I have always preferred a fast way to figure out my characters and NPCs/monsters hit rolls during play. The faster and smoother the process the better a mechanic can be, the more useful it is to me. Now I am not going to debate the benefits of all ascending numbers versus descending scores (that is for another place and time). Rather what I am presenting is a simple conversion (that I expected to see in a recent publication but found lacking). The gem of the conversion is that it works both ways, thus allowing a wider access to material.
The above being said, a small commentary is needed concerning THAC0. This really is not difficult to neither grasp nor learn. It is simply a quick reference tool and has in all reality has been inherent in a particular set of mechanics since the creation of the first attack matrix. If referencing the following matrix is not your style, the simple conversion is twenty minus proficiency bonus equals to hit armor class zero (THAC0).
Now this is also assuming you are using descending armor classes. Again, this is not debating the pros or woes of either system, it is simply providing a conversion; which is done by taking the current system armor class and subtracting it from twenty. Both systems start with a base ten armor class for an unarmored target. Bonuses that would normally add to a targets armor class (like that from dexterity) are simply subtracted instead.
Hopefully the above will prove to be a useful tool for those that would use it. I have always likened the choice of ascending and descending to simply a matter of taste. Both have been around for a long time (I played in many a 1e game that used ascending armor classes back before it was called 1e). Personally I like the sound of the descending armor class and thus will continue to use it in my future games, but to each his own. Again it is only my hope to present a simple conversion tool. May your dice continue to roll and your imagination run ahead of them.
Copyright 2014 John Hazen