Pharmacy Measurement Conversions Chart
Updated
Almost every calculation on the PTCE begins with a conversion. Before you can divide a bottle into doses or work out a days supply, the container and the directions have to be in the same unit. This chart collects the conversions worth memorising cold, grouped so you can learn them in sets: metric, household to metric, apothecary, and temperature.
These values sit inside the exam's Order Entry & Processing domain, the home of pharmacy calculations. Once the numbers are second nature, put them to work on the PTCB math practice test and the guide on calculating days supply, where conversions decide the answer. This chart is one stop on our PTCB study guide.
How to use this chart on the exam
The metric ladder is the backbone: kilograms, grams, milligrams, and micrograms each step down by a factor of 1,000, and litres to millilitres do the same. Get the direction right by asking whether the answer should be a bigger or smaller number, then multiply or divide by 1,000. Most exam slips are decimal-shift errors, moving the point one or two places too far, so a quick sanity check of the size of the answer catches them. For household measures, remember the chain that a tablespoonful is three teaspoonfuls and a fluid ounce is two tablespoonfuls, which lets you rebuild the volume table from a single anchor. Keep weight and volume ounces apart: a weight ounce is about 28.4 g, but a fluid ounce is 30 mL, and mixing them is a favourite distractor. The apothecary grain is the one legacy unit that still appears, most often with aspirin and nitroglycerin, so know that a grain is about 65 mg. When a question gives its own conversion factor, use that number rather than the one you memorised, because the exam sometimes rounds a grain to 60 mg on purpose.
Metric system
| Unit | Equals / note |
|---|---|
| 1 kilogram (kg) | 1,000 grams (g) |
| 1 gram (g) | 1,000 milligrams (mg) |
| 1 milligram (mg) | 1,000 micrograms (mcg) |
| 1 litre (L) | 1,000 millilitres (mL) |
| 1 microgram (mcg) | 0.001 mgWrite micrograms out in full; the abbreviation ug is error-prone. |
Volume (household to metric)
| Unit | Equals / note |
|---|---|
| 1 teaspoonful (tsp) | 5 mL |
| 1 tablespoonful (tbsp) | 15 mLEquals 3 teaspoonfuls. |
| 1 fluid ounce (fl oz) | 30 mLEquals 2 tablespoonfuls. |
| 1 cup | 240 mLEquals 8 fluid ounces. |
| 1 pint (pt) | 480 mLEquals 16 fluid ounces. |
| 1 quart (qt) | 960 mLEquals 2 pints. |
| 1 gallon (gal) | 3,840 mLPharmacy convention using 480 mL per pint; the exact US gallon is 3,785 mL. |
Weight (household to metric)
| Unit | Equals / note |
|---|---|
| 1 kilogram (kg) | 2.2 pounds (lb)Convert lb to kg by dividing by 2.2. |
| 1 pound (lb) | 454 gMore precisely 453.6 g. |
| 1 ounce (oz), weight | 28.4 gThis is weight. A fluid ounce (volume) is 30 mL. |
Length
| Unit | Equals / note |
|---|---|
| 1 inch (in) | 2.54 cm |
| 1 centimetre (cm) | 10 mm |
| 1 metre (m) | 100 cm |
Apothecary and grains
| Unit | Equals / note |
|---|---|
| 1 grain (gr) | 65 mgCommon exam value. The exact figure is 64.8 mg, and older texts round to 60 mg, so read each question for the value it uses. |
| gr 1/150 | about 0.4 mgClassic nitroglycerin example using 65 mg per grain. |
| 1 fluid dram (fl dr) | 5 mLEquated to a teaspoonful in pharmacy practice. |
Temperature
Convert between the scales with two formulas. To find Celsius from Fahrenheit, use C = (F - 32) × 5/9. To find Fahrenheit from Celsius, use F = (C × 9/5) + 32. The reference points below are worth knowing outright, especially the 2 C to 8 C refrigerator range used for vaccines and insulin.
| Celsius | Fahrenheit | Reference point |
|---|---|---|
| 0 C | 32 F | Water freezes |
| 2 C to 8 C | 36 F to 46 F | Refrigerator (vaccine and insulin storage) |
| 25 C | 77 F | Controlled room temperature |
| 37 C | 98.6 F | Normal body temperature |
| 100 C | 212 F | Water boils |
Roman numerals
Apothecary prescriptions still use lowercase Roman numerals for quantities, so a dose written gr v means 5 grains. These are the symbols you will see.
| Symbol | Value |
|---|---|
| ss | 1/2 |
| i | 1 |
| v | 5 |
| x | 10 |
| l | 50 |
| c | 100 |
Frequently asked questions
How many millilitres are in a teaspoon and a tablespoon?
A teaspoonful is 5 mL and a tablespoonful is 15 mL, which is three teaspoonfuls. These are the household measures the PTCE uses, so a 10 mL dose equals two teaspoonfuls.
How do you convert pounds to kilograms?
Divide the weight in pounds by 2.2, because 1 kg = 2.2 lb. A 176 lb patient weighs 80 kg. Weight-based doses in mg/kg must use kilograms, so this conversion comes first.
How many milligrams are in a grain?
About 65 mg is the value most exam questions use, though the exact apothecary grain is 64.8 mg and some older references round to 60 mg. Always use the figure the question provides.
How do you convert Fahrenheit to Celsius?
Use C = (F - 32) × 5/9. For example, 98.6 F becomes (98.6 - 32) × 5/9 = 37 C. To go the other way, use F = (C × 9/5) + 32.
How many millilitres are in a fluid ounce?
One fluid ounce is 30 mL. A pint is 480 mL (16 fluid ounces) and a cup is 240 mL (8 fluid ounces).
Keep studying
Conversions are only useful once you apply them. Pair this chart with the pharmacy math formulas cheat sheet, then drill both on the pharmacy math practice test and inside the workflow context of the order entry quiz. For the step-by-step method, read how to calculate days supply and follow the full 2026 PTCB study guide.
Official sources checked
PTCB Quiz Prep is an independent study resource, not affiliated with the Pharmacy Technician Certification Board. Household and apothecary equivalents are the standard conventions used for exam calculations; exact laboratory values can differ slightly. See our editorial standards.