What's a browser cache, how do I "clear" it, and why would I want to?

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Summary: "Clear your browser's cache" is a frequent first response when encountering web page problems. We'll look at why, and how.

The browser cache comes up a lot in answers, more than questions, but in doing so it probably causes even more questions. Even when following instructions to clear it, it's not at all clear what this piece of magic really is, and why clearing it would do anything at all.

Let's review the browser cache; what it is, and why it exists. Along the way we'll review the steps to clear it, in both Internet Explorer and Firefox, and try to dream up some reasons why that sometimes helps.

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The browser cache exists because of a basic assumption made by the folks who design browsers: the internet is slow. Perhaps more correctly, your internet connection is slower than your computer.

What that means is that it's faster to get something to display from your hard disk than it is to get it "over the wire" by downloading it from the internet. Even with today's faster internet speeds, that still holds very true.

When current browsers were being designed, what people noticed is that many web pages contained the same elements over and over, and many sites had the same elements over and over on all their pages. For example, if you look at this page you'll see a logo at the top of the page. It's actually at the top of every page on this site. So the thinking was basically this: why download the same thing for every page? Why not just download it once, and then keep it so we can use it again if it's needed again?

The cache is nothing more than a place on your hard disk where the browser keeps things that it downloaded in case they're needed again. The first time you visit a page on this site, for example, the browser will download the logo so that it can be shown. Every time thereafter when you visit a different page on the site as long as the same logo is displayed it doesn't need to be downloaded again - it's already here.

The cache has a limit of how big it can get and you can usually configure how much space to set aside for it. Essentially when the cache gets full, the items in it that haven't been used in a while are discarded to make room for whatever new items you're looking at now.

There's a little more to it than that - for example ways for me to update the logo on my site and have that override whatever is in your cache so what you see is always up to date. But by and large, that's all it is: a place to keep things locally so that you don't always have to download the same things over and over and over again.

And it's all transparent to you.

Until something breaks, of course.

And that's where "clearing the cache" comes in.

For reasons that I simply can't explain - other than by saying "stuff happens" - the cache can sometimes get confused. This seems to happen to most browsers, and at random times. What you'll see are partially loaded web pages, badly formatted web pages, incomplete pictures, or in some cases, the wrong picture in the wrong place.

It's not always a caching problem, but since it happens often enough, "clear your browser cache" is often one of the first diagnostic steps you'll hear from people like me.

Here's how.

Internet Explorer

Click on the Tools menu, then the Internet Options menu item. In the resulting dialog box, click on the Delete... button:

In the resulting "Delete Browsing History" dialog, click on the Delete Files... button:

Finally, click on Yes in the confirmation:

Your browser cache is now empty.

FireFox

Click on the Tools menu, Clear Private Data... menu item (or type CTRL+SHIFT+DEL). In the resulting dialog, ensure that the Cache item is checked:

You can of course check other items to be cleared at the same time.

Press Clear Private Data Now.

Your browser cache is now empty.

Your browser cache is empty - so what?

An empty cache means there's no confusion. As you visit web pages hereafter the browser will download fresh copies of everything that you see on each page. Effectively you've simply forced your browser to rebuild its cache from scratch as it loads or re-loads web pages. Any cache-related issues should be cleared up.

Until the next time.

Related:

• How does flushing DNS help resolve some issues, and while you're at it what's DNS? DNS transforms domain names into IP addresses. Your PC caches or remembers much of the DNS information but sometimes, though, it needs to forget.

• What are all these other pictures that appear when I view a picture from email? Viewing a picture from email causes the email program to place that image on your hard disk, along side other pictures from other places.

• What are those strange folders in my Temporary Internet Files?

Article C3708 - April 18, 2009