Every casino game, from roulette to a modern video slot, begins as a maths model long before it gets graphics, sound or a theme. Knowing how that model is built is the best protection against myths: once you see where the house edge comes from, it becomes obvious why no theme, soundtrack or “hot streak” changes it. This article walks through the main design steps and then builds a tiny dice game — played for points, not money — to show the numbers in action.
House edge and RTP: one number seen from two sides
Return to player (RTP) is the share of all stakes a game pays back over the long run. The house edge is what remains: house edge = 1 − RTP. A slot with 96% RTP has a 4% house edge; over a very large number of rounds it keeps, on average, €4 of every €100 wagered.
In table games the edge comes straight from the gap between true odds and payouts. In European roulette a single number has a 1 in 37 chance and pays 35:1, so a win returns 36 units. The expected return is 36/37 ≈ 97.30%, which gives the familiar house edge of 2.70%. The single green zero is the whole edge.
The building blocks of a game
Paytable
The paytable defines how much each outcome pays. It is the half of the design that players can see.
Symbol weights and reel strips
The other half is hidden. In a slot, each reel is a virtual strip on which symbols appear different numbers of times. The random number generator (RNG) picks a stop position on every reel, so a symbol that appears once on a long strip lands far less often than one that appears many times. Multiply the probability of every combination by its payout, add everything up — including free spins and other features — and you get the RTP.
Hit frequency and volatility
Hit frequency is the share of rounds that return anything at all. Volatility describes how widely results are spread: frequent small wins mean low volatility, rare large wins mean high volatility. Two games with exactly the same RTP can feel completely different to play — the dice example below shows this with real numbers.
Simulation
Designers calculate the RTP exactly wherever possible and then confirm it by simulating millions or even billions of rounds. Simulation also shows how the results are distributed: how often a session of a given length ends in profit, how long dry spells last, how often the maximum win can occur.
Certification and RNG testing
In regulated markets a game is tested by an independent laboratory before it goes live. Well-known labs include GLI (Gaming Laboratories International), BMM Testlabs, eCOGRA and iTech Labs. They check that the RNG produces statistically random, unpredictable results and that the game's maths really delivers the stated RTP. GLI publishes technical standards that many regulators reference, such as GLI-11 for gaming devices and GLI-19 for interactive (online) gaming systems; which standards apply is decided by each regulator.
Once a game is certified, changing its maths generally means testing and approval all over again. That is one reason why no algorithm can quietly “learn” to make a certified slot pay you more or less — see what AI can and cannot do in online casinos.
Build a game yourself: “lucky seven”
Points, not money. Depending on your country, organising gambling for money at home can be illegal or require a licence. Play this game for points, matchsticks or bragging rights — it works just as well as a maths lesson.
Rules: roll two ordinary dice. The player stakes 1 point that the total will be 7. If it is, the “house” pays 4:1 — the player gets the stake back plus 4 points. Any other total loses the stake.
Step 1 — count the outcomes. Two dice give 36 equally likely combinations. Six of them add up to 7: 1+6, 2+5, 3+4, 4+3, 5+2 and 6+1. The probability is 6/36 = 1/6.
Step 2 — expected return. A win returns 5 points (stake plus 4). Expected return per point staked = (6/36) × 5 ≈ 0.833. The RTP is 83.3% and the house edge 16.7% — far harsher than any common casino game.
Step 3 — tune the paytable. Changing only the payout changes the edge:
| Payout on a 7 | Returned on a win (incl. stake) | RTP | House edge |
|---|---|---|---|
| 3:1 | 4 points | 66.7% | 33.3% |
| 4:1 | 5 points | 83.3% | 16.7% |
| 9:2 (4.5:1) | 5.5 points | 91.7% | 8.3% |
| 5:1 | 6 points | 100% | 0% — a fair game |
| 6:1 | 7 points | 116.7% | −16.7% — the player has the edge |
The true odds against a 7 are 5 to 1 (five losing outcomes for every winning one), so a 5:1 payout is fair. Every casino bet works the same way: the house pays a little less than the true odds and keeps the difference.
Step 4 — change the volatility, not the RTP. Add a second bet: “double six”, paying 29:1. Its probability is 1/36 and a win returns 30 points, so the RTP is 30/36 ≈ 83.3% — exactly the same as the 4:1 bet on 7. But it wins in only 1 round out of 36 instead of 1 in 6, with much larger wins when it does. Same RTP, much higher volatility: that is precisely how two slots with identical RTP can behave so differently.
Step 5 — simulate. Roll a million times in a spreadsheet or a few lines of code. The overall return of the 4:1 bet will usually land within a few tenths of a percentage point of 83.3%, while short runs of 20 or 50 rolls can look like anything. That gap between short sessions and the long run is what every player experiences.
What you can read in a real game's info panel
- RTP. Some studios supply several RTP versions of the same game and the operator chooses which one to run; the info panel should show the version you are actually playing.
- Volatility. Usually the studio's own rating (low, medium, high, or a scale). It is not standardised across studios, but it is a useful hint.
- Maximum win. A cap on what a single round can pay, not a promise of how often it happens.
- Paytable and feature rules. What each combination pays and how bonus features work.
- Malfunction rule. Most games state that a malfunction voids pays and plays — worth knowing before you start.
Unfamiliar terms are explained in our glossary.
Why the theme does not matter
Ancient tombs, fruit, outer space or a famous film: the theme is a skin. The same maths model can sit under completely different graphics, and a dramatic theme or soundtrack has no influence on the result of any round. If you compare games, compare what actually determines your costs — RTP, volatility and your own stake — and remember that even big documented wins are rare outliers, as explained in why big-win stories mislead.
Play responsibly — 18+ only. Every casino game is designed with a house edge, so over time the player is expected to lose. Treat gambling as paid entertainment and set limits before you start. If it stops being fun, get support: GamCare (UK) on 0808 8020 133 or gamcare.org.uk, or free international online support at gamblingtherapy.org.
Frequently asked questions
What is the difference between RTP and house edge?
They are the same number seen from two sides. A game with 96% RTP has a 4% house edge: over a very large number of rounds it pays back about €96 of every €100 staked.
Who checks that a slot really pays its stated RTP?
In regulated markets, independent laboratories such as GLI, BMM Testlabs, eCOGRA or iTech Labs test the RNG and the game's maths against the standards the regulator requires, and the regulator then approves the game.
Does a high-volatility game have a lower RTP?
Not necessarily. RTP and volatility are separate properties. In the dice example, the double-six bet at 29:1 has the same 83.3% RTP as the 4:1 bet on 7, but wins far less often and pays much more when it does.
Can I play the dice game for money with friends?
That depends on your country: private gambling for money can be illegal or regulated. Playing for points avoids the problem and teaches exactly the same maths.