IV solutions can be administered by an IV pump or by gravity.   Most pumps are set in a milliliter per hour flow rate (mL/hr).  Although an IV flowing by gravity flow may be ordered as mL/hr, to assure accurate flow you need to count the flow in drops per minute (gtt/min).  

IV tubing comes in a variety of sizes in relation to how many drops it takes to equal one milliliter.  A macrodrip administration set delivers 10, 15, or 20 drops per mL depending on the manufacturer.  A microdrip administration set delivers 60 drops per mL.  Other terms you may see for microdrip tubing is minidrop tubing, or pediatric administration set.

This objective involves converting between mL/hr and gtt/min by use of a "magic number" or calibration factor.  The magic number is derived from the relationship of the number of drops per mL of the tubing to the number 60.

     Magic Number
   10 drop tubing = 6
   15 drop tubing = 4
   20 drop tubing = 3
   60 drop tubing = 1

• To convert from mL/hr to gtt/min, you divide by the magic number. 
• To convert from gtt/min to mL/hr, you multiply by the magic number.

HINT:  For something big to get little, it must divide.  For something little to get big, it must multiply.  A  mL is bigger than a drop and an hour is bigger than a min; therefore, mL/hr is bigger than gtt/min.


Example problem #1:   Convert 75 mL/hr to gtt/min with 15 gtt/mL tubing.

"Magic number" for 15 drop tubing is 4.   75 divided by 4 =  18.75  = 19 gtt/min

{Remember PCC calculation rules---IV mL/hr and gtt/min flow rates must be to nearest whole number.}


Example problem #2:  The patient's IV is infusing at 42 gtt/min with minidrop tubing.  How many mL/hr is the patient receiving?

Remember that minidrop tubing is the same as 60 gtt/mL tubing with a "magic number" of 1.  42 X 1 = 42 mL/hr


NOTE:  The "magic number" conversion works only between mL per hour and drops per minute.  If you have mL/min,  mL in 8 hrs,  gtt/cc,  gtt/hr or any other labels, the magic number will not work.