Educational exam prep, not medical advice. This page teaches what certification exams test. Follow your facility’s policies, your supervising clinician and the manufacturer’s instructions in practice. If you are a patient with a health question, talk to your clinician or see MedlinePlus.
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Conversions to memorize
| From | To |
|---|---|
| 1 kg | 2.2 lb |
| 1 tsp | 5 mL |
| 1 tbsp | 15 mL |
| 1 fl oz | 30 mL (29.57 exact) |
| 1 g | 1,000 mg |
| 1 mg | 1,000 mcg |
| 1 L | 1,000 mL |
| 1 grain | 65 mg |
Days supply
Days supply = quantity dispensed / quantity used per day. Round down.
Example: #90 tablets, sig 1 tab tid. 90 / 3 = 30 days.
Liquid: 150 mL, 5 mL bid. 150 / 10 = 15 days.
Dilution: C1V1 = C2V2
How much 20% stock makes 500 mL of 5%? V1 = (5 x 500) / 20 = 125 mL of stock, plus 375 mL diluent.
Alligation
Mix 10% and 2% to get 5%. Parts of 10% = 5 - 2 = 3; parts of 2% = 10 - 5 = 5; total 8 parts. For 400 g: 10% = 3/8 x 400 = 150 g; 2% = 250 g.
IV flow rate
mL/hr = volume / hours. gtt/min = (volume x drop factor) / minutes.
1,000 mL over 8 hr = 125 mL/hr. With a 15 gtt/mL set: (1,000 x 15) / 480 = 31.25 -> 31 gtt/min.
Percentage and ratio strength
w/v % = grams per 100 mL: 0.9% NaCl = 0.9 g/100 mL = 9 g/L. Ratio strength 1:1,000 = 1 g in 1,000 mL = 0.1%.
Practice
CNA, PTCB and CCMA prep course - $19
1,297 original practice questions for CNA, PTCB and CCMA, with explanations, timed exams and domain score reports. One-time $19 for 90 days, emailed to you after purchase - no subscription.
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Frequently asked questions
What math is on the PTCB exam?
Formulas, ratios, proportions, conversions and sig codes for days supply, quantity, dose, concentration and dilutions (PTCE outline 4.1).
Can you use a calculator on the PTCB?
An on-screen calculator is provided at the Pearson VUE test center; check PTCB’s candidate guide for current rules.
How do you calculate days supply?
Divide the quantity dispensed by the amount used per day, and round down.
Sources
- PTCB - PTCE Content Outline effective January 6, 2026 (accessed 2026-09-23)