--- title: "Lesson 4 — Dosage Conversions | AlabamaPharmTech.com" description: "Dosage unit conversions for pharmacy techs: mg to mL, lb to kg, and ordered vs dispensed dose math. Step-by-step practice and quizzes." lang: en json-ld: | { "@context": "https://schema.org", "@type": "LearningResource", "name": "Dosage Conversions", "description": "Dosage unit conversions for pharmacy techs: mg to mL, lb to kg, and ordered vs dispensed dose math. Step-by-step practice and quizzes.", "url": "https://www.alabamapharmtech.com/lessons/lesson4-dosage-conversions.html", "inLanguage": "en", "learningResourceType": "Interactive lesson", "educationalLevel": "Beginner", "isPartOf": { "@type": "Course", "name": "Pharmacy Math Mastery", "url": "https://www.alabamapharmtech.com/pharmacy-math-practice" }, "provider": { "@type": "Organization", "name": "AlabamaPharmTech", "url": "https://www.alabamapharmtech.com" } } --- AlabamaPharmTech .com An Allied Health Academy company Pharmacy Math Mastery ALBOP-Approved · PTCB & NHA Prep Overall progress 57% Lessons 1 Math Foundations 25 min · Complete ✓ 2 Dimensional Analysis 30 min · Complete ✓ 3 Day Supply Calculations 40 min · Complete ✓ 4 Dosage Conversions 35 min · In progress 5 Ratio, Proportion & Percent 35 min 6 Sig Interpretation 30 min Final exam ★ Bringing It All Together Cumulative · All 6 lessons AlabamaPharmTech.com › Pharmacy Math Mastery › Lesson 4 of 6 Core # 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. [Back to Course](/pharmacy-math-practice) [Back to Dashboard](/modules) Overview Examples Practice 42 Quiz 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 ✓ 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. 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. 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 **★ 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. Easy Retail 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 Medium Hospital 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 Medium Patient 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 Medium Hospital 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 Hard Hospital 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 Hard Retail 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 Easy Medium Hard Context Retail Hospital Patient Easy Retail Dose to Volume Rx: amoxicillin 400 mg/dose. Available: 400 mg/5 mL. How many mL per dose? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **Answer: 4 mL** 40 ÷ 10 = **4 mL**. Easy Hospital Dose to Volume Dose: ondansetron 4 mg IV. Available: 2 mg/mL. How many mL? Check Show solution **Answer: 2 mL** 4 ÷ 2 = **2 mL**. Easy Retail Dose to Volume Dose: 10 mg cetirizine. Available: 5 mg/5 mL. How many mL? Check Show solution **Answer: 10 mL** 10 × (5/5) = **10 mL**. Easy Hospital Dose to Volume Order: metoclopramide 10 mg IV. Available: 5 mg/mL. How many mL? Check Show solution **Answer: 2 mL** 10 ÷ 5 = **2 mL**. Easy Retail Dose to Volume Dose: 250 mg phenytoin suspension. Available: 125 mg/5 mL. How many mL? Check Show solution **Answer: 10 mL** 250 × (5/125) = **10 mL**. Med Hospital Dose to Volume Order: morphine 6 mg IV. Available: 4 mg/mL. How many mL? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **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. Med Retail Reconstitution Reconstituted amoxicillin-clavulanate: 400 mg/5 mL. How many mg are in 7.5 mL? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **Answer: 15 mL** 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? Check Show solution **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**. Med Patient scenario Pediatric Weight-Based Child: 22 lb. Ibuprofen 10 mg/kg. Available: 100 mg/5 mL. How many mL per dose? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **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**. 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? Check Show solution **Answer: 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? Check Show solution **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**. Hard Hospital Pediatric Weight-Based Child: 66 lb. Tobramycin 2.5 mg/kg q8h IV. Available: 10 mg/mL. How many mL per dose? Check Show solution **Answer: 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? Check Show solution **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**. Hard Hospital Pediatric Weight-Based Child: 55 lb. Gentamicin 2.5 mg/kg q8h IV. Available: 40 mg/mL. How many mL per dose? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **Answer: 0.1 mL** 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? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **Answer: 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? Check Show solution **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**. Hard Hospital Units/mL Dosing Vasopressin: 20 units in 100 mL D5W. Order: 0.04 units/min. Pump rate in mL/hr? Check Show solution **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**. 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? A. 50 mL is correctB. Correct is 5 mL — student multiplied instead of dividingC. Correct is 5 mL — student divided instead of multiplyingD. Correct is 50 mL but concentration is too high for oral use Show solution **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. 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? A. Yes — 0.1 mL of 10 mg/mL is correctB. No — should be 10 mLC. No — morphine cannot be given IVD. No — should be 1 mL (10 mg ÷ 10 mg/mL) Show solution **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. 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? Check Show solution **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. Hard Retail Sanity Checks A technician calculates a 150-day supply for 30 tablets taken once daily. What is the error? A. No error — 150 days is correctB. Correct is 30 days — divide qty by daily dose: 30 ÷ 1 = 30 daysC. The bottle must contain 150 tablets for this to happenD. Patient must take 5 per day — the Rx was read incorrectly Show solution **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. Easy Retail Total Quantity Amoxicillin 500 mg capsules. Sig: 1 cap TID for 10 days. What quantity should be dispensed? Check Show solution **Answer: 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? A. 15 tabletsB. 20 tabletsC. 25 tabletsD. 30 tablets Show solution **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**. 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? A. 4 tabletsB. 5 tabletsC. 6 tabletsD. 8 tablets Show solution **Answer: 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? Check Show solution **Answer: 22 units** 4 + 4 + 8 + 6 = **22 units** total. **How this quiz works:** Pick your question count and difficulty, then start. After answering, click _Reveal explanation_ to see the full step-by-step solution — answers never show automatically. Score and type breakdown appear at the end. ⓘ This lesson has 42 questions in the bank. A 10-question quiz draws a random subset. Run multiple sets to cycle through all questions. How many questions? 102030 Question difficulty Random mixEasy onlyMedium onlyHard only Options Randomize question order Shuffle every time you start Timed mode 45-second countdown per question Number of sets Each set draws a fresh pool − 1 + 10 Questions 1 Sets 10 Total ~10 min Est. time ▶  Start quiz — Dosage Conversions Question 1 of 10 0 correct · 0 skipped \--% Score Quiz complete! Lesson 4 · Dosage Conversions · AlabamaPharmTech.com \-- Correct \-- Wrong \-- Skipped \-- Time Performance by question type \--% Overall accuracy \-- Weakest area Miss rate by question type Retry missed + skipped Next set New quiz Answer review — every question this session [Home](/) [Back to Dashboard](/modules) Previous [Back to Course](/pharmacy-math-practice) Lesson 4 of 6 Next lesson