Use the bonus bet calculator to find how much cash to bet on the other side of a bonus bet or free bet to guarantee a profit.
They're the same thing. Most US sportsbooks now call them bonus bets, while others (and most UK books) still say free bets. Either way, if it wins you get the winnings but not the stake, so this calculator works for both.
A risk-free bet or second-chance bet is different. You bet real money and get a bonus bet back if you lose. Use the risk-free bet calculator for those.
A bonus bet only pays out the winnings, not the stake. That makes long odds more valuable: bet the bonus on an underdog, then hedge the favorite with cash so you profit whichever side wins. Your conversion rate is that guaranteed profit as a percentage of the bonus amount.
For example, a $100 bonus bet at +500 hedged with $429 at -600 returns about $71 either way, a 71% conversion.
Any conversion rate above 70% is considered good. You can consistently get a higher rate by using the Parlay Buddy Chrome extension on the Crazy Ninja Odds Positive EV page. A parlay of the top three legs is usually above 90%.