Bonus rounds: types and principles of work
1. What is the bonus round
Bonus round - a separate game mode that starts when certain characters drop out (usually Scatter or Bonus). In it, the player gets additional chances to win outside the main game, often with increased multipliers or unique mechanics.
Integration into RTP: Payouts from bonus rounds are accounted for in the overall RTP of the slot.
A separate mathematical model: within a round, its own paytable, its multipliers and volatility levels can operate.
2. Triggers and trigger conditions
1. Scatter-symbols
Most often, you need to drop N≥3 anywhere on the screen.
Sometimes combined with additional conditions: falling out next to Wild or on active lines.
2. Bonus-symbols
Require exact alignment of positions (for example, on the first and third reels).
Less common, give more difficult rounds.
3. Direct Entry (Bonus Buy)
Round purchase option for a fixed fee (usually 50-100 × rate).
Increases volatility and accelerates access to potentially large payouts.
3. Types of bonus rounds
3. 1 Freespins (Free Spins)
Mechanics: The player receives a fixed number of free spins (5-50).
Features:
3. 2 Pick-and-Win
Mechanics: a set of closed cells appears on the screen - the player sequentially opens N positions.
Types of prizes: fixed amounts, bet multipliers, additional freespins.
Balancing:
3. 3 Wheel Bonus
Mechanics: wheel rotation with sectors, each sector contains a prize: multiplier, freespins or jackpot.
Integration:
3. 4 Board Game
Mechanics: the figure moves across the field in accordance with the numbers drawn (for example, cubes or an arrow).
Cell prizes: bonus amounts, freespins, multipliers.
Risks: Balancing complexity: Number of moves and field structure directly affect variance.
3. 5 Cascade / Tumbling bonus
Mechanics: after each win, the symbols participating in the combination disappear, new ones fall from above; a multiplier is charged for each stage.
Application: more often in slots with Cluster Pays, increases the dynamics of payments.
3. 6 Progressive jackpot
Mechanics: the round is rarely launched, requires a maximum bet.
Jackpot payout: separate from standard payouts, can have several levels (Mini, Minor, Major, Mega).
Participation formula: A percentage of each bet is accumulated in the jackpot pool.
4. Algorithms for calculating payments in rounds
1. Free Spins
$$
ext{RTP}_{FS} = \frac{\sum (m_i imes p_i)}{ext{ставка}} imes 100%
$$
where $ m _ i $ is the payout multiplier in the back, $ p _ i $ is the probability of the combination.
2. Pick-and-Win / Wheel
$$
\ text {Expectation} =\sum (ext {prize} _ jimesext {probability of choice} _ j)
$$
It is adjusted so that the total contribution to RTP corresponds to the slot design.
3. Cascades
The multiplier grows according to the formula $ M _ k = 1 + (k - 1 )imes\alpha $, where $ k $ is the cascade number, $\alpha $ is the multiplier step.
Simulations of tens of millions of batches implement an exact contribution to total RTP.
5. Round design and balancing
Frequency vs value:
Bonus round - a separate game mode that starts when certain characters drop out (usually Scatter or Bonus). In it, the player gets additional chances to win outside the main game, often with increased multipliers or unique mechanics.
Integration into RTP: Payouts from bonus rounds are accounted for in the overall RTP of the slot.
A separate mathematical model: within a round, its own paytable, its multipliers and volatility levels can operate.
2. Triggers and trigger conditions
1. Scatter-symbols
Most often, you need to drop N≥3 anywhere on the screen.
Sometimes combined with additional conditions: falling out next to Wild or on active lines.
2. Bonus-symbols
Require exact alignment of positions (for example, on the first and third reels).
Less common, give more difficult rounds.
3. Direct Entry (Bonus Buy)
Round purchase option for a fixed fee (usually 50-100 × rate).
Increases volatility and accelerates access to potentially large payouts.
3. Types of bonus rounds
3. 1 Freespins (Free Spins)
Mechanics: The player receives a fixed number of free spins (5-50).
Features:
- Increased multipliers (× 2, × 3 and higher).
- Expanding or sticky Wild.
- Additional repeat backs when Scatter falls inside the round.
- Mathematical model: native RTP, often above baseline, is offset by a rarer run.
3. 2 Pick-and-Win
Mechanics: a set of closed cells appears on the screen - the player sequentially opens N positions.
Types of prizes: fixed amounts, bet multipliers, additional freespins.
Balancing:
- The prize table is set by the developer; total expected gain = claimed "frequency" contribution to RTP.
3. 3 Wheel Bonus
Mechanics: wheel rotation with sectors, each sector contains a prize: multiplier, freespins or jackpot.
Integration:
- The probability of starting the wheel corresponds to the slot declared in the literature.
- The average value of the prize on the wheel is included in the total RTP.
3. 4 Board Game
Mechanics: the figure moves across the field in accordance with the numbers drawn (for example, cubes or an arrow).
Cell prizes: bonus amounts, freespins, multipliers.
Risks: Balancing complexity: Number of moves and field structure directly affect variance.
3. 5 Cascade / Tumbling bonus
Mechanics: after each win, the symbols participating in the combination disappear, new ones fall from above; a multiplier is charged for each stage.
Application: more often in slots with Cluster Pays, increases the dynamics of payments.
3. 6 Progressive jackpot
Mechanics: the round is rarely launched, requires a maximum bet.
Jackpot payout: separate from standard payouts, can have several levels (Mini, Minor, Major, Mega).
Participation formula: A percentage of each bet is accumulated in the jackpot pool.
4. Algorithms for calculating payments in rounds
1. Free Spins
$$
ext{RTP}_{FS} = \frac{\sum (m_i imes p_i)}{ext{ставка}} imes 100%
$$
where $ m _ i $ is the payout multiplier in the back, $ p _ i $ is the probability of the combination.
2. Pick-and-Win / Wheel
$$
\ text {Expectation} =\sum (ext {prize} _ jimesext {probability of choice} _ j)
$$
It is adjusted so that the total contribution to RTP corresponds to the slot design.
3. Cascades
The multiplier grows according to the formula $ M _ k = 1 + (k - 1 )imes\alpha $, where $ k $ is the cascade number, $\alpha $ is the multiplier step.
Simulations of tens of millions of batches implement an exact contribution to total RTP.
5. Round design and balancing
Frequency vs value:
- Rare but highly profitable rounds increase volatility.
- Frequent, but with low return - on the contrary, reduce it. Interactivity:
- Increases engagement; better suited for social slots. Automatic and semi-automatic rounds:
- Some slots combine several types (freespins + wheel).
- The Buy option allows the player to adjust the exposure to bonuses.
6. Developer Tips
1. Transparency of rules: be sure to publish the conditions for launching and calculating prizes.
2. Testing: Conduct separate simulation of bonus rounds on a fresh mathematical model.
3. Audit and certificates: bonus mechanics are subject to certification along with the basic RTP calculation method.
4. Adaptability: Adjust the number of freespins and multipliers based on target audience and regulatory requirements.
Conclusion
Bonus rounds are a key tool for increasing slot engagement and diversity. The correct choice of round type, well-thought-out launch mechanics and accurate mathematical calculation guarantee a balance between the player's interest and the preservation of the declared RTP and volatility indicators.
