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RMIT University Library - Learning Lab

Drop factor: converting between millilitres and drops


Drops are just another unit for measuring the amount of fluid flowing into a patient’s system. Discover what ‘drop factor’ means, and how to convert between millilitres and drops. 

In getting medication, we are constantly talking about millilitres. It helps to understand what a ml actually is, a ml measures a volume of liquid. If you can imagine a cube of water that measures one centimetre… by one centimetre... by one centimetre... then this is 1 ml. One cubic centimetre of liquid is one millilitre.

Now, imagine that same cubic centimetre divided into 20 separate drops, drops that are large enough to pack tightly into that same cubic centimetre of space. Here we have 20 drops in that space, we have 20 drops for that 1 ml of liquid. In other words, we have 20 drop per ml. This is your drop factor for macro drops, take note of the units here because the drop factor has units, the units of drop factor are drops per ml.

The unit of drops on its own is another way to express a volume of liquid, just as you can express a price in dollars or you could express it in Japanese yen or euros, both describe the same value but they are in different units, mls or drops. You can convert one to the other if you have the conversion rate - which is a little bit like the drop factor here. But you need to clearly state what you have - weather it’s drops or mls or euros or dollars.

So, if you have 1 ml of liquid you the equivalent of 20 drops. If you have 2 ml of liquid you have 2 times 20, that is 40 drops. If you have 5 mls of liquid, you have 5 lots of 20, that is 100 drops.

See if you can make these conversions: 7 ml is equivalent to 7 times 20, which is 140 drops. 10 ml will give you 10 times 20, which is 200 drops. 50 ml, each containing 20 drops gives you 5 times 2 is 10, and 2 extra zeros, that’s a 1000 drops. Same thing with the 80 ml, 8 times 2 is 16 and then 2 extra zeros, becomes 1600 drops. Finally, 120 mls multiplied by 20, gives us 2400 drops.[Stop]

In short, if you want to convert from mls to drops and this is when dealing with an adult patient and you are using macro drops, you multiply the number of mls by 20. If on the other you have drops and you want to convert back to mls. You do the opposite to multiply, which is to divide by 20.

For example, 600 drops converted back to mls, requires you to divide 600 by 20, after cancelling zeros we get an answer of 30. So 600 drops is equivalent to 30 mls. If you have 430 drops and you want to convert them to mls, you again divide the 430 by 20, it doesn’t go evenly this time, it is equal to 21 and a half, which should be expressed as 21.5. 430 drops equals 21.5 mls.

Finally, please remember that we have been dealing with macro drops, which are the larger size drops for adult infusions. If you are dealing with a paediatric patient, the infusion may be in micro drops, these drops are smaller and they are so small that 60 of them fit into the 1 ml of liquid (where before we had 20). So that means we have 60 drops per ml when dealing with micro drops, and 60 drops per ml is your drop factor for paediatric infusions. This means that converting between mls and drops for micro drops would require multiplying and dividing, not by 20 but rather by 60.