Retention rate is an indication of how much of your bonus you will retain after placing a bet.
A bet with 100% retention rate means that if you bet your entire bonus on that bet your entire bonus will be intact. That is, if you have a €50 bonus and bet the whole amount on one outcome, you will still have €50. A bet with 95% retention rate and a €50 bonus means that if you bet the whole amount you will have €47.50 left of your bonus (95% of 50).
This calculation applies to free bets where the whole bonus amount can be wagered and cleared in a single bet. A bonus with rollover requirements is calculated a bit different. (Rollover means you need to bet the money a fixed number of times before the bonus can be withdrawn). A bonus of €50 with a rollover requirement of 5 times means that a total amount of €250 has to be wagered. When wagering €250 with a retention rate of 95% you will have €237.50 left (95% of 250). This means that you have lost €12.50 when clearing your bonus. Since the bonus was €50 you have retained €37.50 (50-12.50).
Another way of calculating this is as follows. You loose 5% (100 – 95) each time you bet your €50 bonus, which calculates to €2.50. Loosing €2.50 for 5 consecutive times equals €12.5.
Now, imagine that you wager your bonuses on bet with retention rates higher than 100%. This means that you will keep your entire bonus, and possibly also earn a bit more on each bet. Select bets with as high retention rate as possible since it will let you keep as much of your bonus as possible when meeting the rollover requirements.
MatchedBetting often displays these positive retention rates because we have access to arbitrage odds.