d20 Conversion Guidelines

This is a work in progress!

This article attempts to create conversion guidelines for creatures written for the D&D 3E/3.5, d20, and Pathfinder rules. Online stat blocks for such creatures can be found here:

Note, however, that conversion work cannot be an exact science - if you don't think something quite fits for a particular monster, change it!

Attributes

Primary Attributes

ST

Use the following formula for Medium-sized and smaller creatures:

GURPS ST = d20 Strength

Larger creatures will end up comparably weak with this formula. Increase their GURPS ST based on their d20 size category:

d20 Size Category ST Modifier
Large +25%
Huge +50%
Gargantuan +75%
Colossal +100%

DX

Use the following formula:

GURPS DX = 10 + d20 Dexterity Modifier

IQ

Use the following formula:

GURPS IQ = 10 + d20 Intelligence Modifier

HT

Use the following formula:

GURPS DX = 10 + d20 Constitution Modifier

Big monsters in D&D have high Constitution scores since Constitution determines hit points. In GURPS, those HT scores will be so high that they'll never fail a check, which is neither realistic nor playable. Modify as follows:

d20 Size Category HT Modifier
Large -2
Huge -4
Gargantuan -6
Colossal -8

Secondary Attributes

HP

Hit Points should generally be set equal to ST.

Will

Per

FP

FP should generally be set equal to HT.

Speed

Derive from DX and HT as normal for GURPS

Move

SM

DR

Use the following formula for the creature's "innate" armor:

GURPS DR = (d20 "natural armor" bonus) / 2

Worn armor should use standard GURPS armor types.

Dodge / Parry / Block

These should be derived as normal for GURPS

Attacks

Afflictions

For attacks that require a reflex save, a Dodge roll should be required to avoid the attack. Treat attacks that require a Fortitude or a Reflex save as Afflictions resisted by HT or Will. The resistance roll should be modified depending on the DC of the attack:

d20 save DC GURPS Attribute Modifier
11 or less +1
12-13 0
14-15 -1
16-17 -2
18-19 -3
20-21 -4
22-23 -5

Traits

Skills

Class

Combat Effectiveness Rating