The short answer
The ‘En Prison’ rule reduces the house edge on even-money roulette bets to 1.35%, giving players a second chance to recover a bet when the ball hits zero.
The full calculation
Under ‘En Prison,’ your bet isn’t lost or split when a zero hits; it is ‘imprisoned’ for the next spin.
- Initial Spin hits 0: $1/37$ probability.
- Recovery Spin: You need your original choice to hit.
- If it hits, you get your stake back (EV = 0 for that portion).
- If it misses, you lose (EV = -1).
- If it hits 0 again, rules vary, but usually, it stays in prison.
Mathematically, this averages out to the same benefit as ‘La Partage’: $$Edge = \left(\frac{18}{37}\right) - \left(\frac{18}{37} + \frac{1}{37} \times \text{Risk}\right)$$ The presence of the second chance reduces the standard 2.70% edge by exactly half: 1.35%.
What this means at the table
In practical terms, ‘En Prison’ acts as a volatility buffer. Over a two-hour session wagering $25 per spin on Red, you can expect the zero to show up 1–3 times. Without this rule, those are instant $25 losses. With it, you’ll statistically recover half of those ‘lost’ bets, keeping your session alive longer than at a standard American wheel.
Common mistakes around this number
Players often mistake ‘En Prison’ for a win. It is not. You don’t get paid any winnings if your imprisoned bet hits on the second spin; you only get your original chips back. It is a ‘break-even’ recovery, not a bonus. Furthermore, this rule is nearly extinct in modern casinos outside of high-limit French tables—don’t expect to find it on a standard $10 floor game.
See also
For related reading, see Roulette En Prison Rule, Roulette House Edge French Roulette, and Roulette European vs American.
In Detail
En Prison turns the zero from a full punch into a delayed argument. Your even-money bet is not instantly dead; it may get locked for another spin. That small mercy is enough to change the long-run cost in a real way.
What the edge is measuring
Roulette House Edge With En Prison is about the long-run price of the En Prison rule. En Prison can hold an even-money bet after zero, giving it a chance to be saved on the next spin. House edge does not predict the next spin. It predicts the average cost after enough spins for luck to stop shouting over the numbers.
Roulette is priced with a simple expected-value idea:
$$EV = (P(win) \times Net\ Win) - (P(loss) \times Stake)$$
For a standard one-unit bet, the house edge is the negative side of that value:
$$House\ Edge = -EV_{player}$$
The important trick is that roulette payouts are based as if the zero did not hurt the player as much as it really does. A European wheel has 37 pockets: 18 red, 18 black, and one zero. An American wheel has 38 pockets: 18 red, 18 black, one zero, and one double zero. The extra losing pocket is not decoration. It is the price tag.
For session cost, use total action, not buy-in:
$$Expected\ Loss = Total\ Amount\ Wagered \times House\ Edge$$
If a player bets 25 units per spin for 80 spins, that is 2,000 units of action. On a 2.70% European edge, the theoretical cost is 54 units. On a 5.26% American edge, the theoretical cost is about 105.20 units. Same player, same bets, same excitement, very different bill.
The key calculation
With En Prison, an even-money bet may be held after zero instead of losing immediately. If the next spin wins, the original stake can be released. Depending on the house procedure, the long-run effect can sit close to the La Partage cost:
$$Approximate\ Reduced\ Edge \approx 1.35%$$
That is not magic. It is simply a rule that lets the player lose less often or less severely when zero appears.
Why players underestimate it
The percentage looks small because roulette sells the result one spin at a time. A 2.70% or 5.26% edge does not feel painful on one chip. It becomes painful through repetition. Every re-bet, every chase, every extra spin before dinner, every “one more color” moment adds to total action.
This is the casino-floor secret most players miss: the edge does not need to beat you quickly. It only needs enough decisions. A roulette table with steady pace and steady action can turn a tiny-looking mathematical advantage into a very reliable business result.
What the player can actually control
You cannot control the ball, but you can control the wheel choice. European is better than American. French rules can be better still for even-money bets. Lower total action costs less than higher total action. Smaller bets survive longer than emotional jumps. Those are real controls. “Red is due” is not a control. “I always win after three blacks” is not a control. “This dealer spins my number” is not a control.
The bottom line
Roulette House Edge With En Prison matters because it tells you whether the game is charging a fair entertainment price or an expensive one. The smartest roulette move is usually made before the spin: choose the lower-edge wheel, understand the rule, and decide what the session is allowed to cost before the ball starts flirting with your bankroll.
The clean way to use this information is not to chase the wheel harder. It is to choose the better version of the game, size bets honestly, and stop treating a lucky spin as proof of a system. Roulette can be fun, loud, elegant, and cruel in the same hour. Respect the math, and the game becomes entertainment instead of a trap dressed as a pattern.