RNG and game fairness

What a random number generator does

An RNG is software that produces numbers which cannot be predicted from earlier ones. Most are algorithms that start from a hidden value, called a seed, and generate a very long sequence. For gaming, the sequence must pass statistical tests for randomness.

In a slot, the number drawn when you press spin is mapped to symbol positions on each reel. In a virtual roulette or card game, it decides where the ball lands or which card is dealt. The result is fixed at that moment and the animation follows.

Because the generator is not tied to your history, each round is independent. How online slots work shows this in a slot, and slot myths covers the beliefs that come from forgetting it.

Fair does not mean even odds

A fair game, in the regulatory sense, is one that does what its rules say. It does not mean the player has an equal chance. Every casino game has a built-in house edge, and a slot with a 96% RTP keeps 4% of stakes over the long run, with the game working exactly as designed.

So fairness is about honesty and consistency. The outcomes must be random, the published rules must match the real ones, and the payout percentage must be what the game says it is.

How independent testing works in general

Many licensing regimes require operators or game studios to have their games tested by an accredited outside laboratory. The labs and the exact procedures differ by jurisdiction, and we do not describe any one lab's methods here. In general terms, testing covers these areas:

• The RNG. Output from the generator is examined statistically to check that it is unpredictable and evenly spread.
• The game maths. The tester checks that the paytable, symbol frequencies and features produce the stated RTP over a very large simulated sample.
• Game behavior. The software is checked to see that it follows the published rules, handles errors correctly and records results properly.
• Changes. When a game's software is updated, it may need to be re-tested before release.

Some regulators also require periodic checks of live games, and some labs publish the list of games and versions they have certified.

What a certificate does and does not prove

A certificate or seal on a casino site tells you that a lab examined a specific version of a game or system at a point in time. It is a useful signal, and its absence on a site that claims to be licensed is worth questioning.

Certification also does not tell you about bonus terms, withdrawal practices or customer support. Those are judged separately, as described in our methodology.

What you can check yourself

1. Confirm the casino holds a licence from a regulator and read what that licence covers. Our guide to casino licences explains the basics.
2. Look for a statement on game testing in the site's footer or fairness page, and check that it names a lab rather than only using a logo.
3. Open the game's info screen and note the RTP. Some providers offer several versions, so the casino's setting is the one that applies.
4. Read the terms for the game and for any bonus, since rules can restrict which games count.
5. Use complaint channels listed in the licence if something looks wrong.

Be aware that small samples of play cannot prove or disprove fairness. A few hundred spins say very little about a game that is built around a long-run average.

Where trust still matters

Testing covers the game, not the whole business. A fair slot at an operator that delays withdrawals or applies hidden terms is still a poor experience, and that is why we review those factors too.

If something seems off, such as results that contradict the stated rules, take screenshots and contact support in writing, then escalate to the regulator if the answer is unsatisfactory. As always, play only with money you can afford to lose, and use the tools on our responsible gambling page.