THAC0 for 5e

Many things have changed over the long and storied history of Dungeons & Dragons. Some changes were bad. Some changes were good. And some changes were so good that few people ever look back on the old rules with any semblance of nostalgia.

THAC0 is one of those. While it actually makes a great deal of sense from a mathematical point of view, the d20 system accomplished most of what it meant to do with quite a bit more modularity and elegance. Still, some people (not many, mind) remember THAC0 fondly, if only as proof that D&D used to have a barrier to entry, which makes them feel good about themselves for having played it back in the day.

To those people who wish to relive those days, I present this document. If you choose to use it, do so at your own risk.

Calculating Armor Class

"THAC0" stands for "To Hit Armor Class 0." As the name suggests, the system is primarily concerned with how the target's defense factors into the calculation.

To calculate AC under this system, use the standard rules for 5e D&D, with the following changes:

Whenever a plus (+) appears in an AC calculation, replace it with a minus (-).

Whenever the initial number in the calculation is not 10, subtract 10 from it. Then, subtract the resulting number from 10. This is the new value.

To calculate monster AC, simply apply rule 2.

AC Calculation Examples The AC of a character wearing leather armor (normal calculation: 11 + Dex modifier) is now 9 - Dex modifier.

The Unarmored Defense AC of a monk (normal calculation: 10 + Dex modifier + Wis modifier) is now 10 - Dex modifier - Wis modifier.

A shield reduces your AC by 2.

A Solar (normal AC: 21) now has an AC of -1.

Calculating THAC0

Now that we know everybody's new AC, it's time to figure out what number we'd need to hit it if it were 0.

Sensible THAC0

Use this rule if you intend to preserve 5e's balance as intended by the designers (hah, lame).

Under this simple system, a character's THAC0 is equal to 20 - their Proficiency Modifier.

Those of you who are not braindead will be asking about ability score modifiers right about now. They will be added at a different point in the process than players of newer editions are used to. More on that in a bit.

Single-Class THAC0

Use this rule if you want to recreate the experience of playing older editions in greater detail. Alternatively, if you agree with the old philosophy that training from different classes is often incompatible.

Under this mildly complex system, you calculate THAC0 as normal, but only use whatever class you have the most levels of. This is because multiclassing is for munchkins and elves. But I repeat myself.

Classic THAC0

For the truly discerning grognard, accept nothing but this rule. It is as close as you will get to the clusterfuck that was AD&D, unless you're even more bored than I am right now.

Locate your class in the table below. If you have multiple classes, locate all of them. Find your class level(s) and figure out the THAC0 it gives you. If you have multiple classes, use the lowest value.

If you have multiple classes belonging to the same class group, only use the highest-level of the bunch.

THAC0 By Class Group