So this isnít perfect, but it should get you very close to being able to use any Pathfinder or 3.5 monsters in D&D 5e. It should even allow you to run a 3.5 or Pathfinder adventure on the fly, including NPCís. This document tries to keep it to a very small amount of adjustments, which you can mentally do and pencil in. For all the calculations in here; round down.
I went and got the entire database of Pathfinder monsters from the web. I grouped them by Challenge Rating and calculated Median Hit Points, Median Damage, Median AC etc.
You should be able to go to www.d20srd.org or www.d20pfsrd.com and get monsters. You can try on your own by downloading the raw data of all of Pathfinderís monsters here: www.d20pfsrd.com/bestiary/tools/monster-filter
Subtract the CR (Challenge Rating) from the Armor Class to get the D&D 5e appropriate AC.
For example: An Iron Golem has an AC of 30 and a Challenge Rating of 13. Its D&D 5e AC should be 30-13 = AC:17
Note: Pathfinder has an Iron Golemís AC as 28, while the 3.5 system has the AC at 30.
To figure out a monsterís Attack Bonus for melee and ranged attacks vs AC. Cut the Challenge Rating in half and add 3 to that number. Usually you should round down, but if the monster has an extremely high Strength score you may want to round up or even add 1.
For example: A CR:7 Flesh Golem should have a melee attack that is +6 vs AC. Thatís half of 7(rounded down) +3.
Leave the damage just as listed on the stat block. If a monster gets multiple attacks with declining attack bonuses, keep them all at the same Attack Bonus calculated in Step 2 above, with the damage as listed.
For example: The Challenge Rating:20 Balor attack line says:
+1 vorpal unholy longsword +31/+26/+21/+16 (2d6+13),
+1 vorpal flaming whip +30/+25/+20 (1d4+7 plus 1d6 fire and entangle)
This would be 4 longsword attacks at +13 vs AC for 2d6+13 damage each, plus 3 attacks with the flaming whip at +13 vs AC for 1d4+7+1d6 damage. Thatís an average damage of 119 per round.
Thatís a good target for a CR:20 D&D 5e creature. Youíll notice itís a little higher than the Balor in the D&D 5e Bestiary, but thatís because this is CR:20 and the Bestiary has the Balor at Level 16. We arenít making level adjustments. If you are converting a Pathfinder adventure that calls for a DC:20 Balor, we want to make sure it stays as challenging as the adventure calls for, not how D&D 5e thinks it should be in the Pantheon.
Allow monsters to keep their extended Crit Ranges, but make critical hits do max damage for monsters instead of double. Donít roll to confirm critical hits, assume they always confirm.
If you want to double the damage, and make fewer attacks, feel free to do so. For instance you could make 2 longsword attacks, each for double damage. Itís tough to do that if you have a monster that makes an odd number of attacks.
To figure out what the characters need to roll to make a saving throw against something the monster does to them; Take the Save DCís of monster attacks or trap that is listed, subtract 10 from it, divide that number in half, and add it back to 10. For example: An Adult Silver Dragon has a breath weapon that requires a DC:23 saving throw. Take 23-10 = 13. Divide the 13 in half to become 6, and add that back to 10, so the saving throw DC becomes 16.
Note: Yes, these monsters will have more HP than the ones in the Bestiary. A major complaint Iíve been seeing on the forums is monsters die too easily. If you graph damage output of the players for D&D 5e side by side with Pathfinder it is spot on at higher levels.
I also checked for differences in Hit Points between the 3.5srd and d20srd, and surprisingly the Medians for HP are all the same! Pathfinder only shifted things, like increasing Outsiders to a d10 Hit Die from a d8, but they reduced Ooze Hit Die down to d8 from d10. All the adjustments must have been in parallel. For every Hit Die size Pathfinder increased over 3.5srd, they must have decreased another one.
If you use half the Fort, Ref, Will save bonuses for everything it will save a LOT of calculation.
For example: A CR:6 Girallon has Fort+9, Ref+8, Will+5. If it has to make a DEX saving throw it would get half the Ref save of +4. If it had to make a STR check to escape being grabbed, it would use half the Fort save of +4.
Even if D&D 5e calls for STR or a CON save, you can use half the listed Fortitude save bonus for both. If the creature has a negative save, donít divide it in two, leave it as a negative.
You may think this cheats the players a bit. If you start to dig youíll notice a few things.
There are no spells that call for an INT saving throw.
Itís also extremely rare for a monster to ever need to make an INT check.
Anything that calls for DEX saving throw, simply use half the listed Reflex save bonus.
There are no spells that call for a STR saving throw. STR usually only comes into play when grappling, escaping, disarming or other maneuvers. Usually STR and CON are either both high or both low, so itís ok to use half the Fortitude save bonus for both, even resisting maneuvers or escaping. There are very rare cases of extremely low STR combined with high CON; those are all incorporeal, swarms, or formless creatures and players wonít be disarming or grappling with them anyway.
There are only a tiny handful of spells that require a Charisma saving throw. These are mostly charm spells, or banishing spells. I think it's ok to lump Charisma and Wisdom saves together into Will save.
Also use half the Will save bonus to detect hidden creatures, to detect lies etc.
If a monster needs to use a skill, simply use half the relevant Fort, Ref, or Will save as the base ability modifier, and if the monster has the skill listed in their stat block give them a +5 for being trained. For example: A Cloaker has Fort+6, Ref+5, Will+7 and has the skills listed of Disguise, Perception, Stealth. If it needs to make a Disguise Check, use half the Will save bonus of +7/2=+3 and add +5 for being trained. So a total of 3+5 = +8 for Disguise checks.
Most of the initiative bonuses are very small in Pathfinder, the ones that arenít are usually because the monster has the Improved Initiative feat. This is normally because going first is important to the monster and helps define how the monster plays.
Monsters that can Sneak Attack, can get Advantage if any of their allies is adjacent to the target, and then give up the Advantage to take a Sneak Attack (just like a Rogue). They also have Advantage on anyone they beat in Initiative, and when hidden, target is prone, etc.
Depending on your knowledge of feats, you could allow monsters with Spring Attack to be able to move 10 ft. without provoking an opportunity attack. You should give out other general feat abilities like that, but donít worry about feats too much, ignore most of them.
Level Drain: -5 to max HP, -1 to all d20 rolls, -1 to Spell DC's
Ability Damage:Keep the same dice (i.e. a Shadow does 1d6 STR damage) Each number gets translated straight to the below. 1d6 hours duration.
STR dmg: penalty to damage rolls - skill checks/Saves
CON dmg: penalty to max HP - skill checks/Saves
DEX dmg: penalty to AC, and skill checks/Saves
INT dmg: penalty to Spell DCs skill checks/Saves
WIS dmg: penalty to Spell DCs and Skill Checks/Saves
CHA dmg: penalty to Spell DCs and Skill Checks/Saves
For any paralyze/stun/frighten effect a player can make a saving throw if they spend an Action to do so.
If a creature has Magic Resistance, simply let them roll twice on all Saving Throws vs. Spells.
If a creature has Damage Reduction, simply let them take half damage from that source: “Resistance” as itís called in D&D 5e terms.
Any durationís that are listed in 1d4+3 rounds or some such, simply change to 1 minute. Also, give all players and monster the ability to get another Saving Throw if they spend an Action to do so.
Breath Weapon, says it recharges every 1d4 rounds, simply roll a d6 every round and recharge on 5 or a 6, the same as 4th edition recharges.
Any Petrify, is restrained for the first round, and if they donít spend an Action to save against, it becomes Petrified.
Any “Save-or-Die” simply does Caster Level*d8 in damage, save for half.
Any Charmed or Dominated person gets another saving throw if they take any damage, or if they are forced to do something that would cause themselves harm.
Most Poison you can change into flat damage, instead of Ability Damage.
Any damage to equipment, such as Black Pudding damaging armor and weapons, simply stack -1 penalties to damage or AC.
grab = whenever you see an attack that "grabs" the target must make a DC of (1/2 monster's Fort save) STR check or become "grabbed". The monster would get Advantage on all future attacks vs that target. The grabbed target must spend an Action to make STR or DEX check to escape.
If a creature has “constrict” then they immediately do that extra damage listed when the grab succeeds. They also get to automatically inflict the constrict damage every round when they act.
rend = if both hit, do an extra X damage (itís listed)
trip = simply make a quick opposed STR or DEX check, playerís choice, or be knocked prone.
pounce = as an Action the creature can move its speed and make a full attack. This means they can Move, and then spend an Action to Move and Attack, essentially double moving (like a charge).
rake = consider this free automatic damage that happens to grabbed foes at the start of creatures turn.
Try your best on spells, some work just the same.
In older editions of D&D (before 4E) a “standard” encounter was targeted to be 50% of the partyís strength. If you had a party of six players with a total of 18 class levels between them all, a standard encounter would be around 9 total Hit Dice of monsters. A “Tough” encounter could be 100% of the partyís strength, and a super-duper hard could even be up to 200%. (Source: Frank Mentzer Dragon#101)
The following is Speculation: ----What they did with Challenge Rating is say: a monster with this “Challenge Rating” is a “standard” difficulty encounter for a party of 4 players of a level equal to that Challenge Rating. Remember, “standard” difficulty means: 50% of the strength of 4 players. What does that tell you? That means that a monster of a given Challenge Rating is roughly equal to 2 players. I should say that again, because if itís true, it makes thinking of creating encounters very easy! A monster of a given Challenge Rating is roughly equal to 2 players.
Pick any single monster of a Challenge Rating, and it is equal to 50% of the strength of 4 players of that level. It will have the Hit Points and damage output of 2 players. So this was my “Eureka” moment. Say you want to figure out what the damage output and HP of a single player is expected to be. If you divide the HP and damage output of a monster in half, you will get the average expected damage and hit points of a single character of the level equal to its Challenge Rating. Donít assume Fighter is the baseline; Fighters have immense advantages to damage and HP. These numbers are meant to be the average of the party where Fighters do a lot of damage, Clericís swing their 1d8 mace, Thieves donít get Dex bonus to damage, wizards do 1d6+0 most rounds. Also, these numbers represent the average ho-hum party, not the super optimized, all 18ís in their primary stats, type party.
One more Challenge Rating fact to remember: if you ever combine 2 monsters of the same Challenge Rating, the Challenge Rating of the entire encounter becomes 2 higher. Combining 2 monsters of 50% difficulty creates an encounter of 100% difficulty. Therefore if you ever see a Challenge Rating 2 higher, it means this encounter is 100% of the strength of the party.
This assumes what they call a Standard encounter is 50% of the partyís strength, and a Tough Encounter is 100% of the partyís strength and that Challenge Rating is roughly 2 lower than the equivalent D&D 5e Level.