Keyword density is believe to be a myth by many people. It isn't because you repeated a word 10 times instead of 20 times that you will get a better ranking than someone else. Just don't spam your keywords in your content and you should be good. However if you still want to know what the keyword density should be, it should stay between 2% - 5%. To calculate it, add up the total amount of words in your content, divide your amount of keywords by the amount of total words and multiply by 100.
Make sure you are covering your bases with proper use of Titles, Metas, and on-page formatting to draw attention to your keywords. Beyond that, make sure you aren't getting too spammy. As far as the acceptable amount, I have seen pages rank in the top spot of Google with a density below 1% for a keyword while others have been close to 10%. I will agree with metaseo and say that 2-5% would be a good goal. Use keywords that have relevance to pad your content and make it more readable. Google likes seeing relevant words because it looks more like your page knows what it is talking about. You may want to check out the free SEO-Quake extension for FireFox, if you haven't got it already. It has a Density function that lets you see various keywords from a page and the density they have, allowing you to do some research into what other sites have for keyword density. Edit: Want to clarify my comment on 2-5% keyword density, as I realize now that I may not have been clear and that metaseo didn't say what my feelings are on what should make up that density level. In my opinion, over 2% for a single keyword is getting into "spammy" territory. Use synonyms and closely related terms to get a Google friendly, and reader friendly, 2-5% density level.
Thank You @metaseo I was little bit wondering before read your post !! how much keyword density should be? but now I know about keyword density stuff !!
keyword density the more times a keyword appears on the web page compared to the total number of word on the page.use as the factor of search engine optimization.
When keywords are optimally utilized, it gets a positive response from google whereas when it is repeated unnecessarily, the site itself will be considered as spam and accordingly penalized in the SERP.
2-5% You may want to calculate it manually and compare it with the kw density tool to make sure it's accurate
I managed to make one of my pages vanish from google's results a few months ago, and the only thing that seemed to change was the content. Did a re-write late in the evening, feeling tired and not very creative, and when I reread it the following week (wondering where the page was!), realised it read like the biggest piece of keyword stuffed rubbish ever! It reappeared fairly quickly when I rewrote the whole thing and submitted it again for crawling. Lesson learned!