O5R: converting TSR-D&D monsters to 5e-D&D... WITHOUT changing the stat-line

This is my (very easy) OSR/TSR to 5e conversion, without changing the actual stats:

Suggested stat line (inspired by RC and S&W):

Goblin. AC 6 [13], HD 1d6; Move 30'; Atk Spear (1d6); Save F0; Morale 7.
Ogre. AC 5 [14], HD 4d10; Move 30'; Atk Club (1d6+2); Save F4; Morale 7.

5e conversion:
AC: as written [in brackets].
Move: as written [in brackets, if there are two numbers].
HP: maximum possible HP. If the HD indicates a single number instead of a type of die (for example, 3 inttead os 3d6), multiply HD by a number based on size: 6 for goblins, 8 for human, 10 for ogre/troll, 12 for giants/dragons, 20 for gargantuan creatures.
Damage: always deal maximum damage.
Bonus to hit: equal to HD.
Saves: bonus equal to the number indicated in two saves, as appropriate; usually Strength & Constitution for F, Wisdom & Intelligence for M, Dexterity & Intelligence for T, etc. Otherwise, halve the bonus.

Examples:

A monster that saves as F8 would have +8 bonus to Strength and Constitution saves, +4 bonus otherwise.

The goblin mentioned above would have a 6 HP and deal 6 damage per attack, while the ogre would have 40 HP and deal 8 points of damage per attack.

by Russ Nicholson

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And this is the (horribly difficult and convoluted) way I used to reach these conclusions:

I'm turning my old one page dungeon into a 30ish-pages module, and since I play both 5e and B/X, I was considering making it compatible with both (plus Dark Fantasy Basic, which is supposed to be compatible with B/X anyway).

I'm playing with the idea of having a single line of stat for BOTH systems. Thought it might be possible with some abbreviations and conversion. So I started playing with this idea.

Not that such type of conversion is necessary: we already have plenty of monsters for BOTH systems, and the easiest way to "convert" is finding a similar monsters. The thing is, I LIKE playing with numbers. So here we go!

I'll usa a goblin, an ogre and a large/adult red dragon as examples, just because. The numbers are from the RC, and the math is mostly done in my head as I write so... let me know if I got it wrong.

Armor Class

This one is actually not hard: just use an abbreviation such as "as leather" or "as plate+6", or even "as leather+1". Of course, armor and shields function differently in each edition - armor and shields provide a bigger bonus in 5e, which makes things more difficult.

Converting the actual numbers isn't hard either: jus subtract RC AC from 19 and you're good to go. The results are far from perfect:

AC	RC	5e (RAW)	5e (19-original)	5e (armor based)
Goblin	6	15	13	Scale (14)
Ogre	5	11	14	Chain (16)
Red Dragon	-3	19	22	Plate+6 (24)

One problem is that that dragons have AMAZING AC in old school D&D - a purple worm (also 15 HD) has AC 6 ("scale"), for example (9 points worse than the dragon), while in 5e the AC is almost the same (18 versus 19).

Another problem is that in 5e armor scales with CR, which doesn't happens as much in the RC. So, the purple worm has the same AC as a Minotaur in the RC, but in 5e, it beat the Minotaur 18 against 14.

Sigh. Let's just stick to the formula for now. So, the dragon has AC -3 [22], if we're using S&W notation.

Hit Dice (and "THAC0")

Here is the crux of the matter: 5e doesn't use HD in exact the same way as TSR-D&D. But it's there - and its useful. Let's compare:

HD	RC (d8)	5e (RAW)
Goblin	1-1 (S)	2d6
Ogre	4 + 1 (L)	7d10+21
Red Dragon	15 (L)	19d12+133

As you can see, 5e numbes are heavily inflated when compared to RC. The number of HD is nearly doubled (the 5e dragon has 19 HD, but lots of extra HP).

However, HD is used in two ways in the RC: to generate hit-points and to-hit bonuses (equivalent to "THAC0").

HP in 5e is WAY higher than the RC, specially at higher levels. However, if we break from the RC conventions and make HP vary with size like 5e suggests (say, 1d6 for goblins, 1d8 for humans, 1d10 for ogres, 1d12 for adult dragons, 1d20 for ancient dragons and purple worms), we could use the HD listed in the RC but giving MAXIMUM HP to each creature. So, the 15 HD dragon would have 180 HP (15x12). With the added AC, he would be a bit tougher than the 5e one, as long as we improve his saves, too.

HP	RC	5e (RAW)	5e (our formula)
Goblin	3.5	7	6
Ogre	19	59	41
Red Dragon	67	256	180

"To-hit" can simply be based in HD, like many OSR enthusiasts suggest: 5 HD means +5 to-hit etc.

"To-hit"	RC	5e (RAW)	5e (our formula)
Goblin	~+1	+4	+1
Ogre	~+5	+6	+4
Red Dragon	~+10	+14	+15

If a "Save DC" is needed, we will use the same number, plus 10.

Move

This is easy; 5e uses the number in brackets. 

So a Move 90' (30') ogre would move 30' in 5e. Maybe just get rid of the 90' and say you move three times faster when outdoor, etc. "As human", "as human x 2", etc, would work wonders in this case.

Attacks/damage

Damage tends to be higher in 5e, since HP is also a lot higher. This isn't that easy to convert. The best simple/elegant solution I could find is this: like with HP, 5e monsters deal the same damage as written in RC, but always maximum damage. So instead of 1d6+1 we got 7, and instead of 2d4+4, 12.

Saves

The RC simply indicates a class and level instead of a number; for example, F12 for a monster that saves as a 12th level fighter. 5e has certain classes being proficient in certain saves: Strength & Constitution for Fighters, Wisdom & Intelligence for Wizards ("Magic-users"), etc. 

We could always use Ref/Fort/Will , but that would be neither here (5e) nor there (OSR/TSR).

There are just so many monsters that have simply "Fx" in their saves, "x" being their HD, that I'm tempted of just getting rid of this line too.

XP/Treasure/etc

No conversion. I'd rather used circumstances/location to find treasure, and HD to calculate XP.

In conclusion...

Not an easy task, but doable. Now I can convert old school monsters on the fly. It is unlikely that I can publish a book with the same stats for B/X and 5e, but I could easily run an old school module for 5e characters without preparing the stats.

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