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24XX is a tiny tabletop RPG system by designer Jason Tocci. A complete 24XX game fits on the two sides of a single sheet of paper, and every roll works the same way: roll one die for your skill, add dice for help, and keep the highest. Tocci built it for his 2400 series of science fiction microgames, then released the rules as a free System Reference Document (SRD) under a Creative Commons license. Hundreds of other designers have since used it to make their own 24XX RPG games.
The quick answer: 24XX is a rules-light system where you roll a d6 by default, a d8, d10, or d12 if you are skilled, or a d4 if you are hindered. Keep the single highest die. On 1 or 2 it goes badly, on 3 or 4 you get a partial success, and on 5 or more you succeed. The SRD is free on itch.io under CC BY 4.0, so anyone can build and even sell a game on it with credit.
What is 24XX, exactly?
The name covers two related things, which confuses people at first.
| Name | What it is | Who makes it |
|---|---|---|
| 2400 | A series of about twenty microgames, mostly sci-fi, each a complete game on two sides of a sheet | Jason Tocci |
| 24XX | The free SRD: the shared rules behind 2400, released for anyone to use | Jason Tocci |
| 24XX games | Games other designers build with the SRD | Hundreds of designers on itch.io |
Think of 2400 as the band and 24XX as the chord chart anyone can play. Tocci’s own games are mostly science fiction, but the SRD page says plainly that you can use it for any setting, and the community has made fantasy, horror, mystery, and slice-of-life games with it.
Tocci bills the 2400 collection as “lo-fi sci-fi.” That is a good summary of the feel: fast, loose, and focused on what characters do rather than on numbers.
How does the 24XX dice mechanic work?
Here is the whole core of the game, in our own words.
1. Say what you do, and hear the risk
Players describe their actions. The game master (GM) says if something is impossible, what it will cost, or what could go wrong. You hear the risk before you commit, and you can change your plan. You only roll when there is a real risk.
2. Pick up your skill die
| Your skill | Die you roll |
|---|---|
| Hindered (hurt, overloaded, bad conditions) | d4 |
| No relevant skill | d6 |
| Skilled | d8 |
| More skilled | d10 |
| Most skilled | d12 |
3. Add dice for help
- Helpful circumstances (the right tool, a good position) add a d6.
- An ally who helps rolls their own skill die alongside yours. The catch: a helper shares the risk, so if it goes wrong, it goes wrong for them too.
4. Keep the highest single die
No adding, no modifiers, no target numbers. Look at the best die on the table.
| Highest die | Result | What happens |
|---|---|---|
| 1 to 2 | Disaster | You suffer the full risk, and the GM decides whether you get anything at all |
| 3 to 4 | Setback | A lesser consequence, or a partial success |
| 5 or more | Success | You do it; higher rolls are better |
When the risk is death, the stakes rise: a disaster means death and a setback leaves you maimed. Because you always hear the risk first, you can choose another approach before it comes to that.
What are the odds?
This is where the bigger dice matter. Here are the chances for a single skill die, and with one extra d6 from help or circumstances:
| Skill die | Disaster | Setback | Success | With +d6: Success |
|---|---|---|---|---|
| d4 (hindered) | 50% | 50% | 0% | 33% |
| d6 | 33% | 33% | 33% | 56% |
| d8 | 25% | 25% | 50% | 67% |
| d10 | 20% | 20% | 60% | 73% |
| d12 | 17% | 17% | 67% | 78% |
Two things jump out. A hindered character cannot succeed alone, so a friend or a good tool is the way out of trouble. And one extra d6 is worth more to an unskilled character than a skill upgrade, which pushes the table to work together.
What else is in the 24XX rules?
The SRD is short, but it covers the parts a campaign needs:
- Load. Carry what makes sense. Too many bulky items can hinder you.
- Defense. When you would take a hit, you can break a piece of gear instead. The blow becomes a brief hindrance, and the item is useless until repaired.
- Harm. Injuries need time or medical care. If a character dies, the player makes a new one and joins quickly.
- Advancement. After each job, raise one skill a step (d6 to d8 to d10 to d12) and roll a d6 for credits.
- Bad luck. When trouble might arrive and nobody is acting, roll a die: 1 or 2 means trouble, 3 or 4 means signs of it, and anything higher means all clear.
- GM principles. Describe threats by what they do, not by stats. Agree on lines the table will not cross. Present dilemmas without a single right answer. Make rulings and revisit them during breaks.
In the SRD’s default sci-fi setup, characters pick a specialty (Face, Muscle, Psychic, Medic, Sneak, or Tech) and an origin (Alien, Android, or Human), choose a few skills, and grab starting gear. It takes a few minutes.
How is a two-page 24XX game structured?
Tocci’s 2400 games each fit on two sides of a letter-size sheet. The SRD gives designers the same building blocks. A typical 24XX game uses its space like this:
| Section | What it holds |
|---|---|
| Premise | Two or three sentences: who you are, and why you stick together |
| Characters | Specialties or roles, origins, a skill list, and starting gear |
| Rules | The core roll, load, defense, harm, and advancement, often trimmed or tweaked to fit the setting |
| GM advice | A few principles for running the game |
| Tables | Random tables for names, places, jobs, and complications |
The tables do the heavy lifting. The SRD notes that half of a letter-size sheet can hold four tables of 20 entries each. Roll once on each and you have a job for tonight: a client, a target, a twist, and a place.
This is why 24XX games are so easy to run. The GM needs no prep beyond a few rolls, and players need no more than the one rule about keeping the highest die.
What are the best 24XX games to try?
Start with Tocci’s own 2400 series, sold as a bundle of about twenty games on itch.io. Two are free demo downloads, which makes them the obvious first step.
| Game | Premise | Why try it |
|---|---|---|
| Emergency Rules | A slightly expanded version of the core rules and principles | Free; the best reference for running any 24XX game |
| Inner System Blues | Cyberpunk freelancers in a grainy retro-future | Free; a complete game and a great first session |
| Cosmic Highway | Space truckers keep a battered ship flying | A working-class crew with a ship to keep running |
| The Venusian Job | A casino heist in the clouds of another world | A ready-made one-shot caper |
| Orbital Decay | A generator for space-horror scenarios | Tables that build a horror night for you |
| Junior Hybrid Battle Cryptids | Animal heroes save the city | Lighter and sillier, good for mixed groups |
| Legends | A classic high-fantasy world overseen by DEUS | Proof the system works outside sci-fi |
Beyond Tocci’s own work, the community has built hundreds more. The 24XX page on itch.io links to a community collection of more than 200 games from more than 100 designers, plus a 24XX Jam that drew over 60 entries. Search “24XX” on itch.io and sort by rating to find settings in nearly every genre, from westerns on Mars to fantasy and horror reskins.
Can you try 24XX in your browser?
Yes. The Ludarium’s RPG Demo Library has a free 24XX demo called “The Silence at Well Nine”. It is a rescue mission: fourteen people work the pumps at a station four kilometers under the ice, they stopped answering thirty-one hours ago, and now their voices are counting down on the radio.
The page plays the GM and you make the choices. Every roll follows the 24XX rules: you pick your skill die, decide whether to bring in a helper who shares the risk, break gear to absorb a hit, and watch an adventure clock called the Count tick toward zero. Because it is solo, it uses an “Ice Oracle” for questions nobody at the table can answer, rolled with the same keep-the-highest dice.
Act One is free and takes about 20 minutes. You can show or hide rule notes as you play, and choose whether to click each roll yourself. The full three-act adventure is a PDF for group play, available in our store. The demo is built on the 24XX SRD and is not an official 24XX product or part of the 2400 series.
How do you make your own 24XX game with the SRD?
This is the best part of 24XX. The SRD exists so you can make your own game, and the license lets you share or sell it.
1. Download the SRD
It is free (pay what you want) on Jason Tocci’s itch.io page. The download includes the plain-text rules plus layout templates for Affinity Publisher, InDesign, and Google Docs and Slides, so you do not have to design a page from scratch.
2. Pick a premise with a reason to stick together
The SRD asks you to explain the setting and give characters a reason to work as a group. “Night-shift paramedics in a haunted city” is a complete premise. So is “Retired heroes running a bakery.”
3. Rename the specialties and skills
Swap Face, Muscle, and Tech for roles that fit your world. A cozy fantasy hack might use Baker, Brewer, and Hedge Witch. Keep the skill list short; the SRD’s default has seventeen.
4. Write your tables
This is where your setting comes alive. Four d20 tables (who needs help, what they need, what goes wrong, where it happens) give a GM endless sessions.
5. Credit the rules and follow the license
The SRD is released under Creative Commons Attribution 4.0 (CC BY 4.0). You must credit it, and the SRD suggests a line like “24XX rules are CC BY Jason Tocci.” You can say your game is compatible with 2400, but do not dress it up as part of the official 2400 series. The SRD also asks that its text, logo, and Tocci’s name never appear in work that promotes bigotry.
6. Playtest, then publish
Run it for friends who have never seen it. Then post it on itch.io, or enter a 24XX or one-page game jam. Our guide to one page RPGs covers jams and where small games find an audience.
Is the 24XX RPG good for beginners?
Very. If you are brand new to the hobby, our tabletop RPG explainer covers the basics first. New players learn one rule, “roll your die and keep the highest,” and the risk is always spelled out before they roll. Nobody does math.
It does ask more of the GM than a heavier game. The SRD is brief by design and expects you to fill gaps with judgment calls. If that sounds daunting, play the browser demo first to see the rules in action, or start with a game whose premise does more of the work, like Lasers & Feelings. Our guide to easy tabletop RPGs for beginners compares 24XX with seven other simple systems.
FAQ
Who made 24XX?
Jason Tocci, an independent designer who also wrote the 2400 series of sci-fi microgames. 24XX is the system behind those games, released as a free SRD.
Is the 24XX SRD free to use commercially?
Yes. The SRD is released under CC BY 4.0, which allows commercial use as long as you give credit, for example “24XX rules are CC BY Jason Tocci.” Do not present your game as part of the official 2400 series.
What dice do you need for 24XX?
A set of d4, d6, d8, d10, and d12. Extra d6s help, since helpful circumstances add one. A standard polyhedral dice set covers it.
How is 24XX different from Lasers & Feelings?
Lasers & Feelings uses one number and counts successes across several d6. 24XX gives you a die size per skill and keeps the single highest roll. 24XX also has gear, injuries, and skill growth, so it handles short campaigns better.
Can you play 24XX solo?
Some 24XX hacks include solo rules, and the SRD’s random tables suit solo play well. Our 24XX demo is a solo adventure with an oracle for answering questions, and our guide to solo RPGs covers how solo play works in general.
Roll the highest die
24XX proves a whole RPG can fit on two sides of a page and still feel like a real adventure. Play a scene of “The Silence at Well Nine” to feel the roll, then grab the free SRD and write the setting only you would think of. For more small games, read our guide to one page RPGs, and Ludarium Letters, just below, sends guides like this to your inbox.
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