I used to be more vigilant about keeping my Gmail inbox clean and organized, but the inbox tabs that Gmail rolled out earlier this year have made me lazy. The tabs help filter promotional e-mails, social media notifications, and other such messages that aren't of primary importance. While these e-mails are effectively hidden with Gmail's Web client, they are still marked as unread on my iPhone. That is, my Mail app icon constantly has a badge alert with a large number because of the voluminous number of unread messages I have hiding in my Social and Promotional tabs.
You can go through your tabs, page by page, marking messages as read, but if you have hundreds or -- gasp! -- thousands of unread messages, this process can quickly because tedious. There is a better way.
For starters, you'll need to disable your inbox tabs temporarily. To do so, click on the gear icon in the upper-right corner of your Gmail inbox page and select Settings. Next, click on the Inbox tab and in the Categories section, uncheck any inbox tabs you checked (you can't uncheck the grayed-out box for the Primary tab, but that's OK). Next, scroll down and click the Save Changes button, and you'll be returned to your inbox.
Now, all of your messages are grouped together, but if you have hundreds or more unread messages, they'll be spread across multiple pages of your inbox. So, we have to find a way to select not just all of the unread messages on the first page of your inbox, but all of the unread messages throughout your entire inbox. In fact, we will need to select all messages, both read and unread. To do so, click on the box in the upper-left corner of your inbox that lets you select different types of messages. You can just click to check the box or you can click the down arrow and choose All from the drop-down menu.
What you've done with this last action is select all 50 messages on the current page of your inbox. But at the top of your inbox, Google gives you an option to select all messages -- conversations, in Google's parlance -- in your inbox. In my case, it asks if I want to "Select all 16,548 conversations in Inbox."
Click that link and then from the More button above your inbox, choose Mark as read. Lastly, click OK on the confirmation pop-up window to confirm your decision to mark each and every e-mail in your inbox as read. Afterward, you can jump back into Settings and enable any inbox tabs you like to use on the Inbox tab.