Drawing up

Desired/on hand.

Your ordered dose is 0.5 mg. On hand, you have 5 mg in 1 mL. 0.5mg/5 mg/mL = X mL. The mg cancel each other out and you solve for X mL.

FYI - make sure you always add the 0 in front of any decimal place!

Sometimes it's easier to look at it without formulas. I say how many ml are in each mg. So 0.2ml in 1mg and you need 0.5mg so how many ml is that?

0.5mg x 1mL/5mg = mL

I'm definitely a fan of equations in these examples. We do "desired"/"have" x quantity. We shorten it to D/H x Q.

For your example it would be 0.5 mg/5 mg x 1 ml. This then multiples out to 0.5 ml.

You always have to be careful that you have the same measurements (i.e. mg, mcg, etc) but otherwise this is a quick, easy way to work out this type of problem.

The formula is (O / OH)V = x where O is ordered amount (mass), OH is on hand amount (mass) and V is on hand volume.

Order is for 0.5mg

On hand qty is 5mg/mL in 1 mL

So: (0.5mg/5mg)1mL = 0.1mL (the masses cancel, besides you know you're looking for volume)

Draw it up in a little tiny syringe...

Also, you know that the desired amount is exactly one tenth the available full amount in 1mL...so, one tenth of a mL...

Honestly, I rarely do the math. Once you're used to it, most of the time you just "see it".