weight  length  volume  
grams g  meters m  liters L  

centimeters cm 

In the above table we can see the basic units of measurement for each category, along with the abbreviations for each unit.
In this table we have included some older abbreviations for micrograms and milliliters (or cubic centimeters) that used to be common, but that should no longer be used. These outdated abbreviations are included because you may still encounter them on drug labels or syringes, and because it is important to be aware when reading drug orders that some doctors may use outdated abbreviationsif you see any of these abbreviations when reading a drug order, it is a good idea to question the drug order, just to be sure!
Most drug dosages are measured by weight in grams, milligrams or micrograms; however certain special drugs have other metric units that measure properties other than weight:
Drugs such as insulin, penicillin, and heparin, all of which we will encounter in this course, are all measured in Units; vitamins, hormones, and blood products in general are also often measured in units. However, it is very important to note that the volume or weight of a Unit of insulin is usually very different from a Unit of penicillin or a Unit of heparin. So while 1 mL of insulin may have 100 Units of insulin in it, 1 mL of heparin may have 10,000 Units of heparin in it!
When we write dosage orders for drugs in the metric system, we have to follow a certain set of rules that have been devised to minimize the most common mistakes that people often make when reading drug orders. These rules are:
The unit should come after the number.
For example, to write five grams, we would write 5 g, not g 5.
All numbers should be written using the HinduArabic decimal system. This is just a fancy way of saying that we should use regular numbers (as opposed to Roman numerals).
For example, to write three micrograms we should write 3 mcg, not iii mcg.
We should always use decimals instead of fractions.
For example, to write one and one half milliliters, we should write 1.5 mL, not 1 1/2 mL
When writing decimals that are smaller than 1, we should always put a leading zero before the decimal point.
(This is important because it removes the possibility that someone will misread .3 as 3 or .42 as 42.)
For example, to write three tenths centimeters, we should write 0.3 cm, not .3 cm.
When writing decimals, we should leave off any unnecesary zeros after the decimal point.
(This reduces the chances that someone could misread 1.50 as 150 or 20.0 as 200.)
For example, when writing ten and one half kilograms, we should write 10.5 kg, not 10.50 kg.
The L for liters or in mL should always be capitalized: L or mL (not l or ml)
Be careful not to confuse the abbreviations mg for milligrams with the abbreviation mcg for micrograms.
When writing the abbreviation for micrograms, always write mcg instead of μg. You should be able to recognise that the abbreviation μg stands for micrograms because it is often used on printed labels, but when handwritten, μg can too easily be mistaken for mg and should never be used!
When writing the abbreviation for milliliters, always write mL instead of cc for cubic centimeter, even though milliliters and cubic centimeters are the same. You should be able to recognise that the abbreviation cc stands for cubic centimeter which is the same as a milliliter because you may still see it on printed labels or hear the term used in hospitals, but when handwritten, cc can too easily be mistaken for zeros and should never be used!
When writing the abbreviation for Units, always write units instead of U. You should be able to recognise that the abbreviation U stands for units because you may still see it on printed labels, but when handwritten, cc can too easily be mistaken for zeros and should never be used!
The traditional system of measurement used in the medical profession in the West has been the apothecary system, which has its roots in Latin. It is no longer commonly used because it contains script symbols that can be hard to read and because its units are much older and therefore more irregular, requiring more work to convert from one unit to another.
In the US this system has been largely discontinued; however, some drugs labels still have some of these units printed on their labels, and occasionally you make work with a doctor who still writes some drug orders in this system, especially for particular drugs that have been traditionally been measured using this system (for example, certain dessicated thyroid medications and asperin have both traditionally been measured in grains). Because of this, it is important that you are aware of this system so that you can identify if a drug order has the potential to be misinterpreted. If you see a drug order written using the apothecary system, it would be reasonable to question it to be sure that you are interpreting it correctly.
weight  length  volume 
grains gr  feet ft  ounces 
pounds lb  inches in  drams dr or 
minims min 
In the above table we can see the basic units of measurement for each category, along with the abbreviations for each unit.
When we write dosage orders for drugs in the apothecary system, we have to follow a certain set of rules that were traditionally used with apothecary abbreviations so that we minimize the chances that they could be confused with metric measurements. These rules are:
The unit should come before the number.
For example, to write five grains, we would write gr v, not v gr.
All numbers should be written using Roman numerals. Notice that when Roman numerals are used in drug orders, they are usually lowercase and when the letter i is written, it is written with a line over the i instead of a dot, like this: ι. (This is because it is too easy to mistake ii for 11 if the i's are written in uppercase without dots or with regular dots that can be easy to miss.)
(The use of Roman numerals makes sense because the apothecary system was based in Latin, where Roman numerals were the standard way of writing numbers.)
For example, to write three minims we should write min ιιι, not min 3 .
We should always use fractions instead of decimals.
One special abbreviation for
1 
2 
1 
2 
1 
2 
Be careful not to confuse the abbreviations for ounces with the abbreviation for drams.
Be careful not to confuse the abbreviations g for grams (or mg for milligrams) with the abbreviation gr for grains.
weight  length  volume 
pounds lb  meters m  ounces oz 
inches in  tablespoon T or tbl  
teaspoon t or tsp  
drops gtt 
In the above table we can see the basic units of measurement for each category, along with the abbreviations for each unit.
When we write dosage orders for drugs in the household system, we have to follow a certain set of rules that have been devised to make them easy for the average patient to read and understand. These rules are:
The unit should come after the number.
For example, to write five drops, we would write 5 gtt, not gtt 5.
All numbers should be written using the HinduArabic decimal system. This is just a fancy way of saying that we should use regular numbers (as opposed to Roman numerals).
For example, to write three tablespoons we should write 3 T, not iii T .
We should always use fractions instead of decimals.
For example, to write one and one half ounces, we should write 1
1 
2 
The abbreviation gtt for drops seems strange because it does not share any of the same letters (this is because it comes from Latin), so be careful, and don't confuse the abbreviation gtt for drops with dr for drams!
Be careful not to confuse the apothecary abbreviation of for ounces with the household abbreviation oz for ounces  when writing instructions for a patient, one should use oz, not , because it is not in common usage among the general public.
For an overview of discontinued abbreviations, take a look at the Institute for Safe Medication Practice's List of ErrorProne Abbreviations. This list explains which older abbreviations and symbols are no longer permitted, and explains why each of these practices was discontinued.
Write one and one half grams using the correct rules.
Because the units used here are grams, we know that we must use the rules for the metric system.
So we must put the number before the units.
The metric system also requires that we use arabic numbers.
The metric system also requires that we use decimals instead of fractions.
We must be careful not to use any unnecessary zeros after the decimal point.
So, the correct way to write this is:
1.5 g
Write one and one half grains using the correct rules.
Because the units used here are grains, we know that we must use the rules for the apothecary system.
So we must put the number after the units.
The apothecary system also requires that we use roman numerals.
The apothecary system also requires that we use fractions instead of decimals.
So, the correct way to write this is:
gr ι
1 
2 
(because ss is one way to abbreviate onehalf in the apothecary system)
Write one half milligram using the correct rules.
Because the units used here are milligrams, we know that we must use the rules for the metric system.
So we must put the number before the units.
The metric system also requires that we use arabic numbers.
The metric system also requires that we use decimals instead of fractions.
We must be careful not to use any unnecessary zeros after the decimal point.We must also sure that we put a leading zero before the decimal point, since this decimal is smaller than one, because we don't want anyone who reads it to mistake it for a whole number.
So, the correct way to write this is:
0.5 mg
Write one half microgram using the correct rules.
Because the units used here are micrograms, we know that we must use the rules for the metric system.
So we must put the number before the units.
The metric system also requires that we use arabic numbers.
The metric system also requires that we use decimals instead of fractions.
We must be careful not to use any unnecessary zeros after the decimal point.We must also sure that we put a leading zero before the decimal point, since this decimal is smaller than one, because we don't want anyone who reads it to mistake it for a whole number.
So, the correct way to write this is:
0.5 mcg
Write two and two fifths ounces using the correct rules.
Because the units used here are ounces, we know that we must use the rules for the apothecary system.
So we must put the number after the units.
The apothecary system also requires that we use roman numerals.
The apothecary system also requires that we use fractions instead of decimals.
So, the correct way to write this is:
ιι
2 
5 
Write two and two fifths drams using the correct rules.
Because the units used here are drams, we know that we must use the rules for the apothecary system.
So we must put the number after the units.
The apothecary system also requires that we use roman numerals.
The apothecary system also requires that we use fractions instead of decimals.
So, the correct way to write this is:
ιι
2 
5 