Appendix 3 Systems of weights and measures
In 1960 the Système International d’Unités (SI system), based on the metric system, was adopted as the standard. Since 1969 all prescriptions in the UK have been dispensed in this system. The older Imperial and Apothecary systems are still found in older books and formularies. This appendix outlines the three systems for weight and volume.
When expressing quantity, it is important to avoid the risk of error or misinterpretation. To reduce this it is best to avoid decimal fractions where possible. Thus, it is better to use 50 mg rather than 0.05 g. Where a decimal point is used, it should be preceded by a 0 (zero); thus it should be 0.1 g rather than .1 g.
The basic unit is the kilogram (kg), which is the mass of the International Prototype Kilogram.
| Name of unit | Abbreviation | Relationship |
| Kilogram | kg | |
| Gram | g | 1/1000 (0.001) kg |
| Milligram | mg | 1/1000 (0.001) g |
| Microgram | μg (or mcg) | 1/1000 (0.001) mg |
| Nanogram | ng | 1/1000 (0.001) μg |
| Picogram | pg | 1/1000 (0.001) ng |
To avoid confusion between mg, mcg and ng it is advisable not to use these abbreviations in dispensing.
The basic unit is the litre (L) which is defined as 1 cubic decimetre.
| Name of unit | Abbreviation | Relationship |
| Litre | L | |
| Millilitre | mL | 1/1000 (0.001) L |
| Microlitre | μL | 1/1000 (0.001) mL |
The basic unit is the mole which is the amount of substance containing as many formula units as there are in 12 g of carbon-12. The formula units may be atoms, molecules, ions, etc.
| Name of unit | Abbreviation | Relationship |
| Mole | mol | |
| Millimole | mmol | 1/1000 (0.001) mol |
| Micromole | μmol | 1/1000 (0.001) mmol |