Medication Dosage Calculations for Nurses
Dosage calculation errors are one of the most common — and most preventable — medication errors in clinical settings. Mastering mg/kg dosing and mg/mL concentration conversions allows nurses to verify physician orders, catch prescription errors, and administer medications with confidence. This reference covers the core formulas and provides a quick-reference table for frequently ordered weight-based drugs.
Clinical Disclaimer: This reference is for educational purposes. Always verify drug doses with a pharmacist and follow your institution's protocols before administering any medication.
How Weight-Based (mg/kg) Dosing Works
Weight-based dosing standardizes drug amounts across patients of different sizes, making it especially important in pediatrics, oncology, and critical care. The prescriber orders a dose in mg per kilogram of body weight, and the nurse must multiply by the patient's weight to find the total dose — then convert that dose to a volume using the available drug concentration.
Step 1 — Calculate Total Dose
Total Dose (mg) = Ordered Dose (mg/kg) × Patient Weight (kg)
Step 2 — Convert to Volume
Volume to Give (mL) = Total Dose (mg) ÷ Stock Concentration (mg/mL)
Combined Formula
Dose (mL) = [Desired Dose (mg/kg) × Weight (kg)] ÷ Stock Concentration (mg/mL)
Example: Order reads Amoxicillin 40 mg/kg/day every 8 hr for a 22 kg child. Daily dose = 40 × 22 = 880 mg/day. Per dose = 880 ÷ 3 = 293 mg. Available concentration = 125 mg/5 mL (25 mg/mL). Volume = 293 ÷ 25 = 11.7 mL per dose.
Quick Reference: Common Weight-Based Drugs
| Drug | Dose | Max Dose | Frequency | Notes |
|---|---|---|---|---|
| Amoxicillin | 25–45 mg/kg/day | 3,000 mg/day | Every 8–12 hr | Otitis media, strep throat |
| Ibuprofen | 5–10 mg/kg/dose | 400 mg/dose | Every 6–8 hr | Analgesic/antipyretic |
| Acetaminophen | 10–15 mg/kg/dose | 1,000 mg/dose | Every 4–6 hr | Max 5 doses/24 hr |
| Morphine | 0.1–0.2 mg/kg/dose | 15 mg/dose | Every 3–4 hr PRN | Moderate-severe pain; monitor resp. |
| Dexamethasone | 0.15–0.6 mg/kg/dose | 10 mg/dose | Every 6–24 hr (indication-specific) | Croup, airway edema, anti-inflammatory |
| Gentamicin | 5–7 mg/kg/day | Per levels (check trough) | Every 24 hr (once-daily dosing) | Renal/hearing monitoring required |
Doses shown are general references. Pediatric/adult ranges differ for many drugs. Always confirm with current prescribing guidelines and pharmacy.
Frequently Asked Questions
How do I calculate a mg/kg dose?
Multiply the prescribed dose (in mg/kg) by the patient's weight in kilograms. For example, if the order is 10 mg/kg and the patient weighs 70 kg, the dose is 10 × 70 = 700 mg. Always confirm the patient's weight in kg — if you only have pounds, divide by 2.2046 to convert.
What is the difference between mg/kg and mg/kg/day?
mg/kg refers to a single dose amount per kilogram of body weight. mg/kg/day is the total daily dose per kilogram, which must be divided by the number of doses given per day to find the individual dose. For example, Amoxicillin 45 mg/kg/day given every 8 hours means each dose is 45 ÷ 3 = 15 mg/kg.
How do I convert a mg dose to mL given a stock concentration?
Divide the desired dose in milligrams by the stock concentration in mg/mL. For example, if you need to give 250 mg and the available concentration is 125 mg/5 mL (= 25 mg/mL), then Volume = 250 ÷ 25 = 10 mL. Always double-check the label concentration and use the formula: Volume (mL) = Desired Dose (mg) ÷ Stock Concentration (mg/mL).