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Maximum value of z-index

Z-index is used to determine the stacking order of positioned elements on a page. The CSS 2.1 spec says the value of z-index should be an integer, but it doesn’t define any maximum values for it. So the actual max is often determined by what variable type the browser uses to store the z-index.

I made a simple test page to find these limits and figure out what happens when you exceed them.

Browser Max z-index value When exceeded, value changes to:
Internet Explorer 6 2147483647 2147483647
Internet Explorer 7 2147483647 2147483647
Internet Explorer 8 2147483647 2147483647
Firefox 2 2147483647 *element disappears*
Firefox 3 2147483647 0
Safari 3 16777271 16777271
Safari 4 2147483647 2147483647
Opera 9 2147483647 2147483647

The results show IE, Firefox and Opera all use a 32-bit signed integer to store the value of z-index. Safari 3 on the other hand hits the max at 16777271. I’m not really sure what that number corresponds to, but it looks like they’ve fixed the issue in Safari 4.

Update: Eric Seidel, one the developers behind Webkit, had the answer to where 16777271 comes from.

Using google to compute the log base 2 (lg) = lg(16777271) gave me the answer.

It must have been a 24-bit bitfield. I expect this would have been
on RenderLayer, and was for saving space.

It’s also pretty interesting what happens when that limit is exceeded. Most browsers just assume the maximum allowed value, but FF3 actually converts it to 0 and FF2 doesn’t show the element at all (as it does with any elements with negative z-index values).

Comments Post a Comment

2^24 + 55 = 16777271.
No idea where the 55 is from.

[…] further, I came acrossthe best summary of the situation, published recently. It also highlights the fact that it’s not just the maximum value we […]

[…] specifically, it depends on the web browser. According to Eric Puidokas, who did some z-index testing in a number of web browsers, “The results show IE, Firefox […]

Just wanted to send you a belated Thank You for the information and all the research & testing you did on this.

For some reason, I always felt “slightly crazy” when I was giving a z-index a value of 99, but what little did I know, eh?

With full credit to your article (and Cameron Moll, who alerted me of this): http://www.webgyver.com/web-design-and-development/whats-the-max-value-for-the-z-index/

Thanks again!

Now, there are a lot of other things stored as values in CSS, height and width might be the most obvious. What happens to those? Do you think you could set up a larger test for testing everything which uses numbers in CSS?

Very cool! I love it when people put decent tests together and share the results. Good question from Zirro… what happens when this is applied to other properties of CSS objects??

[…] Maximum value of z-index » Eric Puidokas » Programming and Designing for the Web (tags: css z-index) Socio-Encapsule this: […]

I’m skeptic about the idea that the z-index for Safari 4 has been fixed to 2147483647 because of the following situation. I’ve a div that gets generated through script and it has a z-index 1410065407, another div with z-index 1410065408 or 2000000000 doesn’t go to overlay the first one. The first has flash-content, that’s through but both elements are absolute positioned… so I’m really stuck here :s Does anyone has enlightments…?

SORRY! It is a misunderstanding, it seems dat in Safari another z-index was generated, 9999999999, so it could never have been underneith the second div. Many sorry’s! Delete my posts ; ) Thanks for your information because it was very handy after all!

[…] 0 MAR 12 2147483647 – highest possible z-indexposted by bill VERY handy to know. We all need to shove a layer right above everything else going on. No more adding on zeros until your layer appears, just use the magic number. 2147483647 is the highest possible z-index in browsers to date. One note, safari 3 apparently will reduce it to 16777271. Read more here: http://www.puidokas.com/max-z-index/. […]

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This research is brilliant, thanks for writing this all up. I work on a site that chronically receives client content with embedded high z index numbers. Increasing our own index numbers to the upper limit has eliminated this problem completely.

Thanks for this! I had a question and please excuse my ignorance. Is it correct to assume that I can just add 2147483647 for my z index and it will appear over everything no matter what?? Are there consequences when setting the higher z index?

[…] Maximum value of z-index » Eric Puidokas Vad är det högsta z-index du kan sätta för olika webbläsare? Oftast 2147483647 (via @westmaaan). […]

[…] you still like to use high indexes, check here for how high you can go! Tags: css, […]

Thanks for this. Now we know for sure :)

As a side note, the article says “z-index is used to determine the stacking order of absolutely positioned elements on a page.”, but actually, z-index applies to *positioned* elements, not only absolutely positioned ones.

@christina:

No, you cannot assume this as it depends on stacking *contexts*. A simple example is to imagine that the parent of the element you want to style with a large z-index value is in a lower stack than one of its siblings - that would be enough to send it below other elements.

I have a page online that may help you try this for yourself: http://tjkdesign.com/articles/z-index/teach_yourself_how_elements_stack.asp

@thierry
I fixed the wording. thanks

Maybe some update? I’m curious about how it works in new browsers:).

[…] So I googled around with a simple question: What is the highest Z-index value one can use? Can it be infinitely high or is there a previously unwritten ceiling? My search for answers landed me Eric Puidokas’s blog – where the answer was somewhat surprising. Did you know that in Firefox 2 if you set an element to have an index value over 2147483647 it will … […]

Hey, if i pay you a monthly fee will you display my banner ad? Say $10-20/mo. Drop us an email, Cheers.

First up, Eric - thank you for this. Your research was a huge help to me a few years back, and saved me many headaches.

I recently decided I’d run a few tests to find what is the optimal z-index in current (2012) browsers - it’s all listed here:
http://joeist.com/2012/06/what-is-the-highest-possible-z-index-value/

I hope it’s helpful…

-Joe

Is there a maximum value for z-index?…

Yes. it’s 2147483647.

This seemingly random number is the maximum value of a 32-bit integer. The CSS spec doesn’t mention anything about a maximum value for z-index.

Eric Puidokas tested pushing the z-index to the limits in different browsers:
http…

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