Converting 5e stat blocks to OSR

ForgottenF · March 24, 2025, 12:45:59 PM

I'm considering running a (3rd party) 5th edition module via the new Tales of Argosa rules, which will mean converting a fair few stat blocks over. There are a few different ways I could go about the conversion, but I'm wondering if any one has a standard set of guidelines they use for 5e to OSR stats.

AC seems like it could probably be left as is, and ToA has an unusual way of calculating HP, but it's easy to apply. I'm wondering whether/how much to tweak attack bonuses or damage, how to apply multi attack, saving throws, that sort of thing.

Eric Diaz · March 24, 2025, 02:31:29 PM

Short answer:

There is no exact formula to convert 5e monster to OSR games.

You can use this:

- Divide HP by 10. This is your number of HD.
- Divide damage by half.
- Keep AC as written [if using AAC], or add +1.
- Attacks and HP are defined by HD.
- Saves as a fighter of equal HD (e.g., 9HD monster saves as a 9th-level fighter).

But it is much better to find a monster of a similar type in your "main" monster manual and change as desired.

Long answer:

https://methodsetmadness.blogspot.com/2023/05/converting-5e-monsters-to-osr-games.html

ForgottenF · March 24, 2025, 05:12:42 PM

Thanks Eric. That's exactly the kind of thing I was asking for

Venka · March 25, 2025, 01:07:05 AM

You know, reading that blog, something just seems a bit off. In 5ed, the monsters kind of have a design that is based on their challenge rating, but which "tier" (from I to IV) they fall into seems to have a big effect. As such, I wouldn't expect that a method that works well for things like orcs and ogres would work as well for dinosaurs and giants.

There is also the manner of 5e's bounded accuracy, in which monster AC ramps up slower than in other versions of the game. OSR AC is also slow, but wouldn't that have some distortions at higher HD monsters?

Whatever, he's done the work and I haven't. I would just expect these 5edisms to show up somewher.

Eric Diaz · March 25, 2025, 07:12:47 AM

Quote from: ForgottenF on March 24, 2025, 05:12:42 PMThanks Eric. That's exactly the kind of thing I was asking for

My pleasure!

Quote from: Venka on March 25, 2025, 01:07:05 AMYou know, reading that blog, something just seems a bit off. In 5ed, the monsters kind of have a design that is based on their challenge rating, but which "tier" (from I to IV) they fall into seems to have a big effect. As such, I wouldn't expect that a method that works well for things like orcs and ogres would work as well for dinosaurs and giants.

There is also the manner of 5e's bounded accuracy, in which monster AC ramps up slower than in other versions of the game. OSR AC is also slow, but wouldn't that have some distortions at higher HD monsters?

Whatever, he's done the work and I haven't. I would just expect these 5edisms to show up somewher.

Yes, I see what you're saying. There is no perfect way to covert it. You could probably make this more precise by adding more rules and exceptions, but it would take more work IMO, and I havent seem it anywhere.

Man at Arms · March 25, 2025, 10:25:51 AM

I first thought, that a good solution would be to purchase an OSR compatible Monster Manual or Bestiary of some type. Just one, perhaps. Then you'd have some examples handy, at a moment's notice.

Converting 5e stat blocks to OSR

ForgottenF

Eric Diaz

ForgottenF

Venka

Eric Diaz

Man at Arms