For my BECMI D&D campaign I want to move more 3d6 roll low GURPS-like without full GURPS. I've already done this for Turn Undead, Thief Skills, and General Skills (from Rules Cyclopedia).


THAC0 17, AC 7 is 50% to hit (Fighter 4 has THAC0 17)

Equivalent 3d6 Skill = 7 with modifier = +(AC/2)

Skill 7, AC -4/-5 = 4.6% (3d6) vs 5% (d20)
Skill 7, AC -2/-3 = 9.3% vs 10%
Skill 7, AC 0/-1/1 = 16.2% vs 20%
Skill 7, AC 2/3 = 25.9% vs 30%
Skill 7, AC 4/5 = 37.5% vs 40%
Skill 7, AC 6/7 = 50.0% vs 50%
Skill 7, AC 8/9 = 62.5% vs 60%
Skill 7, AC 10 = 74.1% vs 70%


THAC0 19 = Skill 6 (starting characters have THAC0 19)

Skill 6, AC -4 = 1.9% vs 5%
Skill 6, AC -2 = 4.6% vs 5%
Skill 6, AC 0 = 9.3% vs 10%
Skill 6, AC 2 = 16.2% vs 20%
Skill 6, AC 4 = 25.9% vs 30%
Skill 6, AC 6 = 37.5% vs 40%
Skill 6, AC 8 = 50.0% vs 50%
Skill 6, AC 10 = 62.5% vs 60%


Seems ok until AC drops below -2/-4 which would be a rather high-level monster. The other thing is having 3 or 4 (on 3d6) be auto-hit and 17 or 18 be auto-miss (I don't yet do critical hits or misses).

This is a one-roll to hit system unlike GURPS attack then defense roll. Also there are no maneuvers or tactics like trading attack skill to lower enemy's defense skill.

There is also the part in the D&D to-hit chart where 20 repeats 5 times before going to 21 and up. It's sort of a "20 always hits unless the AC is truly good" but I've never really liked that discontinuity. In my 3d6 to hit system I ignore it since I do want that there's always a chance to hit.