What does clearing cache do, and what happens to your logins?

What the cache is, in the makers’ words

Google puts it in one line: “The cache remembers parts of pages, like images, to help them open faster during your next visit.” Mozilla says the same of Firefox: the cache stores temporary files, such as web pages and other online media that the browser downloaded, to speed up loading of pages and sites you have already seen. A cache is not only a browser matter: the MDN guide to HTTP caching says that a browser’s cache is a private cache, “a cache tied to a specific client”, because the stored responses are not shared with other clients.

MDN adds that stored responses are either fresh or stale. A fresh one can be reused as it is; a stale one has expired, and the browser can ask the server whether the stored copy is still good rather than download it again. A site that has changed can look old, which is the case the University of Iowa describes: files saved in the cache may conflict with what is actually on the site.

Cookies are a different thing, kept for a different reason. Google calls them files created by the sites you visit, which save browsing data, and Mozilla names what a site keeps in them: login status, site preferences, or a shopping cart.

What happens when you clear them

Google lists three effects after you clear the cache and cookies in Chrome. Some settings on sites get deleted, so that if you were signed in you will need to sign in again. If sync is on in Chrome, you stay signed into the Google Account you are syncing to, so that the deletion reaches your other devices. And “Some sites can seem slower because content, like images, needs to load again.” The choice of time range is yours: Google’s steps start with picking one, such as Last hour or All time.

Microsoft says of Edge that deleting all cookies and site data for the time range you chose “signs you out of most sites”. Apple is more guarded about Safari on a Mac: “Removing the data may reduce tracking, but may also sign you out of websites or change website behavior.” It adds a warning: “Removing cookies and website data in Safari may change or remove them in other apps.” On an iPhone, Apple says that Clear History and Website Data “doesn’t change your AutoFill information”.

Mozilla draws a line between two kinds of login. In Firefox, clearing Active Logins affects only logins to sites that use HTTP basic authentication, and “It will not log you out of websites that use cookies to store login status.” On Microsoft’s page, what signs you out of most sites is clearing cookies and site data.

Why people do it

Google gives three reasons: it can resolve website loading or formatting issues, it can give you the most up-to-date version of the website you are visiting, and it can free local storage space on your device. The University of Iowa calls it an important first step for almost any troubleshooting of internet browsing, and tells you to close and restart the browser after clearing. The pages we read give no schedule, so there is no figure for how often to do it; the three reasons are the occasions.

If the reason is a page that will not load, Google’s article on connection and loading errors lists deleting browsing data as step 6 of its nine, after the Incognito test and the extensions; the page on the memory error follows that order.

Less than everything

You do not have to clear everything. MDN describes two ways to get a current page without touching the cache of every site. A reload, in its words, is a validation: the browser asks the server whether its stored copy is still current. A force reload goes further: “force reload is an additional way to bypass cached responses”, and it makes sure of a 200 response from the origin server. Neither deletes anything.

When you do want to delete, the makers offer narrower choices. Chrome’s dialog starts with a time range, and Google points to a separate setting for deleting the cookies of a specific site. Edge has the same option, under Cookies, See all cookies and site data. Firefox clears the last hour, two hours, four hours, the current day or everything, and its Forget About This Site removes all history items for one site, including cookies, cache, active logins and passwords. Safari on a Mac lets you select one or more websites and remove them, and on an iPhone you can clear website data but keep your history.

The steps for Safari are on the pages for the iPhone and the Mac; Chrome’s cookie settings are on the page on cookies in Chrome.

On a phone, for an app

Android Help separates two buttons. “Clear cache: Deletes temporary data. Some apps may be slow the next time you open them.” And Clear storage “Permanently deletes all app data”, with the advice to try to delete the data within the app first. Google adds that the settings can vary by phone.

Apple’s guide to managing storage on an iPhone, as we read it, has no button for the cache of a single app. It offers offloading: when storage is low, iPhone can automatically remove unused apps while keeping their data, and the icon stays on the Home Screen with a cloud symbol to reinstall. Offload Unused Apps is under Settings, Apps, App Store.