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nurse administrating medicine into patient

Metric (SI) units

Care when administering medicines is crucial when it comes to avoiding serious errors. Learn more about how to convert metrics accurately and ensure the safety of your patients.

Metric SI unit

Base unit

Type of measurement Unit    Symbol 
Volume litre  l or L
Length metre   m
Weight gram  g

Kilo = one thousand times greater than the base unit (x1,000)

Type of measurement  Unit  Symbol  
Length  Kilometre (1,000 metres)  km            
text  Kilogram (1,000 grams)  kg

milli = one thousandth of a base unit (1/1,000)

Type of measurement Unit   Symbol 
Volume millilitre (1/1,000 of a litre)  ml
length millimetre (1/1,000 of a metre)  mm
Weight milligram (1/1,000 of a gram)  mg

micro = one millionth of base unit (1/1,000,000) 

Type of measurement Unit  Symbol
Weight microgram (1/1,000,000 of a gram)  mcg (it is recommended that you write microgram in full)

nano = one billionth of base unit (1/1,000,000)

Type of measurement Unit  Symbol
Weight nanogram (1/1,000,000,000 of a gram)  ng (it is recommended that you write nanogram in full)


Medication administered to patients is often in small doses. We commonly use prescriptions written in grams and milligrams. For infants, prescriptions in micrograms are common, and sometimes nanograms are used. When administering medicines, serious errors can result when units get mixed up. Consider for example the consequences for a baby if milligrams were confused with micrograms. It would result in an error of one thousand times magnitude, which could have catastrophic consequences – so care is very important.

A mistake in a drug calculation is never an acceptable occurrence, however a medication error made for a child may have significantly greater affect than the same one made for an adult. This situation is compounded by the fact that many drug dose calculations will be determined on a child’s weight, often leading to the necessity for additional calculations to be performed and therefore increased stages at which an error can be made.

For any type of measurement, you should use 'the unit that requires the least decimal places', as this minimises the possibility of simple errors (e.g. accidentally putting a decimal place or zero in the wrong position).  For example, instead of writing 0.75 litres you would put 750 ml.  Similarly you would choose to write 7 litres instead of 7000 ml, as 7000 ml has more zeros to write and one zero could be missed out by accident.

Converting between metric units

One of the more common calculations performed in nursing is converting between units of the same type, e.g., converting from litres to millilitres or from grams to milligrams. 

Earlier we saw that moving up to the next biggest unit increases the value by a factor of 1000. Moving down to the next smallest unit decreases the value by a factor of 1000. 

So, to convert between units you just need to multiple or divide by 1000. The slide-show below shows how this is done. To begin, select either the ‘larger to smaller’ button or the ‘smaller to larger’ button. The examples below are based on weight, but the principle also applies to volume and length.

Converting between very large and very small numbers

To convert to a unit that is two 'levels' higher, you must divide by one million (1,000,000).
Example - change 1420 nanograms to milligrams
1420 ÷ 1,000,000 = 0.001420mg

To convert to a unit that is two 'levels' lower, you must multiply by one million (1,000,000).

Example - change 0.067g to micrograms

0.067 x 1,000,000 = 67,000micrograms

Now try the practice examples below which require you to convert between the metric units shown.

Converting from imperial to metric units

The most common measurements used in health care are weight and volume. It is not uncommon for patients (particularly older ones) to feel more comfortable using the imperial system of measurement when talking about these (stones and pounds for weight and feet and inches for height). 

You may need to ‘translate’ imperial measurements given to you by patients to metric units, or convert metric units to imperial measurements when discussing height and weight with patients.

There is a link to a useful tool that provides conversion tables and calculators for a range of imperial to metric units in the 'Useful resources' section. If you don't have access to these tools and charts, it is handy to know these conversion factors:

  • 1 ounce (oz) = 28.3g
  • 2.2. pounds (lbs) = 1 kg
  • As there are 14 pounds in a stone, one stone = 14 divided by 2.2 pounds = 6.35 kg per stone

Here’s an example:

A child weighs 1 stone 8 pounds, the parents would like to know what this is as a metric weight.

Step 1:

You need to change the weight in pounds by multiplying the stones by 14.
1 stone  8 pounds = (1 x 14) + 8 pounds = 22 pounds.

Step 2:

You then need to convert the pounds into kilograms by dividing by 2.2.
22/2.2 = 10 kilograms

So, the child weighs 10 kgs.

Page last updated - 26/02/2024