As a general rule, I recommend calculating IV rates in milliliters per hour (mL/hr) as in most instances, that is the information you will need. If you need drops per minute (gtt/min), once you get mL/hr you can use the "magic number" and convert to gtt/min.
For these problems, the left side of the ratio will the the physician's order. As usual, add in units of measurement on right side to match left side, "plug in" numbers into appropriate location on right, cross multiply, and solve for X.
Example problem #1: Ordered is 1000 mL D5W q 8 hr. What is the mL/hr flow rate?
1000 mL = X mL
8 hr 1 hr
8 X = 1000
8 X = 1000
8 8
X = 125 mL/hr
Example problem #2: You have an IV of Keflex 2 g mixed in 50 mL of D5W. To infuse the IV in 20 minutes with 15 gtt/mL tubing, what is the drop per minute flow rate?
HINT: The 2 g Keflex is insignifant. What matters is that you have 50 mL that you want to complete in 20 minutes. Remember if you calculate a mL/hr flow rate, you can convert to get gtt/min.
50 mL = X mL
20 min 60 min (You can't put 1 hour here because units much match; therefore, put 60 min.)
20 X = 3000
20 X = 3000
20 20
X = 150 mL 150 mL/60 min or 150 mL/hr
The "magic number" for 15 drop/mL tubing is 4.
150 divided by 4 = 37.5 = 38 gtt/min (PCC rule: drops must be whole number.)
Example problem #3: The patient has an IV of 1000 mL of D5RL with 20 mEq KCl infusing at 40 mL/hr. The pump malfunctions and you need to change the IV to gravity flow. Using an administration set with a drop factor of 20 drops per mL, what should be the drop per minute flow rate?
This problem is a typical clinical situation with a lot of numbers that you don't need to work the problem. It doesn't matter that you have an IV of 1000 mL or that IV has 20 mEq of KCl. What matters is that you have 40 mL/hr that you need to change to a gtt/min flow. All you have to do is use the "magic number" and convert.
"Magic number" for 20 drop tubing is 3. 40 divided by 3 = 13.33 = 13 gtt/min