The number of opens vs. the number of emails sent. To calculate the open rate, you take the number of opens and divide it by the number of emails sent. For instance, if we sent 1000 emails, and 50 people opened the messages, the open rate is 5%. To calculate this, we take 50 / 1000, which gives us .05. Multiplying by 100 gives us the whole number percentage: .05 ** 100 = 5%.