How a new online slot game is built and tested before launch

By Clinton Courier | November 13, 2026 |

A slot game on a UK licensed site has usually spent months in development before it goes live. Much of that process is set by the Gambling Commission. Knowing those steps makes it easier to see what’s already been checked by the time you try a new slot game at a licensed site.

It starts with the maths model

Before any design work begins, a studio builds the maths model. This sets the paytable, the volatility and the return to player (RTP) percentage. RTP is the theoretical share of all stakes a game returns over a very large number of spins.

A 96% RTP doesn’t mean you’ll get 96 credits back for every 100 staked in a session. It’s a long-term average across millions of game rounds, and a single session could land well above or below it. The outcome of each spin is still decided by chance.

Design and development

Once the maths is fixed, artists and developers build the game around it. That covers symbols, animations, sound and the rules shown on screen. Games are usually built in HTML5, so the same code can run on desktop and mobile.

UK rules shape this stage too. Since 31 October 2021, online Slots offered in Great Britain have had to meet game design requirements in the Gambling Commission’s Remote Technical Standards. These include:

A minimum of 2.5 seconds between the start of one game cycle and the next
No autoplay
No turbo, quick spin or slam stop features that shorten the time before a result appears
No celebratory sounds or graphics for a return that’s equal to or less than your stake
A clear display of your net position and the time elapsed during the session

Random number generator checks

Every spin outcome comes from a random number generator (RNG). According to the Commission’s testing strategy, RNG testing includes a review of the documentation, a source code review and statistical testing of the raw output. Test houses also assess systems against published technical standards, such as Gaming Laboratories International’s GLI-19 standard for interactive gaming systems.

Independent testing before release

Studios run their own quality assurance first. But for the British market, game and RNG testing must be carried out by an approved test house, and the report has to be submitted to the Commission before the game is released for play.

The test house checks the maths, the artwork and the rules as they’re displayed, along with the theoretical RTP. It also measures the actual RTP through simulation, emulation and manual testing. The final report records details including the RTP, the software number and a digital signature, so the version that goes live can be matched to the version that was tested.

Once submitted, the game is logged on the Commission’s games register and given a unique reference that operators use when they offer it.

Monitoring after launch

Checks don’t stop on release day. Licensees must monitor live RTP for both underpayments and overpayments by comparing the actual figure against the advertised one. The Commission notes that one measurement a month won’t account for a popular game, and gives daily measurements based on the last 30 days of play as an acceptable method.

Later updates that don’t affect fairness can go live without external retesting. They still have to be logged in change control records, and the process is reviewed in an annual audit by an approved test house.

What this means when you play

Any slot game on a UK licensed site has been through this process. A game’s RTP and rules can usually be found in its help or information section.

Testing confirms a game works as described. It doesn’t change the fact that every outcome is random and a payout is never guaranteed. Setting a budget for your bankroll before you start, and stopping once it’s reached, is one way to keep gaming in check. If you have concerns about your gaming, support is available from trusted organisations.