I got a lot of great feedback about my previous article, How to Convert D&D monsters to 5e, so it's obviously time for the same article for Third Edition (or 3.5, realistically).
For the longest time, I never really bothered converting from 3.5 to 5e, because I found them to be relatively easy to convert in real time. I've run several Pathfinder and 3e modules for 5e, and all I've had to do is adjust the obvious stuff: instead of a Disable Device skill check, you ask for a Dexterity check, and so on. Any DM is used to having to invent a DC target in the moment, so adjusting for that comes naturally. Enemies tend not to last too long anyway, so hasty conversions of an unusual Pathfinder-only NPC class to whatever's "close enough" in 5e tends to work pretty well.
Sometimes, approximation just doesn't feel right. Maybe you run a strict game based heavily on stats, where even just 1 extra hit point could mean the difference between player character death and victory. In that case, you need to do a careful conversion, and the Wizards of the Coast conversion document explains this in detail. If you embark on a conversion to 5e, don't rely on my notes here: read the whole conversion document carefully! Assuming you promise to do that, here are notes from my time converting monsters and player characters from 3.5 to 5e.
A quick note: I refer to 3e, Third Edition, 3.5, and Pathfinder interchangeably because in terms of lineage, spirit, and math, there's essentially no difference, and in real life, I use books from both 3.5 and Pathfinder.
The conversion guide spells this out. The math is simple: you can either use the average of touch AC and AC, or 20% lower than in 3e. The maximum is 22.
For example, assume a creature has AC 17 and Touch AC 11 in Third Edition. That's (17 + 11) divided by 2, which resolves to 28/2 for 14.
Alternately, you can take 20% off of just the Third Edition AC (17 in this example). Percents are harder to calculate, but you can use a calculator if needed. Since you are subtracting 20%, you are by definition keeping 80% (or .8, converted to decimal), and 17 times .8 is 13.6. That rounds up to 14.
Who says math is hard?
Ability scores between 3.5 and 5e usually stay the same. Sometimes you might have to adjust numbers to account for a flavourful special skill. You shouldn't build a creature with a +3 bludgeoning attack bonus, but a STR of 10 just because 3.5 granted the attack through a feat that 5e doesn't have. Generally, though, the ability scores are a no-brainer.
The conversion document fails to mention this, but the hit points (derived from a creature's hit die, just like a player character' HP is) are provided in the D&D Monster Manual on page 7, organized by size. For instance, a large creature uses d10. That answers what kind of die to use for hit die, but it doesn't address how many hit die the monster should be assigned. For that, turn to page 274 of your Dungeon Master's Guide, containing a table called Monster statistics by Challenge Rating. This table gives you everything you need to know.
If the monster you're converting was CR 4 in 3.5, reference the CR 4 line in the table: AC 14 is confirmed, and a total of 116 to 130 HP is expected. Call it 120 for the sake of this example (because round numbers are easier). Not all of that HP comes from hit die, though. The Monster Manual states that a monster's CON modifier gets multiplied by the number of its hit die, and the result is added to its HP. It's easiest to incorporate this into your initial calculation (you can do it at the end, instead, but then you have to go back and adjust, which is doing all the work twice or thrice). What the rule averages out to is this: the CON modifier of a monster gets added to its hit die roll.
Assume the monster you are converting has 14 CON for a +2 modifier. This means that the monster's HP roll is actually 1d10+2, not just a d10 as indicated in the table.
The average of a d10 is 5.5 (according to the Monster Manual table, and also math) but for this monster, the average is 5.5+2 CON, for 7.5 each roll. To get to 120 with average rolls of 7.5 each time, the monster requires this formula: 120 (your target HP according to the DMG) divided by 7.5 (hit die average) for a result of 16 hit die.
That's expressed as the number of hit die plus [the CON modifier times the number of hit die]. In this example: 16d10 + 32.
Check your work by multiplying the hit die by the average roll, and then add the bonus. For example: 16 times an average of 5.5 is 88. Add the 32 CON bonus for 120, which is exactly your target HP.
In summary, without all the narrative:
Get the target Challenge Rating (CR) of a monster from your 3.5 source material.
Get your target HP from the Monster Statistics table on page 274 of DMG.
Get the monster's hit die type from page 276 of DMG or page 7 of the Monster Manual.
Get the average roll of the hit die from the same table.
Add the monster's CON modifier to the average hit die roll. For example, if your monster has a CON mod +1 and its hit die average is 4.5, then your new working average is 5.5.
Divide the target HP by the hit die average (Example: 88 HP / 5.5 average hit die roll).
Express the result as hit die plus (CON modifier times number of hit die). In this list's example of 88 HP, the result is expressed as 16d8+16).
Check your work by multiplying the number of hit die by the average roll, and then adding the CON bonus. For example: 16 times 4.5 is 72, plus 16 CON bonus equals 88.
Now, that is a lot of math, and I admit it can be overwhelming if you're not great with numbers. However, it's something that can be programmed into a spreadsheet pretty easily. Obviously I wouldn't just mention a spreadsheet without providing one, so here is a very simple HP calculator for LibreOffice and OpenOffice:.
To extract the sheet, you may need to download and install the free and open source 7-zip archive tool (that's
p7zip for Linux users, available from your software repository). Just click the icon below:
That's all of the math out of the way. The rest of the build is translating special abilities and feats and attacks from 3.5 mechanics into 5e mechanics. Sometimes that means adapting a spell, or dropping the "feat" term, or changing a FORT save to a CON save, while other times it means reworking some aspect of the monster to fit into 5e sensibilities. For example, there is no Base Attack Bonus in 5e, so if an attack was boosted in Third Edition, then it must be translated in some meaningful (or arbitrary!) way for 5e.
There's so much diversity in monster design that there is no template or conversion script for this part of the exercise. It's the art half of the art and science of monster conversion.
Maybe what you're converting isn't a monster but an NPC with a class. Because Third Edition was famous for expansion (and obviously Pathfinder adds to this legacy), it can be difficult to take a highly specific class and force it to fit in the comparatively sparse class system of 5e. The conversion document offers a few tips on this, and it mostly boils down to multi-classing. If a Third Edition NPC is written to have skills and feats that don't normally get built into a single class in 5e, then just start smashing 5e classes together. Unlike a player character, an NPC can afford to sacrifice expertise in minor skills they'll likely never use, so the drawbacks of multi-classing are negligible.
If you still can't get the right mix from multi-classing, consider mixing in racial traits if that's an option. For instance, who says an NPC written as a human has to be human? If it would help you approximate the right mix of stats and attributes, draw from another source book for extra races and backgrounds.
Unlike the math conversions, this is not a science. You're creating an artist's rendering of a character, and usually an approximation is all you really need.
In my next blog post in this mini-series about conversion, I'll take a look at a 3-to-5 conversion by a pro in the industry. It's a direct conversion of a monster created by some guy named gary Gygax, in a book published for both Pathfinder and 5e. Stay tuned!