Three core setups — master all three
Volume = Dose ordered ÷ Concentration (dose/mL)
Also expressed as: dose × (mL / concentration dose). The fraction format prevents errors — always check that your unwanted units cancel.
Dose to volume (available dose)
Setup: dose ordered × (mL / dose on hand)
Example: 400 mg needed, have 500 mg/10 mL
400 × (10/500) = 8 mL
Check units: mg cancels, mL remains ✓
Example: 400 mg needed, have 500 mg/10 mL
400 × (10/500) = 8 mL
Check units: mg cancels, mL remains ✓
Reconstitution
Powder + diluent = reconstituted solution
Total volume = powder volume + diluent added
Concentration is on the label after reconstitution
Always check: does the label say how much water to add?
The final volume ≠ the water volume added.
Total volume = powder volume + diluent added
Concentration is on the label after reconstitution
Always check: does the label say how much water to add?
The final volume ≠ the water volume added.
Pediatric weight-based
Step 1: lb ÷ 2.2 = kg
Step 2: kg × mg/kg = dose in mg
Step 3: (if divided): mg/day ÷ frequency = mg/dose
Step 4: mg × (mL/mg) = volume
Never skip Step 1. Never skip Step 3 for TID/BID/q8h.
Step 2: kg × mg/kg = dose in mg
Step 3: (if divided): mg/day ÷ frequency = mg/dose
Step 4: mg × (mL/mg) = volume
Never skip Step 1. Never skip Step 3 for TID/BID/q8h.
Units-based dosing (insulin & heparin)
Insulin: U-100 = 100 units/mL
10 units = 0.1 mL on a U-100 syringe
Heparin: ordered in units, not mg
Penicillin G: also in units
Always verify: units ≠ mg for these drugs
10 units = 0.1 mL on a U-100 syringe
Heparin: ordered in units, not mg
Penicillin G: also in units
Always verify: units ≠ mg for these drugs
★ PTCB tip: Dosage conversion questions appear in Order Entry & Processing. The PTCB commonly tests: (1) amox/azithromycin suspensions with a volume to dispense, (2) reconstitution of a vial then how many mL to draw, (3) pediatric weight-based dose in mL, (4) insulin syringes in units vs mL. Reconstitution is still tested even though compounding was removed — this is dispensing, not compounding.
4 critical errors to avoid: (1) Using mg/mL without setting up the fraction correctly. (2) Forgetting to divide by dose frequency for TID/BID orders. (3) Adding water volume = total volume (wrong) — the powder occupies space too. (4) Confusing U-100 insulin concentration with dosing in units — 10 units is 0.1 mL, not 10 mL.
Six worked examples covering the full range of dosage conversion types you will see in real practice and on the PTCB exam.
EasyRetail
Rx: azithromycin 500 mg loading dose. Available: 200 mg/5 mL suspension.
How many mL per dose?
Set up the fraction:500 mg × (5 mL / 200 mg) = 500×5/200 = 12.5 mL
✓ 12.5 mL for the Day 1 loading dose
MediumHospital
Ceftriaxone 1 g vial. Reconstituted to 100 mg/mL. Patient needs 500 mg.
How many mL to draw up after reconstitution?
After reconstitution concentration = 100 mg/mL:500 mg × (1 mL / 100 mg) = 5 mL
✓ Draw up 5 mL from the reconstituted vial
MediumPatient scenario
Child: 44 lb. Amoxicillin 40 mg/kg/day in 3 divided doses. Available: 250 mg/5 mL.
How many mL per individual dose?
Step 1 — lb to kg:44 ÷ 2.2 = 20 kg
Step 2 — daily dose:20 × 40 = 800 mg/day
Step 3 — per dose (TID = 3x/day):800 ÷ 3 = 266.7 mg/dose
Step 4 — to mL:266.7 × (5/250) = 5.33 → 5.3 mL per dose
✓ 5.3 mL per dose, three times daily
MediumHospital
Insulin drip: 100 units regular insulin in 100 mL NS (= 1 unit/mL). Order: 8 units/hr.
What is the pump rate in mL/hr?
Concentration = 1 unit/mL (100 units / 100 mL):8 units/hr ÷ 1 unit/mL = 8 mL/hr
Shortcut: when insulin drip = 100 units/100 mL, units/hr = mL/hr
✓ Set pump to 8 mL/hr
HardHospital
Child in acute seizure: 44 lb. Lorazepam 0.05 mg/kg IV. Available: 2 mg/mL.
How many mL IV immediately?
Step 1:44 ÷ 2.2 = 20 kg
Step 2:20 × 0.05 = 1 mg
Step 3:1 mg ÷ 2 mg/mL = 0.5 mL
✓ Draw up 0.5 mL and give slowly IV
HardRetail
Sanity check: student calculates 50 mL to give 500 mg from amoxicillin 500 mg/5 mL. Is this right?
What is the correct answer and how do you identify the error?
Correct setup:500 mg × (5 mL / 500 mg) = 5 mL
Check the answer against reality:50 mL from a 100 mL bottle = half the bottle for ONE DOSE — that is a red flag.
✓ 5 mL, not 50 mL. Always sanity-check: does this volume make sense?
Sanity rule: If your answer is more than the entire bottle, something is wrong. Use your clinical judgment as a final check after every calculation.
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Lesson 4 · Dosage Conversions · AlabamaPharmTech.com
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