For 5e AC, subtract their AC from 20 (so “7” ought to turn into 13, for instance). That is for AD&D 1 and a pair of, for B/X model video games subtract from 19 as an alternative.
For “to hit” bonus, add half degree or HD to 12. So HD 3 ought to turn into to hit 13.
That’s from the 5e conversion PDF that WotC put out some time again.
Alternatively for creatures that do have a full set of potential scores, you possibly can calculate their bonus the 5e method utilizing dex for ranged, str for mêlée, and the very best of dex and str for finesse and thrown. That is not a simple calculation (half the rating’s offset from ten, plus a fourth of (HD+7), even in the event you memorize that second half: 2 for 1–4, 3 for five–8 and so on. So a degree 2 dork with dex 16 would shoot at +5.
Notice that in 5e’s authentic monster guide, spell casting was derived from HD however proficiency bonus appeared derived from CR (in a sort of round logic that by no means made sense to me). Anyway, having numerous enjoyable wh Arden Vul 💁🏻♀️