. It's the default. For a 70kg patient prescribed gentamicin at 5mg/kg, the dose is 350mg. Done.

Ideal body weight (IBW)

Some drugs distribute primarily in lean tissue and don't penetrate adipose tissue well. Using actual weight in obese patients would overdose them. Aminoglycosides (gentamicin, tobramycin, amikacin) are the classic example. For these, you use IBW instead. IBW for males: 50 + 2.3 x (height in inches - 60). IBW for females: 45.5 + 2.3 x (height in inches - 60). Use our Ideal Body Weight Calculator to get this number quickly.

Adjusted body weight (ABW)

Some drugs partially penetrate adipose tissue. For these, ABW = IBW + 0.4 x (actual weight - IBW). It's the middle ground. Commonly used for aminoglycosides in morbidly obese patients when actual weight exceeds IBW by more than 30%.

Dose capping: the rule you can't skip

Always enter a maximum dose when you have one. A large child or heavy adult can calculate to an amount that exceeds the published maximum for safety reasons. The calculator caps automatically at whatever maximum you enter. If you're unsure of the maximum dose, check our Max Daily Dose Checker first, then come back here.

Drugs that commonly use mg/kg dosing

Aminoglycosides (5-7mg/kg once daily for gentamicin), vancomycin (15-20mg/kg every 8-12 hours), enoxaparin (1mg/kg twice daily for treatment-dose DVT), most chemotherapy agents, paediatric antibiotic doses, and heparin infusions (weight-based protocol). For IV drugs, pair this tool with the IV Drip Rate Calculator to get the infusion rate from the calculated dose. For patients with renal impairment, always run the Renal Dose Adjustment Calculator before finalising the dose.

Frequently asked questions

How do you calculate a weight-based dose?

Dose (mg) = dose per kg x patient weight in kg. For a 70kg adult at 5mg/kg: 5 x 70 = 350mg. Always cap at the published maximum dose for that drug, and confirm which body weight (actual, ideal or adjusted) should be used for your specific medication.

When do I use ideal body weight instead of actual weight?

Use ideal body weight (IBW) for drugs that don't distribute well into adipose tissue, most commonly aminoglycoside antibiotics in obese patients. Using actual weight for these drugs in obese patients would result in overdose. Use our IBW Calculator to get the correct value, then confirm with a pharmacist which weight to use for your specific drug.

What is adjusted body weight?

Adjusted body weight (ABW) = IBW + 0.4 x (actual weight - IBW). It's used for drugs that partially penetrate adipose tissue. Commonly applied to aminoglycosides in morbidly obese patients where actual weight exceeds IBW by more than 30%.

Which drugs commonly use weight-based dosing?

Common examples include aminoglycosides (gentamicin at 5-7mg/kg once daily), vancomycin (15-20mg/kg per dose), enoxaparin (1mg/kg twice daily for treatment), heparin infusions (weight-based protocols), most chemotherapy agents, and virtually all paediatric antibiotic doses. The appropriate dosing weight varies by drug.