It depens on how many word your article has. You should not go over 3%. Otherwise you would get bad ideas because of overoptimisation.
Some people doing fine with 1% also.

Typically people say 3.5%. Generally ... just read your article. Does it flow naturally? Does it SOUND like you are stuffing the keyword into the article for search purposes? Write for your audience. You'll rarely get above that 3.5% and your end results will be better from the traffic that does hit your page.

try this keyword density checker from seobook Free Keyword Density Analyzer Tool

It used to be a great topic before google Zoo animals stepped in ,I used to be very peculiar about Keyword density ,But now it is not of much relevance ,Try to write as naturally as possible without thinking much about keyword desnity,,After that check your keyword desnity it will be as perfect as it gets

Hope this helps
Chyan007

1 to 2% is optimal. Anything higher can look unnatural and you will find most natural articles often hit around that mark. Don't "try" to put your keywords throughout the article..

Is keyword density really that important nowadays? Personally i think that the placement of your keywords matters far more than their frequency but that´s just me.. Btw, who says the optimal way to go is 1%, 2% or 3%? Where do you guys find these numbers? From google HQ or from blogs or sites selling seo products?

This is something that has changed a lot over the last few years. I would say keep the density fairly low 1%'ish, but it is more about the overall subject matter. Big G likes to see extensive content that is media rich and covers a subject broadly. So, make sure the content stays on subject and use lots of different variations of the keywords / phrases, lots of long tail key phrases, LSI's and synonyms. You will then find you rank for a whole multitude of target and related terms. Simples.

I would say keeping it low, about 1% . Someone said 3.5%, I think that is a bit high. You should write your articles with the keywords in mind, but don`t overdo it. After you finish your article, read it a few times. Be critic about it, if you think that you overdid it, rewrite some parts because keywords should flow naturally.
Also, I think that the placement of keywords is more important than the number of them. Use keywords or variation of them in the title of the page, in headers or in the alt attribute of the images

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@a1Derek 's suggestion is great and the keywords density should be near 1% (not a strict rule) and you can use synonyms to create keyword diversification. But in special cases like in classified sites sometimes the keyword density of the searched terms reache 70%+ and they still ranked high in SERPs.

# of repeats of the keyword / total number of words on the page * 100 = Keyword Density

Probably around 0.5% keyword density in a 400 word article i would say. And use a few LSI keywords with that also. This isn't an exact science but having to high keyword density can set off alarms... also known as keyword stuffing.

Its about total number of words in your content, as per google algorithm you should use only 4% of keywords in whole content in your web page, beyond that limit results in keyword stuffing & it will be considered as spamming.

As per the rules of Google algorithm the keyword density would be around 1% to 2% within an article and a website as well. Excess insertion of keyword would violate the rules of Google and it will not be effective for getting good rank in Google search engine easily.

Adding different keywords within your website is also helpful to get it ranked in different search engines easily.

Its about total number of words in your content upon total number of keyword that you use multiply by 100. Your keyword density must be less then 3%

1% will be fine and also use this keyword in just starting of the content that will be far enough.

As per Google’s recommended keyword density ranges from 1-2%. You don’t need to really count if you have a 2% ratio in order to rank well.

The keyword density mostly depends upon the total word count of your article. There is a limit for keyword density that has to be included within the articles. It should range anywhere between 1% to 3% and not more than that.

Including more keywords within the article will make readers and search engines feel that websites are doing it just for the sake of improving the search rankings rather than providing readers with relevant information.

You should have your keyword in the:

-Title
-Meta Description
-H1 and H2 Tags
-First Paragraph
-Last Paragraph
-3% Keyword Density
-Image ALT Text

Just mention the main keyword 3-4 times and sub keywords too. Forget density.