The basic rules of Call of Cthulhu.
Call of Cthulhu runs on a single gesture, repeated all evening: two ten-sided dice, a percentage written on the sheet, and a result you need to roll under. Searching a library, lying to a police officer or shooting at whatever comes out of the cellar are all settled exactly the same way.
Here are those rules gathered in one place, in the order they actually come up at the table. The d100 and the three success thresholds, the eight characteristics, bonus dice, pushed rolls, combat and Sanity.
The rule that governs everything.
Every action with an uncertain outcome is settled the same way: you roll two ten-sided dice for a result from 1 to 100, and you succeed if that result is equal to or lower than the value of the skill you are using. A skill of 60 therefore succeeds 60 times out of 100, and the number on the sheet is your chance of success, plain and simple.
Difficulty does not change the dice, it changes the threshold. An ordinary action asks you to roll under the full value, a hard one under half of it, an extreme one under a fifth. A 01 is a critical success, a 100 a fumble, and so is any result from 96 to 99 when the skill sits below 50. One percentage, one roll, nothing else to remember.
The eight characteristics.
It sets melee damage and what you can force open. Combined with Size, it gives your damage bonus.
With Size, it sets the investigator's hit points. It also resists poison, disease and exhaustion.
It weighs on hit points and on the damage bonus, but also makes you visible. A large investigator hides badly.
It gives the Dodge skill, at half its value, and covers every fast movement. It often decides whether you get away.
It has no effect in a fight, but opens doors and eases interviews. It serves the character, not the sheet.
It provides the Idea roll, that nudge when the table is stuck. It also grants your personal interest skill points.
Starting Sanity equals Power, and Luck often derives from it. The most precious characteristic, and the least visible.
It sets how many points the occupation gets to spend. A professor starts with far more skills than a drifter.
The rolls and their thresholds.
You roll percentiles and want to come in under the skill value. Fifty in Library Use means one chance in two of finding it.
The Keeper calls for it when conditions are clearly against you: you need under half the value. A skill at sixty becomes thirty.
Reserved for feats, it asks for a fifth of the value. In combat it often doubles or maximises damage.
A bonus die adds a second tens die and you keep the better result. Penalty dice do the reverse, and the two cancel one for one.
After a failure you describe how you press on and roll once more. If that second roll fails, the Keeper is entitled to be cruel.
You spend Luck points one at a time to close the gap to the threshold. They do not come back on their own, so every point counts.
Combat and wounds.
Characters act in descending order of Dexterity. A melee attack is an opposed roll: the attacker rolls their skill, the target chooses to dodge or fight back, and the better level of success wins the exchange. Firearms are resolved with a plain roll instead, shaped by range, cover and whether the target is standing still.
Hit points are Constitution plus Size, divided by ten. An investigator holds around ten of them, which is two bad encounters. A wound that takes away half that total in one blow is a major wound: it calls for a Constitution roll to stay conscious, and death stays on the table until someone provides treatment. Nobody walks this off overnight.
Sanity, in numbers.
Starting Sanity equals Power, and its ceiling drops as the Cthulhu Mythos skill climbs: understanding the world means losing the ability to live in it. Every unbearable sight calls for a roll against that score, with a small loss on a success and a markedly heavier one on a failure.
Losing five points or more at once triggers a bout of temporary madness, lasting a few scenes. Losing a fifth of your Sanity within the same day tips the investigator into indefinite madness, which leaves a lasting mark on the sheet. You recover slowly, by closing a case or following treatment, never fast enough to keep up with what you keep finding out.
The rules people get wrong.
If the case stops without that clue, there is no roll to make. The roll is for getting it faster, better, or unnoticed.
The Keeper announces the threshold before the roll, never after. Calling it hard once the die has landed breaks the table's trust.
A pushed roll is not a free second chance: failure has to carry a concrete consequence. Otherwise everyone pushes everything.
You never add two tens dice together, you keep whichever helps. Three bonus dice are three dice to choose from, not a total.
Luck points hoarded to the end of the scenario did nothing at all. They exist to turn a failure into survival.
It comes back by solving the case, by resting or through a psychiatrist. A campaign that only subtracts it burns itself out.
Applying the rules online.
Working out half a skill, then a fifth, then a Sanity loss, then a major wound threshold while the scene waits is the surest way to break an atmosphere that was hanging by a thread.
Tomeus gathers the sheets, the dice roller, the handouts and the music in a single window. You create the game, send the link, and your players arrive with their investigator in front of them, nothing to install and nothing written on a scrap of paper that goes missing.
Go further with Call of Cthulhu.
How to Start Playing Call of Cthulhu
Free quick-start rules, percentage skills, Sanity and a first investigation: the complete guide to start playing Call of Cthulhu in one evening.
The Lore of Call of Cthulhu
The Cthulhu Mythos, its places, its creatures, its Great Old Ones and its grimoires: the lore of Call of Cthulhu explained, and how to use it at your table.
Call of Cthulhu occupations
Call of Cthulhu has no classes but occupations: antiquarian, private investigator, journalist, doctor, their skills, skill points and Credit Rating explained.
Call of Cthulhu Great Old Ones
Azathoth, Yog-Sothoth, Nyarlathotep, Cthulhu, Hastur: the Outer Gods and Great Old Ones of the Mythos, their cults and what they cost in Sanity.
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How to Prepare a Tabletop RPG Campaign
Setting, central stake, factions, NPCs and per-session prep: the method to build a tabletop RPG campaign without writing everything in advance.
The Best Tabletop RPGs for Beginners
Dungeons & Dragons, Call of Cthulhu, Alien, Monster of the Week: six easy tabletop RPGs for a first session, and how to pick the right one.
How to Create a Tabletop RPG Character
Concept, class, ability scores, skills, backstory and bonds: the full method to create a tabletop RPG character who will carry a whole campaign.
Frequently asked questions.
Where can you find the Call of Cthulhu rules for free?
The publisher releases free quick-start rules online. They hold the complete system, ready-made investigators and everything needed to run a full first investigation. The core book mainly adds occupations, creatures and options, not a different system.
Which dice do you need to play?
Two ten-sided dice cover every test, one for the tens and one for the units. A third die of the same kind comes in whenever a roll grants a bonus or a penalty die. Damage then calls for the usual dice, a d4, a d6 or a d8 depending on the weapon.
How does pushing a roll work?
After a failure you may try again once, provided you describe what your investigator does differently: pressing on, taking their time, taking a risk. The second failure does not simply miss: the Keeper turns it into a real consequence, an accident, a door giving way, or something that has now heard you.