Size Limits for Web Pages

by Jakob Nielsen on March 1, 1997

Sidebar to Jakob Nielsen's column on Web response times.

The following table shows the maximum allowable page size in order to achieve desired response times for various connection speeds. The numbers assume 0.5 s latency which is faster than most Web connections these days, so for many realistic purposes, page sizes really need to be even smaller than indicated in the table.

  1 second
response time
10 seconds
response time
Modem or mobile 2 K 34 K
ISDN 8 K 150 K
Cable/DSL 100 K 2 M

The concept of "page size" is defined as the sum of the file sizes for all the elements that make up a page, including the defining HTML file as well as all embedded objects (e.g., image files with GIF and JPG pictures). As further discussed in my main article on Web response times, it is sometimes possible to get away with page designs that have larger page sizes as long as the HTML file is small and is coded to reduce the browser's rendering time.

Note that the 1 sec. response time limit is required for users to feel that they are moving freely through the information space. Staying below the 10 sec. limit is required for users to keep their attention on the task.

In mid-1997, a study found that the mean size of Web pages was 44 kilobytes. This is more than five times too big for optimal response time for ISDN users, so even when more people get mid-band connections, the Web will be much too slow. Also note that 44KB is 30% larger than even the most generous size limit for modem users.

See Also: Website Optimization: Speed, Search Engine & Conversion Rate Secrets by Andrew King. (Europeans: order from )

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