OSR / One-Page Dungeon → D&D 5e Quick Conversion

Fast conversions for old-school adventures. Convert only what you need.

1. Hit Dice → HP

HD5e HP
½ HD5
1 HD10
2 HD18
3 HD27
4 HD36
5 HD45
6 HD55
HP ≈ HD × 9

2. AC Conversion

Old AC5e AC
910
712
514
316
019
-221
5e AC = 19 − Old AC

4. Save DCs

ThreatDC
Minor11
Normal13
Dangerous15
Boss17

CON saves for poison, paralysis, etc.

3. THAC0 → Attack Bonus

THAC0Attack Bonus
19+1
17+3
15+5
13+7
11+9
Attack Bonus = 20 − THAC0

Quick Table Rules

  • Keep damage mostly the same as written. Add +2 damage if the monster should feel scary in 5e.
  • When monsters hit half HP, make a DC 10 Wisdom save or flee/surrender.
  • Rough CR: 1 HD = CR ⅛2 HD = CR ¼4 HD = CR 16 HD = CR 2
Convert only what you need · Keep it moving · Rule of cool beats rule of law