What weight should I use for an obese patient?

For aminoglycosides, vancomycin and heparin, use Adjusted Body Weight (ABW = IBW + 0.4 × (actual − IBW)). For most other drugs use actual body weight unless the drug monograph specifies otherwise. When uncertain, ask pharmacy.

The prescription says mcg/kg — how do I enter that?

Convert to mg/kg by dividing by 1,000. For example, 10 mcg/kg = 0.01 mg/kg. Or enter the dose in mcg and make sure your stock concentration is also in mcg/mL.

What is the adult maximum dose for common drugs?

This calculator does not apply drug-specific maximum doses automatically. You must check the adult maximum in your drug reference (AMH in Australia, Lexicomp or Micromedex in the US) for every weight-based order.

Drug Dose by Weight (mg/kg) — Free Nurses Calculator

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Transfer arrives from ED — 68 kg, gentamicin 5 mg/kg ordered. Stock is 80 mg/2 mL. You need the draw-up volume before the porter leaves.

Drug Dose by Weight (mg/kg)

Medication

Patient weight (kg)

Always use kg

Dose (mg/kg)

Stock concentration (mg/mL)

Route

Total mg = Weight (kg) × Dose (mg/kg) Volume (mL) = Total mg ÷ Concentration (mg/mL) Example: 70 kg patient, 15 mg/kg, stock 50 mg/mL
Total = 1050 mg · Volume = 21 mL

💡 For high-alert medications (e.g. gentamicin, heparin) always perform an independent double-check.

Reference: NPS MedicineWise · ACSQHC Medication Safety

⚕️ Clinical safety: 🇦🇺 Verify with facility drug formulary and senior clinician · Meets AHPRA/ACSQHC standards

1 What this calculator does

Calculates total dose in mg and the volume to draw up from a weight-based (mg/kg) prescription. Supports IV, IM and oral routes. Automatically converts pounds to kilograms in imperial mode.

2 Formula & professional reasoning

Total dose (mg) = Weight (kg) × Dose (mg/kg)
Volume to give (mL) = Total dose ÷ Concentration (mg/mL)

Weight-based dosing accounts for differences in volume of distribution between individuals. The heavier the patient, the greater the volume drugs distribute into, requiring a proportionally higher dose to achieve the same plasma concentration. The two-step calculation — total dose first, then volume — is important: always calculate the intermediate mg value before converting to mL, to prevent unit errors.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Gentamicin 5 mg/kg IV

Given: Weight: 68 kg · Dose: 5 mg/kg · Stock: 80 mg/2 mL (40 mg/mL)

Working: Total: 68 × 5 = 340 mg · Volume: 340 ÷ 40

Answer: 8.5 mL

💡 Gentamicin: always check most recent serum level and renal function before dosing.

Standard

Paracetamol IV 15 mg/kg

Given: Weight: 82 kg · Dose: 15 mg/kg · Stock: 1000 mg/100 mL (10 mg/mL)

Working: Total: 82 × 15 = 1,230 mg — exceeds adult max of 1,000 mg

Answer: Cap at 1,000 mg → 100 mL

💡 Always apply adult maximum dose cap. 82 kg × 15 mg/kg = 1,230 mg but adult IV paracetamol max is 1,000 mg.

Advanced

Vancomycin 25 mg/kg loading dose

Given: Weight: 95 kg · Dose: 25 mg/kg · Stock: 500 mg/vial (reconstituted to 50 mg/mL)

Working: Total: 95 × 25 = 2,375 mg · Volume: 2,375 ÷ 50

Answer: 47.5 mL

💡 Vancomycin loading doses must be infused slowly (≥60 min) to avoid Red Man Syndrome. Pharmacy consultation recommended for doses >3,000 mg.

4 Sanity check

Always apply adult maximum dose

Weight × mg/kg must not exceed adult maximum

Large patients: 90 kg × 15 mg/kg paracetamol = 1,350 mg — cap at 1,000 mg adult max.

Obese patients

Use adjusted body weight, not actual, for most drugs

Aminoglycosides, vancomycin, heparin: use adjusted IBW. Ask pharmacy if uncertain.

Volume sanity

Most IM injections ≤ 3 mL per site

Volume > 3 mL for IM: split dose across two sites or use IV route.

Unit check

mcg/kg vs mg/kg — 1,000× difference

Check the prescription unit carefully. mcg/kg orders are common for fentanyl, digoxin.

5 Common errors

Error	Cause	Consequence	Fix
Using estimated weight instead of measured	Scales not available	Dose error proportional to weight error — 10 kg error = 10% dose error	Weigh every patient before weight-based dosing. For emergencies, use visual estimate as last resort only.
Not capping at adult maximum dose	Applying mg/kg blindly to large patients	Overdose in patients weighing over ~70 kg for many drugs	Always look up the adult maximum dose and cap if exceeded
Wrong units on stock concentration	Reading 80 mg/2 mL as 80 mg/mL	Volume 2× wrong — significant over- or under-dose	Always reduce to mg/mL first: 80 mg/2 mL = 40 mg/mL. Write this intermediate step.
Skipping independent double-check	Perceived time pressure	Calculation error uncaught	Gentamicin, vancomycin, heparin, insulin: two-nurse independent check is mandatory, not optional