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Lesson 4: Dosage Conversions
Convert any dose to a volume to dispense or administer — tablets, suspensions, reconstituted vials, weight-based pediatric doses, and units-based medications.
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.
Difficulty
Context
Easy
Retail
Dose to Volume
Rx: amoxicillin 400 mg/dose. Available: 400 mg/5 mL.
How many mL per dose?
Answer: 5 mL
400 × (5/400) = 5 mL.
400 × (5/400) = 5 mL.
Easy
Retail
Dose to Volume
Loading dose: 500 mg azithromycin. Available: 200 mg/5 mL.
How many mL?
Answer: 12.5 mL
500 × (5/200) = 12.5 mL.
500 × (5/200) = 12.5 mL.
Easy
Retail
Dose to Volume
Dose: 20 mg prednisone oral solution. Available: 5 mg/5 mL.
How many mL?
Answer: 20 mL
20 × (5/5) = 20 mL.
20 × (5/5) = 20 mL.
Easy
Hospital
Dose to Volume
Order: furosemide 40 mg IV push. Available: 10 mg/mL.
How many mL?
Answer: 4 mL
40 ÷ 10 = 4 mL.
40 ÷ 10 = 4 mL.
Easy
Hospital
Dose to Volume
Dose: ondansetron 4 mg IV. Available: 2 mg/mL.
How many mL?
Answer: 2 mL
4 ÷ 2 = 2 mL.
4 ÷ 2 = 2 mL.
Easy
Retail
Dose to Volume
Dose: 10 mg cetirizine. Available: 5 mg/5 mL.
How many mL?
Answer: 10 mL
10 × (5/5) = 10 mL.
10 × (5/5) = 10 mL.
Easy
Hospital
Dose to Volume
Order: metoclopramide 10 mg IV. Available: 5 mg/mL.
How many mL?
Answer: 2 mL
10 ÷ 5 = 2 mL.
10 ÷ 5 = 2 mL.
Easy
Retail
Dose to Volume
Dose: 250 mg phenytoin suspension. Available: 125 mg/5 mL.
How many mL?
Answer: 10 mL
250 × (5/125) = 10 mL.
250 × (5/125) = 10 mL.
Med
Hospital
Dose to Volume
Order: morphine 6 mg IV. Available: 4 mg/mL.
How many mL?
Answer: 1.5 mL
6 ÷ 4 = 1.5 mL.
6 ÷ 4 = 1.5 mL.
Med
Hospital
Dose to Volume
DVT prophylaxis: heparin 5,000 units subcut. Available: 5,000 units/mL.
How many mL to inject?
Answer: 1 mL
5,000 ÷ 5,000 = 1 mL.
5,000 ÷ 5,000 = 1 mL.
Med
Retail
Reconstitution
Add 90 mL water to powder. Final volume = 100 mL. Label states: 125 mg/5 mL.
What is the final concentration after reconstitution?
Answer: 125 mg/5 mL
The label tells you the final concentration: 125 mg/5 mL. The water volume determines total bottle size, but concentration is fixed by manufacturer.
The label tells you the final concentration: 125 mg/5 mL. The water volume determines total bottle size, but concentration is fixed by manufacturer.
Med
Retail
Reconstitution
Reconstituted amoxicillin-clavulanate: 400 mg/5 mL.
How many mg are in 7.5 mL?
Answer: 600 mg
400 mg / 5 mL × 7.5 mL = 600 mg.
400 mg / 5 mL × 7.5 mL = 600 mg.
Med
Retail
Reconstitution
Reconstituted cephalexin: 250 mg/5 mL. Patient needs 375 mg.
How many mL to measure?
Answer: 7.5 mL
375 × (5/250) = 7.5 mL.
375 × (5/250) = 7.5 mL.
Med
Hospital
Reconstitution
Ceftriaxone 1 g vial reconstituted to 100 mg/mL. Patient needs 500 mg.
How many mL to draw up?
Answer: 5 mL
500 mg ÷ 100 mg/mL = 5 mL.
500 mg ÷ 100 mg/mL = 5 mL.
Med
Hospital
Reconstitution
Vancomycin 500 mg vial reconstituted to 50 mg/mL. Need to draw up 750 mg.
How many mL?
Answer: 15 mL
750 ÷ 50 = 15 mL.
750 ÷ 50 = 15 mL.
Hard
Retail
Reconstitution
Child: 33 lb. Azithromycin 10 mg/kg loading dose. Available: 200 mg/5 mL.
How many mL per dose?
Answer: 3.8 mL
33 ÷ 2.2 = 15 kg. 15 × 10 = 150 mg. 150 × (5/200) = 3.75 → 3.8 mL.
33 ÷ 2.2 = 15 kg. 15 × 10 = 150 mg. 150 × (5/200) = 3.75 → 3.8 mL.
Hard
Hospital
Reconstitution
Piperacillin-tazobactam 3.375 g reconstituted to 225 mg/mL in 15 mL total.
How many mL for the full 3.375 g dose?
Answer: 15 mL
3,375 mg ÷ 225 mg/mL = 15 mL — the entire vial.
3,375 mg ÷ 225 mg/mL = 15 mL — the entire vial.
Hard
Retail
Reconstitution
Amoxicillin 250 mg/5 mL. Child takes 125 mg TID for 10 days.
What total volume (mL) is needed for the full course?
Answer: 75 mL
Per dose: 125/250 × 5 = 2.5 mL. Per day: 2.5 × 3 = 7.5 mL. 10 days: 7.5 × 10 = 75 mL.
Per dose: 125/250 × 5 = 2.5 mL. Per day: 2.5 × 3 = 7.5 mL. 10 days: 7.5 × 10 = 75 mL.
Med
Patient scenario
Pediatric Weight-Based
Child: 22 lb. Ibuprofen 10 mg/kg. Available: 100 mg/5 mL.
How many mL per dose?
Answer: 5 mL
22/2.2=10 kg. 10×10=100 mg. 100×(5/100) = 5 mL.
22/2.2=10 kg. 10×10=100 mg. 100×(5/100) = 5 mL.
Med
Patient scenario
Pediatric Weight-Based
Child: 33 lb. Amoxicillin 40 mg/kg/day TID. Available: 250 mg/5 mL.
How many mL per dose?
Answer: 4 mL
33/2.2=15 kg. 15×40=600 mg/day. 600÷3=200 mg/dose. 200×(5/250) = 4 mL.
33/2.2=15 kg. 15×40=600 mg/day. 600÷3=200 mg/dose. 200×(5/250) = 4 mL.
Med
Patient scenario
Pediatric Weight-Based
Child: 44 lb. Acetaminophen 15 mg/kg. Available: 160 mg/5 mL.
How many mL per dose?
Answer: 9.4 mL
44/2.2=20 kg. 20×15=300 mg. 300×(5/160)=9.375→9.4 mL.
44/2.2=20 kg. 20×15=300 mg. 300×(5/160)=9.375→9.4 mL.
Med
Hospital
Pediatric Weight-Based
Child: 44 lb. Cefdinir 7 mg/kg/day BID. Available: 125 mg/5 mL.
How many mL per dose?
Answer: 2.8 mL
44/2.2=20 kg. 20×7=140 mg/day. 140÷2=70 mg/dose. 70×(5/125)=2.8→2.8 mL.
44/2.2=20 kg. 20×7=140 mg/day. 140÷2=70 mg/dose. 70×(5/125)=2.8→2.8 mL.
Med
Retail
Pediatric Weight-Based
Child: 55 lb. Prednisolone 1 mg/kg/day once daily. Available: 15 mg/5 mL.
How many mL per day?
Answer: 8.3 mL
55/2.2=25 kg. 25×1=25 mg. 25×(5/15)=8.33→8.3 mL.
55/2.2=25 kg. 25×1=25 mg. 25×(5/15)=8.33→8.3 mL.
Hard
Patient scenario
Pediatric Weight-Based
Child: 44 lb. Augmentin 45 mg/kg/day BID. Available: 400 mg/5 mL.
How many mL per dose?
Answer: 5.6 mL
44/2.2=20 kg. 20×45=900 mg/day. 900÷2=450 mg/dose. 450×(5/400)=5.625→5.6 mL.
44/2.2=20 kg. 20×45=900 mg/day. 900÷2=450 mg/dose. 450×(5/400)=5.625→5.6 mL.
Hard
Hospital
Pediatric Weight-Based
Child: 66 lb. Tobramycin 2.5 mg/kg q8h IV. Available: 10 mg/mL.
How many mL per dose?
Answer: 7.5 mL
66/2.2=30 kg. 30×2.5=75 mg. 75÷10=7.5 mL.
66/2.2=30 kg. 30×2.5=75 mg. 75÷10=7.5 mL.
Hard
Patient scenario
Pediatric Weight-Based
Child: 22 lb. High-dose amoxicillin 90 mg/kg/day BID (AOM protocol). Available: 600 mg/5 mL.
How many mL per dose?
Answer: 3.75 mL
22/2.2=10 kg. 10×90=900 mg/day. 900÷2=450 mg/dose. 450×(5/600)=3.75→3.75 mL.
22/2.2=10 kg. 10×90=900 mg/day. 900÷2=450 mg/dose. 450×(5/600)=3.75→3.75 mL.
Hard
Hospital
Pediatric Weight-Based
Child: 55 lb. Gentamicin 2.5 mg/kg q8h IV. Available: 40 mg/mL.
How many mL per dose?
Answer: 1.56 mL
55/2.2=25 kg. 25×2.5=62.5 mg. 62.5÷40=1.5625→1.56 mL.
55/2.2=25 kg. 25×2.5=62.5 mg. 62.5÷40=1.5625→1.56 mL.
Hard
Patient scenario
Pediatric Weight-Based
Child: 44 lb. Lorazepam 0.05 mg/kg IV for acute seizure. Available: 2 mg/mL.
How many mL to administer?
Answer: 0.5 mL
44/2.2=20 kg. 20×0.05=1 mg. 1÷2=0.5 mL.
44/2.2=20 kg. 20×0.05=1 mg. 1÷2=0.5 mL.
Med
Retail
Units/mL Dosing
Dose: 10 units regular U-100 insulin (100 units/mL).
How many mL on the insulin syringe?
Answer: 0.1 mL
10 ÷ 100 = 0.1 mL. This corresponds to the 10-unit mark on a U-100 insulin syringe.
10 ÷ 100 = 0.1 mL. This corresponds to the 10-unit mark on a U-100 insulin syringe.
Med
Hospital
Units/mL Dosing
Heparin flush: 100 units. Available: 100 units/mL.
How many mL?
Answer: 1 mL
100 ÷ 100 = 1 mL.
100 ÷ 100 = 1 mL.
Med
Hospital
Units/mL Dosing
Insulin drip: 50 units in 50 mL NS (= 1 unit/mL). Order: 6 units/hr.
Pump rate in mL/hr?
Answer: 6 mL/hr
1 unit/mL concentration. 6 units/hr ÷ 1 unit/mL = 6 mL/hr.
1 unit/mL concentration. 6 units/hr ÷ 1 unit/mL = 6 mL/hr.
Hard
Hospital
Units/mL Dosing
Penicillin G reconstituted to 100,000 units/mL. Order: 500,000 units IV.
How many mL?
Answer: 5 mL
500,000 ÷ 100,000 = 5 mL.
500,000 ÷ 100,000 = 5 mL.
Hard
Hospital
Units/mL Dosing
Oxytocin: 10 units in 1,000 mL NS. Order: start at 2 milliunits/min.
Pump rate in mL/hr?
Answer: 12 mL/hr
Concentration: 10 units/1000 mL = 0.01 units/mL = 10 milliunits/mL. Rate: 2 milliunits/min × 60 = 120 milliunits/hr. 120 ÷ 10 = 12 mL/hr.
Concentration: 10 units/1000 mL = 0.01 units/mL = 10 milliunits/mL. Rate: 2 milliunits/min × 60 = 120 milliunits/hr. 120 ÷ 10 = 12 mL/hr.
Hard
Hospital
Units/mL Dosing
Vasopressin: 20 units in 100 mL D5W. Order: 0.04 units/min.
Pump rate in mL/hr?
Answer: 12 mL/hr
Concentration: 20/100 = 0.2 units/mL. Rate: 0.04 units/min × 60 = 2.4 units/hr. 2.4 ÷ 0.2 = 12 mL/hr.
Concentration: 20/100 = 0.2 units/mL. Rate: 0.04 units/min × 60 = 2.4 units/hr. 2.4 ÷ 0.2 = 12 mL/hr.
Med
Retail
Sanity Checks
Student calculates 50 mL to give a 500 mg dose from amoxicillin 500 mg/5 mL suspension.
What is the correct answer and what was the error?
Answer: Correct is 5 mL — student multiplied instead of dividing
500 mg × (5 mL/500 mg) = 5 mL (not 50 mL). Student likely multiplied 500 × (5/50) or did not cancel units correctly.
500 mg × (5 mL/500 mg) = 5 mL (not 50 mL). Student likely multiplied 500 × (5/50) or did not cancel units correctly.
Med
Hospital
Sanity Checks
Student draws 0.1 mL from a morphine 10 mg/mL vial intending to give a 10 mg dose.
Is this correct?
Answer: No — should be 1 mL (10 mg ÷ 10 mg/mL)
0.1 mL × 10 mg/mL = only 1 mg, not 10 mg. Correct volume is 10 mg ÷ 10 mg/mL = 1 mL. This is a 10-fold under-dose.
0.1 mL × 10 mg/mL = only 1 mg, not 10 mg. Correct volume is 10 mg ÷ 10 mg/mL = 1 mL. This is a 10-fold under-dose.
Hard
Hospital
Sanity Checks
Child needs 2 units of U-100 insulin. Correct volume = 0.02 mL. A colleague suggests drawing 2 mL instead.
What is the error in the colleague's suggestion?
Answer: 2 mL = 200 units — a 100-fold overdose
2 units U-100 = 2/100 mL = 0.02 mL. If 2 mL is drawn: 2 mL × 100 units/mL = 200 units — a 100x overdose. Decimal errors in pediatric insulin dosing are a major patient safety event.
2 units U-100 = 2/100 mL = 0.02 mL. If 2 mL is drawn: 2 mL × 100 units/mL = 200 units — a 100x overdose. Decimal errors in pediatric insulin dosing are a major patient safety event.
Hard
Retail
Sanity Checks
A technician calculates a 150-day supply for 30 tablets taken once daily.
What is the error?
Answer: Correct is 30 days — divide qty by daily dose: 30 ÷ 1 = 30 days
30 tablets ÷ 1 tab/day = 30 days. The technician likely multiplied (30 × 5) or made a unit entry error. A 150-day supply for a 30-tablet bottle with 1 QD dosing is impossible.
30 tablets ÷ 1 tab/day = 30 days. The technician likely multiplied (30 × 5) or made a unit entry error. A 150-day supply for a 30-tablet bottle with 1 QD dosing is impossible.
Easy
Retail
Total Quantity
Amoxicillin 500 mg capsules. Sig: 1 cap TID for 10 days.
What quantity should be dispensed?
Answer: 30 capsules
1 × 3 × 10 = 30 capsules.
1 × 3 × 10 = 30 capsules.
Med
Retail
Total Quantity
Prednisone 10 mg tablets. Sig: 20 mg/day × 5d, then 10 mg/day × 5d, then 5 mg/day × 5d. Use 10 mg tablets (round up for 5 mg doses).
Total 10 mg tablets needed?
Answer: 20 tablets
20 mg = 2 tabs × 5d = 10. 10 mg = 1 tab × 5d = 5. 5 mg = 0.5 tab → round up to 1 tab × 5d = 5. Total: 10+5+5 = 20 tablets.
20 mg = 2 tabs × 5d = 10. 10 mg = 1 tab × 5d = 5. 5 mg = 0.5 tab → round up to 1 tab × 5d = 5. Total: 10+5+5 = 20 tablets.
Med
Retail
Total Quantity
Z-pack: 500 mg on Day 1, then 250 mg on Days 2–5. Using 250 mg tablets.
Total tablets needed for the complete Z-pack?
Answer: 6 tablets
Day 1: 500 mg = 2 tabs. Days 2-5: 250 mg × 4 = 4 tabs. Total: 2 + 4 = 6 tablets.
Day 1: 500 mg = 2 tabs. Days 2-5: 250 mg × 4 = 4 tabs. Total: 2 + 4 = 6 tablets.
Hard
Hospital
Total Quantity
Sliding scale QID. Today's glucose readings: 182 mg/dL (4 units), 165 mg/dL (4 units), 267 mg/dL (8 units), 210 mg/dL (6 units).
Total insulin units given today?
Answer: 22 units
4 + 4 + 8 + 6 = 22 units total.
4 + 4 + 8 + 6 = 22 units total.
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Lesson 4 · Dosage Conversions · AlabamaPharmTech.com
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