d12 Combat System

The d12 combat system is a (more) fluid battle model for D&D 3rd edition. It modifies the standard D&D combat rules to make combat more fluid and less turn-based. It also gives the poor, underutilized d12 some love.


The d12 combat system was designed to address the following issues with the D&D 3rd edition combat rules:

  1. Movement is extremely unrealistic

    1. Chases during battle work in a non-intuitive fashion

    2. Interrupting someone’s movement (e.g., tripping him as he runs past) is too difficult, and hence is never done in practice

  2. Interrupting spellcasting is much too difficult

  3. Initiative modifiers do not impact combat order enough (e.g., a fighter with +0 to initiative still has a 16.5% chance to act before a rogue with +8 to initiative)

The standard combat rules attempt to address these problems with prepared actions and attacks of opportunity, but those rules often fall short. The d12 combat system provides an integrated model for battle to occur in a more fluid fashion.


Each round of combat is divided into twelve distinct time units of one-half second each. Any action that a character takes during combat happens in one of those time units, or across a span of time units.


When combat begins, players roll initiative using a d12 (instead of the usual d20). The resultant value determines the time unit during which the character may begin to act. Characters achieving initiatives above 12 are able to act early, whereas characters whose initiatives are less than 1 are too slow to act during the earliest combat rounds.

If a character’s initiative is higher than 12, he earns one or more surprise rounds. To determine the number of surprise rounds earned, repeatedly subtract 12 from the character’s initiative until the value is between 1 and 12. The number of times 12 was subtracted corresponds to the number of surprise rounds earned. For example, if a character has a +7 initiative modifier and rolls a 9, he has an initiative of 16, which translates to a 4 (16 – 12) with one surprise round.

If a character’s initiative is lower than 1, he faces one or more penalty rounds. To determine the number of penalty rounds faced, repeatedly add 12 to the character’s initiative until the value is between 1 and 12. The number of times 12 was added corresponds to the number of penalty rounds faced. For example, if a character has a –3 initiative modifier and rolls a 1, he has an initiative of –2, which translates to a 10 (–2 + 12) with one penalty round.

For two characters with the same initiative, action order is determined by Dexterity scores as normal (and if the Dex scores are equal, by a coin flip).

A more detailed example of initiative follows:

Suppose Luet the rogue and Thorin the fighter enter combat against a pair of elementals–one air and one earth.

Luet has a +8 initiative modifier and rolls a 5 on her d12, yielding a 13 initiative. Thorin has a +3 modifier and rolls a 3, yielding 6. The earth elemental has a –2 initiative and rolls a 2, yielding 0. The air elemental has a +14 initiative and rolls a 12, yielding 26.

Luet’s initiative of 13 translates to 1, plus one surprise round. The earth elemental’s initiative of 0 translates to 12, plus one penalty round. The air elemental’s initiative of 26 translates to 2, plus two surprise rounds.

On the first surprise round, the air elemental begins to act at 2. No one else scored a high enough initiative to act this round.

On the second surprise round, the air elemental acts at 2, then Luet acts at 1. Thorin and the earth elemental have not yet acted and are still flat-footed.

Then the first regular round begins. Thorin acts at 6, followed by the air elemental at 2, followed by Luet at 1. The earth elemental does not act in the first regular round, as it is his penalty round.

On the second regular round, the earth elemental can finally act at 12. Thorin goes again at 6, the air elemental goes at 2, and Luet goes at 1. The combat continues in this fashion until it is complete.


Standard actions and move-equivalent actions take 6 time units to complete. Full-round actions take 12 time units to complete. Rules for character actions allowed per round are unchanged from the normal system (in particular, a character may take only one standard action per round in normal circumstances).

As a general rule, a character begins taking action on his initiative, and finishes the action when the action’s duration is complete. For example, when using a move-equivalent action to obtain an item from a backpack, the character spends six time units rummaging through the pack locating the item, and has the item in hand at the beginning of the next time unit.

When spellcasting as a standard action, the magic user is assumed to be locating material components, chanting and gesturing during the six time units. The spell finally goes off at the beginning of the following time unit (assuming the casting was successful). If the caster takes damage or is otherwise distracted during the casting time, he must make a Concentration check as per the normal rules.

An exception to the rule is the attack action. A character who attacks deals the blow on the first time unit of his action, then spend the remaining five time units recovering, defending and regaining his balance. Because attacks happen more quickly, it becomes much more feasible for an intelligent warrior to interrupt a magic user during spellcasting.

For the full attack action, use the following table to determine the time units upon which attacks fall:

# of attacks 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th
1 X - - - - - - - - - - -
2 X - - - - - X - - - - -
3 X - - - X - - - X - - -
4 X - - X - - X - - X - -
5 X - X - - X - - X - - X
6 X - X - X - X - X - X -
7 X X - X - X - X - X - X
8 X X - X X - X X - X X -
9 X X X - X X - X X - X X
10 X X X X - X X X - X X X
11 X X X X X X - X X X X X
12 X X X X X X X X X X X X

For example, a character taking the full attack action with three attacks per round whose initiative is 7 takes his first attack at 7, his second attack at 3, and his third attack at 11 the following round.


When a character takes a move action, he moves at his movement rate, but unlike standard D&D combat rules, the move happens continuously over the course of its duration. A normal move takes 6 time units, and a double move takes 12 time units. Charging takes 12 time units, but the attack happens as soon as the area being charged is reached. Running in a straight line (at 4x) takes 12 time units, and the character’s movement rate is treated as doubled for that round.

The following table converts normal D&D movement rates into “squares per time unit” measurements. For example, with the table below, it is easy to see that a character with a movement rate of 45 covers three five-foot squares every two time units (or 6.67 feet per half-second).

Movement rate Feet per time unit Squares moved
10 1.67 1/3
15 2.5 1/2
20 3.33 2/3
25 4.17 5/6
30 5 1/1
35 5.83 7/6
40 6.67 4/3
45 7.5 3/2
50 8.33 5/3
55 9.17 11/6
60 10 2/1
65 10.83 13/6
70 11.67 7/3
75 12.5 5/2
80 13.33 8/3
85 14.17 17/6
90 15 3/1