Random number generators

A random number generator produces a constant stream of numbers.

By Ryan Jessop. Last checked 9 October 2026.

Key takeaways

A random number generator produces a constant stream of numbers.
One spin, step by step.
The generator cycles through thousands of numbers a second.
An approved test laboratory, before release.

Short answer: A random number generator produces a constant stream of numbers. The instant you press spin, the current number is taken and mapped to reel positions. The animation then shows a result already decided.

One spin, step by step

You press spin and the stake leaves your balance.
The game takes the generator's current number.
That number is mapped to a stopping position for each reel.
The win, if any, is calculated.
The reels animate to that result and the balance updates.

Why timing cannot be used

The generator cycles through thousands of numbers a second. Pressing a fraction earlier or later gives a different result, but not one you can aim for.

Who checks it

An approved test laboratory, before release. The operator then monitors real returns against the expected figure.

Where to play

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Keep reading: Slot finder: which slot should I play, Retriggers, Animal slots.

Slot figures are developers' published defaults, each confirmed against two sources; a casino may run a lower RTP. Offer terms dated 9 October 2026. How we check. 18+. Free help: 0808 8020 133 or safer gambling.

Frequently asked questions

How does a slot's random number generator work?

A random number generator produces a constant stream of numbers.

What is the maximum stake on a UK online slot?

£5 a spin, or £2 for ages 18 to 24.

Where do I find a slot's rules?

In its help or information screen.

Can I try it free?

Most slots have a demo mode after age verification.

Is any slot due to pay?

No. Each spin is independent.

Where can I get help with gambling?

The National Gambling Helpline, free and open 24 hours, on 0808 8020 133.

When is a slot's result decided?

The instant you press spin.

What is RTP?

The share of stakes a slot returns over a very large number of spins.

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