Systems of Measurement
11 minute read
What you will learn
- Use metric units and prefixes for mass, volume and length
- Convert between metric units
- State the apothecaries' and avoirdupois weights and measures
- Convert between metric and older systems using equivalents
- Use household measures and convert between Celsius and Fahrenheit
- Write units safely on prescriptions and labels
Key points
- The metric system is the standard in pharmacy. Its base units are the gram for mass and the litre for volume.
- Prefixes show size: kilo 1 000, milli 0.001, micro 0.000001. Each step between mass units is a factor of 1 000.
- To go from a larger unit to a smaller one, multiply. To go from a smaller unit to a larger one, divide.
- The apothecaries' and avoirdupois systems are old systems still found in some books and on some labels. The two ounces are different.
- Useful equivalents: 1 grain = 64.8 mg, 1 kg = 2.2 lb, 1 fluid ounce (imperial) = 28.4 mL.
- A household teaspoon is taken as 5 mL and a tablespoon as 15 mL. Use measuring spoons because kitchen spoons vary.
- Write units in full where there is a risk of confusion, for example 'micrograms' and 'units'.
Why systems of measurement matter
Medicines are measured by mass, volume and length. Most of pharmacy now uses the metric system, but older systems still appear in some books, old prescriptions and imported labels. You need to read all of them and convert safely.
The metric system
The metric system is based on powers of ten, which makes conversion simple.
| Quantity | Base unit | Symbol |
|---|---|---|
| Mass | gram | g |
| Volume | litre | L |
| Length | metre | m |
Prefixes
| Prefix | Symbol | Meaning |
|---|---|---|
| kilo | k | 1 000 |
| deci | d | 0.1 |
| centi | c | 0.01 |
| milli | m | 0.001 |
| micro | µ or mc | 0.000 001 |
| nano | n | 0.000 000 001 |
Mass and volume tables
| Mass | |
|---|---|
| 1 kilogram (kg) | 1 000 grams |
| 1 gram (g) | 1 000 milligrams (mg) |
| 1 milligram (mg) | 1 000 micrograms |
| 1 microgram | 1 000 nanograms |
| Volume | |
|---|---|
| 1 litre (L) | 1 000 millilitres (mL) |
| 1 decilitre (dL) | 100 mL |
| 1 millilitre (mL) | 1 cubic centimetre (cm³) |
Converting within the metric system
- Larger unit to smaller unit: multiply (the number gets bigger).
- Smaller unit to larger unit: divide (the number gets smaller).
| Conversion | Working | Answer |
|---|---|---|
| 0.25 g to mg | 0.25 × 1 000 | 250 mg |
| 750 mg to g | 750 ÷ 1 000 | 0.75 g |
| 500 micrograms to mg | 500 ÷ 1 000 | 0.5 mg |
| 1.5 L to mL | 1.5 × 1 000 | 1 500 mL |
| 2.5 kg to g | 2.5 × 1 000 | 2 500 g |
| 0.06 g to mg | 0.06 × 1 000 | 60 mg |
The apothecaries' system
The apothecaries' system was once the standard for dispensing. It is still found in older reference books.
Weight
| Unit | Equals |
|---|---|
| 20 grains (gr) | 1 scruple |
| 3 scruples | 1 drachm (60 grains) |
| 8 drachms | 1 ounce (480 grains) |
| 12 ounces | 1 pound (5 760 grains) |
Fluid measure (imperial)
| Unit | Equals |
|---|---|
| 60 minims (min) | 1 fluid drachm |
| 8 fluid drachms | 1 fluid ounce |
| 20 fluid ounces | 1 pint |
| 8 pints | 1 gallon |
The American pint and fluid ounce are different sizes from the imperial ones. Check which system a book or label uses.
The avoirdupois system
The avoirdupois system is used for everyday goods and is the system found in the shop.
| Unit | Equals |
|---|---|
| 437.5 grains | 1 ounce (oz) |
| 16 ounces | 1 pound (lb), 7 000 grains |
The grain is the same in both systems, but the ounce is not: the apothecaries' ounce is 480 grains and the avoirdupois ounce is 437.5 grains.
Equivalents
| Item | Exact | Approximate |
|---|---|---|
| 1 grain | 64.8 mg | 65 mg |
| 1 minim | 0.0592 mL | 0.06 mL |
| 1 ounce (avoirdupois) | 28.35 g | 28 g |
| 1 pound | 453.6 g | 454 g |
| 1 kilogram | 2.205 lb | 2.2 lb |
| 1 fluid ounce (imperial) | 28.4 mL | |
| 1 pint (imperial) | 568 mL |
Worked conversions
- 5 grains to mg: 5 × 64.8 = 324 mg exact. With the approximate value of 65 mg, 5 × 65 = 325 mg.
- 2 fluid ounces to mL: 2 × 28.4 = 56.8 mL.
- 8 ounces (avoirdupois) to g: 8 × 28.35 = 226.8 g.
- A patient weighs 154 lb. Weight in kg? 154 ÷ 2.2 = 70 kg.
- Half an imperial pint in mL: 568 ÷ 2 = 284 mL.
Household measures
| Measure | Volume |
|---|---|
| 1 teaspoonful | 5 mL |
| 1 dessertspoonful | 10 mL |
| 1 tablespoonful | 15 mL |
Ordinary kitchen spoons vary widely in size. For accuracy, give the patient a 5 mL measuring spoon, an oral syringe or a measuring cup. A bottle of 100 mL at 5 mL per dose holds 100 ÷ 5 = 20 doses.
Temperature
°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
| Example | Working | Answer |
|---|---|---|
| 37°C to °F | 37 × 1.8 = 66.6, + 32 | 98.6°F |
| 25°C to °F | 25 × 1.8 = 45, + 32 | 77°F |
| 104°F to °C | (104 − 32) = 72, × 5/9 | 40°C |
| 2°C to °F | 2 × 1.8 = 3.6, + 32 | 35.6°F |
| 8°C to °F | 8 × 1.8 = 14.4, + 32 | 46.4°F |
A refrigerator for medicines is normally kept between 2°C and 8°C, which is 35.6°F to 46.4°F.
Writing units safely
Unclear units cause serious errors. Follow these habits:
- Write "microgram" in full, or "mcg", rather than "µg". The "µ" can be misread as "mg", which is a thousand times larger.
- Write "units" in full, never "U", which can be read as a zero.
- Write "mL" and not "cc".
- Use a zero before a decimal point (0.5 mg) and no zero after a whole number (5 mg).
- Leave a space between the number and the unit.
Exam focus
- Metric conversions are tested in nearly every paper: mg to g, mL to L, micrograms to mg.
- Know the apothecaries' tables, especially 20 grains = 1 scruple, 3 scruples = 1 drachm and 8 drachms = 1 ounce.
- Know that 1 apothecaries' ounce is 480 grains and 1 avoirdupois ounce is 437.5 grains.
- Temperature conversion between Celsius and Fahrenheit, often for storage conditions.
- Common mistake: multiplying when you should divide. Check that smaller units give bigger numbers.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.0.25 g expressed in milligrams is:
2.500 micrograms is equal to:
3.In the apothecaries' system, one ounce contains:
4.In metric dispensing, one teaspoonful is taken as:
5.37°C expressed in degrees Fahrenheit is: