Tools
These calculators do the arithmetic of a bet so you don't get it wrong at the moment it matters. Each one answers a single question.
Four calculators
The parlay calculator shows what a ticket pays when the legs are multiplied together, so you can check the figure the book gives you. The odds calculator converts a price between American, decimal and fractional and shows the implied probability, which is how often the bet has to win to break even. The hedge calculator works out how much to bet on the other side when a ticket is still alive and you want to lock in a result. The no-vig calculator takes the book's cut out of a two-sided price and shows the fair odds underneath. That one is my favorite.
What they can't tell you
A calculator tells you what a bet pays. It doesn't tell you whether the bet is any good, and the mistake I've seen most is reading the payout and stopping there. I'd read the break-even figure first and ask whether your side really wins that often. More on that under what a sports bet calculator is.
And I'd put in a stake that fits your bankroll, not the one that makes the payout look good.