2015 Update

Back in 2012 this wasn't possible, if you wanted to support all major browsers in-use. Unfortunately, right now this is still a Chrome only feature (a non-standard extension of window.performance).

window.performance.memory

Browser support: Chrome 6+

2012 Answer

Is there a way to find out how much memory is being used by a web page, or by my jquery application? I'm looking for a runtime solution (not just developer tools), so that my application can determine actions based on memory usage in a user's browser.

The simple but correct answer is no. Not all browsers expose such data to you. And I think you should drop the idea simply because the complexity and inaccuracy of a handmade solution may introduce more problem than it solves.

Counting DOM elements or document size might be a good estimation, but it could be quite inaccurate since it wouldn't include event binding, data(), plugins, and other in-memory data structures.

If you really want to stick with your idea you should separate fixed and dynamic content.

Fixed content is not dependant on user actions (memory used by script files, plugins, etc.)

Everything else is considered dynamic and should be your main focus when determining your limit.

But there is no easy way to summarize them. You could implement a tracking system that gathers all these information. All operations should call the appropriate tracking methods. e.g:

Wrap or overwrite jQuery.data method to inform the tracking system about your data allocations.

Wrap html manipulations so that adding or removing content is also tracked (innerHTML.length is the best estimate).

If you keep large in-memory objects they should also be monitored.

As for event binding you should use event delegation and then it could also be considered a somewhat fixed factor.

Another aspect that makes it hard to estimate your memory requirements correctly is that different browsers may allocate memory differently (for Javascript objects and DOM elements).