Character Count Online is a webpage you'll want on your Bookmarks Toolbar.
Visit the Character Count Online webpage and you'll see that it counts:
Characters, Words, Sentences, Paragraphs, and Whitespace.
Counting the Whitespace is especially valuable because some webpages like Twitter limit you to a certain number of characters that include the whitespace.
I use this free web tool to create spreadsheets of Tweets that I upload to Twittimer each month.
All I have to do is make a tweet that I know is 140 characters or less (actually, I make one that is 137 characters), paste it into my spreadsheet, then it's easy to see if any succeeding tweet I paste in is too long.
The Internet offers all sorts of free apps and webpages that can save you a lot of time. Try Character Count Online, and you'll find it just as useful as I.