The RTP can be calculated for any game in theory. The formula is simple: take the total amount returned to players, and divide it by the total amount gambled by players.
On the other hand, RTP on online pokies, for instance, can be misleading since it does not tell you how often you will win.
Things appear pretty simple in games like Blackjack, where you know you will win around half the stakes you place.
However, online pokies, where huge jackpots can be won, are less likely to payout. In short, the end result will always be the same, a return to players slightly below turnover over time.