I want to adapt several creatures from AD&D/2e so I can use them in 5e. Is there an easy way to translate THAC0 (to hit AC 0) to the +Hit bonus in 5e? I'm looking for a formula or a chart if possible. Thanks.
Conversion document suggests using the HD
There is no direct one-to-one correspondence due to the "bounded accuracy" design principle of 5e. Instead of converting the THAC0, the simplest thing to do, would be to use the following method from the Conversion to the 5th Edition D&D document published by the WotC:
Attack roll modifiers are HD/2 + 2, up to +12.
Alternative make-shift attempt of converting directly from THAC0
If that is not what you want and you really desire to convert THAC0 without referring to HD or levels we can try to come up with a rough formula assuming that the to-hit bonuses progress similar to a fighter's progress.
- It is easy to convert THAC0 to the hit bonus in 3/3.5e: Simply subtract THAC0 from 20. So THAC0 of 15, would mean +5 bonus to hit.
- The fighters in 3/3.5e (as well as 2e) improved one point of hit bonus (or THAC0) for each level. In 5e, for every four levels proficiency bonus increases by one. Based on this, I would suggest something like: roundup((20-THAC0)/4)+1.
- Yet, there is one more complication: A monster's THAC0 also took into account its strength and other characteristics that the designers saw appropriate. In 2e, you cannot always figure out those details. I would suggest having at the 3.5e SRD for the 3.5e version of any given monster and do a conversion using them. Conversion from 3e to 5e is probably best left to a separate question.