Enter the number of errors observed and the number of opportunities for errors into the Medication Error Rate Calculator. The calculator will evaluate the Medication Error Rate. 

Medication Error Rate Formula

The following two example problems outline the steps and information needed to calculate the Medication Error Rate.

MER = #E / #O * 100

  • Where MER is the Medication Error Rate (%)
  • #E is the number of errors observed 
  • #O is the number of opportunities for errors 

How to Calculate Medication Error Rate?

The following example problems outline how to calculate Medication Error Rate.

Example Problem #1:

  1. First, determine the number of errors observed. 
    • The number of errors observed is given as: 40.
  2. Next, determine the number of opportunities for errors. 
    • The number of opportunities for errors is provided as: 1000.
  3. Finally, calculate the Medication Error Rate using the equation above: 

MER = #E / #O * 100

The values provided above are inserted into the equation below and computed.

MER = 40 / 1000 * 100 = 4.00 (%)


Example Problem #2: 

For this problem, the variables required are provided below:

number of errors observed = 60

number of opportunities for errors = 500

Test your knowledge using the equation and check your answer with the calculator..

MER = #E / #O * 100 = ?