How online slots are made
How online slots are made starts with a spreadsheet as much as a sketch. A studio pairs a theme and a feature idea with a maths model that fixes the RTP, volatility and max win, builds the art and sound around it, runs the outcomes on a remote game server, has the whole package tested, then ships it to casinos through distribution platforms.
By the Best Slots UK team. Last checked .
Key takeaways
- The maths model, not the artwork, decides how a slot pays: reel strips, paytable and feature odds.
- Results are generated on the studio's game server; your screen only displays them.
- Every slot sold in Great Britain needs independent testing and a GB-compliant build before release.
- Studios usually produce several RTP versions, and each casino chooses which one to run.
- Core document
- The maths model
- Where results are made
- The studio's remote game server
- Front end
- HTML5, for phones and desktops
- Before UK release
- Independent test house report
- Route to casinos
- Direct or via aggregators
How online slots are made: the production line at a glance
A modern online slot is a small software product with several specialist teams behind it. Game designers come up with the concept, mathematicians build the payout model, artists and animators create the look, sound designers score it, developers code the client and server, and compliance staff prepare it for each regulated market. The stages overlap, but they usually run in this order.
- Concept: theme, core mechanic and target audience.
- Maths model: paytable, reel strips, feature rules, RTP, volatility and max win.
- Art, animation and sound built around the mechanic.
- Development: the game client players see and the server logic that produces results.
- Internal testing: simulation, quality assurance and checks against market rules.
- Independent certification by an approved test house for each regulated market.
- Distribution to casinos and configuration of the RTP version each one runs.
- Launch, followed by live monitoring and any updates.
Concept: theme and mechanic come first
Most new slots start with a pairing: a theme people recognise and a mechanic that gives the game its rhythm. A fishing theme with a cash-collecting wild became the Big Bass series; a candy theme with pay-anywhere tumbles became Sweet Bonanza. The mechanic matters more than it looks. It decides how the screen behaves, how features trigger and how much room the maths has to create big outcomes.
Studios also design for a particular kind of player. A low-volatility fruit game, a high-volatility collector slot and a branded TV game each need different maths and different pacing. Some studios are known for a signature mechanic: Big Time Gaming for Megaways, Nolimit City for its xWays family, and ELK Studios for Avalanche and walking wilds. Our slot themes explained guide looks at why certain themes keep coming back.
The maths model: where a slot's behaviour is decided
The maths model is the heart of the game. It sets the paytable, the virtual reel strips (the long list of symbols each reel can land on), the rules and odds for each feature, and the resulting RTP, hit rate, volatility and max win. Change any one and the others move.
A reel strip worked example
Imagine a 5 × 3 game where each reel's strip has 50 positions, and reels 1, 3 and 5 each contain 3 scatter symbols spaced well apart. With 3 rows visible, the chance of a scatter showing on one of those reels is about 3 × 3 ÷ 50, or 18%. The chance of scatters on all three reels at once is 0.18 × 0.18 × 0.18, about 0.58%, or roughly 1 spin in 171. If the designer wants the feature rarer, say 1 in 250, they remove a scatter from one strip or add blanks. If they want it more frequent, they do the opposite.
Splitting the RTP budget
The designer also decides where the return comes from. A 96% game might be split like this:
| Source of return | Share of total RTP | Effect on play |
|---|---|---|
| Base game line wins | 62% | Frequent small wins that keep a balance ticking over |
| Free spins feature | 30% | Rarer, larger results |
| Other features and wild multipliers | 4% | Occasional boosts |
| Total | 96% |
Shift more of the return into the feature and the game becomes more volatile. The mathematicians check the model by simulating the game over a very large number of rounds, then compare the simulated return with the theoretical figure. Our guides to how slot maths models work and how slot RTP is calculated go deeper into the numbers.
Why studios build several RTP versions
Most studios don't ship one model. They produce a family of versions of the same game, typically a top version and one or more lower ones a couple of points apart, by adjusting reel strips or feature odds. A casino chooses which version to run. That's why Big Bamboo is listed at 96.13% with a 94.13% version also available, and Eye of Horus Megaways at 95.49% with 90%, 92% and 93% versions too.
For you, this means the developer's headline RTP is a maximum, not a promise about every casino. The live figure for the version in front of you is shown in the game's help screen. Our slot RTP versions guide explains how to find it.
Art, animation and sound
Once the maths is stable, the visual team builds the symbols, backgrounds, win animations and feature screens. Each symbol has to read clearly at phone size, so high-value symbols usually get the boldest colours and the most detail. Animations are timed to the game's pacing, which in Great Britain has to fit the minimum 2.5 seconds per spin.
Sound is more deliberate than most players notice: reel stops, anticipation cues and win jingles all shape how a game feels. UK rules affect this too, because games can't celebrate a return that is smaller than the stake. The slot sounds and win celebrations guide explains the rule and why it exists.
The technology: game client and remote game server
An online slot has two halves. The client is the part you see: an HTML5 application that runs in a browser or inside a casino app, draws the reels and plays the animations. The server side, often called a remote game server, holds the maths and the random number generator. When you press spin, the client sends the request, the server produces the outcome and records it, and the client then animates a result that has already been decided.
| Component | Runs where | Job |
|---|---|---|
| Game client | Your browser or app | Shows reels, animations, balance and controls |
| Casino platform | The operator's systems | Holds your account and balance, applies limits |
| Remote game server | The studio's or supplier's systems | Generates the result with the RNG and the maths model |
| Game history log | Server side | Records each round for disputes and audits |
This split is why the spinning reels can't influence the result, and why your game history can be checked if anything goes wrong mid-spin.
Building a version for Great Britain
Great Britain has some of the strictest slot design rules in the world, so studios often maintain a GB build of each game. Compared with versions for other markets, it typically has:
- no autoplay and no turbo, quick-spin or slam-stop options;
- a minimum of 2.5 seconds per spin;
- no feature buy button;
- no celebration of returns smaller than the stake;
- a visible balance and net position throughout play;
- stake ranges that work within the £5 per spin limit for players aged 25 and over and £2 for those aged 18 to 24.
The UK slot game design rules page explains each requirement and when it came in.
Testing and certification before launch
Before a new slot can go live in Great Britain, the Gambling Commission's testing strategy requires the game's RNG and its rules and design to be checked by an approved independent test house. That review covers the maths, the artwork, the rules shown to players and the theoretical RTP, using simulation, emulation of rare outcomes and manual play. The test report goes to the Commission before release.
Studios also test internally for bugs and usability. Changes made after launch are sorted into major and minor updates; anything that could affect fairness needs external retesting. The full process is covered in our guide to how online slots are tested in the UK.
Getting a slot into casino lobbies
A finished slot reaches casinos in one of two ways. Larger studios connect to operators directly. Many games travel through aggregators and partner programmes, which bundle titles from several studios behind a single integration. Games Global, for example, distributes more than 1,300 titles from over 40 in-house and partner studios; Relax Gaming runs Silver Bullet and Powered By programmes for partner studios; Yggdrasil runs YGG Masters; and Pragmatic Play distributes Reel Kingdom's Big Bass games.
The studio, the distributor or the casino then chooses the RTP version, the stake range and the lobby placement. That's why the same game can appear at different RTPs on different sites. Our slot providers hub profiles the main studios and how their games reach UK players.
After launch: monitoring, updates and sequels
Release isn't the end. Operators must compare each live game's actual return with its advertised RTP, watching for both underpayment and overpayment, and investigate unusual numbers of complaints. Bug fixes and graphical tweaks continue, under the major and minor update rules. And if a game proves popular, the studio may revisit it with a sequel, a Megaways remake or a 1000 edition, as covered in our guide to how slot sequels and 1000 editions differ.
| Stage | Led by | Main output |
|---|---|---|
| Concept | Game designers and producers | Theme, mechanic and feature outline |
| Maths | Mathematicians | Paytable, reel strips, RTP versions, max win |
| Art and sound | Artists, animators, sound designers | Symbols, screens, animations, audio |
| Development | Client and server developers | Playable build and server logic |
| Certification | Approved test house | Test report for each market |
| Distribution | Studio, aggregators, casinos | Live game at chosen RTP and stakes |
| Live operation | Operators and studios | RTP monitoring, updates, sequels |
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Frequently asked questions
Who decides how often a slot pays out?
The studio's mathematicians decide it when they build the maths model, before the game is released. They set the paytable, the symbols on each reel strip and the odds of each feature, which together fix the RTP, hit rate and volatility. Independent testers then check that the software behaves as the model says. The casino can choose between RTP versions the studio supplies, but can't change how a version pays once it's live. The walkthrough on how slot RTP is calculated explains the maths.
How long does it take to make an online slot?
It varies widely by studio and game. A follow-up that reuses an existing engine and maths approach can be quicker to produce than a game built around a brand-new mechanic, which needs fresh maths, art and code. Every version also needs independent testing before release in Great Britain, which adds time. Studios rarely publish exact timelines, so treat any single figure with caution. Our page on how to judge a new release covers what to check once a game is out.
Are slot results decided on my phone or on a server?
On a server. The game you see on your phone or computer is a client that displays results. When you press spin, it asks the studio's remote game server for an outcome, the server generates it with the random number generator and the maths model, records it, and sends it back to be animated. That's why the reels' movement can't affect the result. The how a slot RNG works guide explains the random number side of the process.
Can a casino change a slot's RTP?
A casino can choose which of the studio's certified RTP versions to run, but it can't adjust the maths of a version or change it during play. UK technical standards ban changing rules, payouts or probabilities while a game is available, unless the rules allow it and players are told. In practice, that's why the same slot can show 96.5% at one site and 94.5% at another. The figure in the game's help screen is the one that applies. Our page on why RTP differs between casinos explains more.
Do studios make different versions of slots for the UK?
Yes, many maintain a GB-specific build. It removes autoplay, turbo and quick-spin options, enforces a minimum of 2.5 seconds per spin, takes out any feature buy button and stops the game celebrating returns below the stake. It also has to show your balance and net position throughout and work within the £5 and £2 stake limits. The theme and features stay the same. The UK slot game design rules guide covers each requirement.
What is a reel strip in an online slot?
A reel strip is the full list of symbols a reel can show, in order, even though you only see three or four at a time. Online slots use virtual reel strips stored in the maths model, which can be much longer than a physical reel. The number of each symbol on a strip decides how often it lands: fewer scatters means a rarer feature. The RNG picks a stopping point on each strip for every spin. The slot symbols explained guide covers what each symbol type does.
Who makes the most popular online slots in the UK?
A mix of large groups and specialist studios. Pragmatic Play makes Gates of Olympus, Sweet Bonanza and, through its partner Reel Kingdom, the Big Bass series. Play'n GO makes Book of Dead, Blueprint Gaming makes Fishin' Frenzy, Barcrest makes Rainbow Riches, and NetEnt makes Starburst. Many of these games reach UK casinos through aggregators as well as direct deals. Our slot providers hub profiles each studio with its best-known games.
Why do so many slots look the same?
Because proven themes and mechanics are less risky to build on. Once a theme like ancient Egypt or a mechanic like cascading pay-anywhere wins succeeds, other studios produce their own takes, and the original studio adds sequels. Underneath similar artwork, though, the maths can be very different, from low-volatility games with small caps to extremely swingy ones. Comparing RTP, volatility and max win tells you more than the theme does. Our slot themes explained guide looks at why certain themes keep returning.
Are slots tested before they go live in the UK?
Yes. Before release in Great Britain, the game's random number generator and its rules and design must be tested by an approved independent test house, and the report has to reach the Gambling Commission before the game goes live. Operators then monitor each game's actual return against its advertised RTP. Changes that could affect fairness need retesting. Our page on who tests UK slots lists the approved test houses and what they check.