There are many online tools are available for checking keyword density. I have mention some tools which are given below:-
seobook.com/general/keyword-density/
smallseotools.com
googletheplanet.com
seoquake.com/
You can check keyword density by using these tools. I had checked my site (Royzez) keyword from seoquake and smallseotools.
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A lot of check keyword density tools available online. Most of the tools premium.
etc.