What is the number of messages sent in Gmail each second?

This question pertains to the scale and capacity of a service like Gmail. It does not seek a precise answer, as only those working with Gmail would know the exact figures. Instead, the interviewer is interested in your reasoning for estimating these numbers. You should think about factors like the total number of Gmail users, the average daily emails sent by each user, and how to convert that figure to a per-second basis. Additionally, you should factor in variations during peak usage times compared to off-peak periods.

We can make a rough approximation. As of 2020, Gmail is said to have 1.5 billion users. If we assume each user sends around 5 emails daily, that totals to 7.5 billion emails sent each day. Given that there are 86,400 seconds in a day, dividing 7.5 billion by 86,400 yields approximately 86,800 emails sent every second. However, this is a very basic calculation. The rate of email sending can fluctuate significantly throughout the day and across different time zones. Moreover, not every Gmail user sends 5 emails daily; some may send more and others none at all. The total number of received emails, which includes spam and automated messages, is not accounted for here and could be considerably higher.

If we want to explore further, we can examine global internet access rates and peak usage times. Let's take the 1.5 billion Gmail users as our basis. Considering that roughly 60% of the global population has internet access, and assuming that the email sending rate doubles during peak hours, we can estimate that an average Gmail user sends around 10 emails during peak hours and about 2 during off-peak hours. With 4 peak hours each day and 20 off-peak hours, this would result in approximately 12 billion emails sent during peak hours and around 18 billion during off-peak hours. Altogether, this amounts to 30 billion emails sent daily. When we divide this by the total number of seconds in a day, we arrive at roughly 347,000 emails sent every second. However, it’s important to note that this figure is a rough approximation, and the actual number could vary significantly.