Is there a way (or tool) to measure a site's content? To find out the keyword density for the whole site?
How will I know when I update the content whether or not it is better (besides following keywords to see if the ranking improves)?
First thing, why do you want to measure site's content?
The things for which you are concerned are less important for a website. Its better to focus on quality of content.
There are tools available to measure the keyword density of the page but, I guess, there are no tools available to measure the keyword density of the website.
There is not fixed time to update content of the website... You can update content on any time of the year.
some firefox addons could help u do this, such as the SearchStatus O0
Website content metrics means i will generally not consider content like devices, sponsorship, or live occasions, in light of the fact that despite the fact that substance without a doubt assumes a part in their objectives' accomplishment, there are likewise different components like client fulfillment and good fortune included which are not straightforwardly identified with content itself or can't be effortlessly measured.
Keyword density is the percentage calculated based on the number of times a keyword occurs inside the content of webpage divided by the total word count. Keyword density / keyword frequency is still a pretty strong indicator to determine the main focus keywords and keyword phrases for a specific webpage.