Passive Perception = 10 + your Wisdom (Perception) check bonus. The check bonus is your Wisdom modifier, plus your proficiency bonus if you are proficient in Perception (double it with Expertise), plus any other bonus that applies to Perception checks. Advantage adds 5 and disadvantage subtracts 5. It works like a spell save DC: a fixed base plus a modifier and proficiency, with no roll.
Both SRDs give the same worked example: a level 1 character with Wisdom 15 who is proficient in Perception has a passive Perception of 14 (10 + 2 + 2), or 19 with advantage (SRD 5.1 "Passive Checks"; SRD 5.2 Rules Glossary, "Passive Perception"). If you are still choosing scores, the point buy calculator shows the modifier for each one.
Passive Investigation and Insight
The 2014 rules define a passive score for any check: 10 + all modifiers that normally apply, with the same ±5 for advantage and disadvantage. That is where passive Investigation (Intelligence) and passive Insight (Wisdom) come from. The 2024 rules only name passive Perception, so using a passive Investigation or Insight score with 2024 rules is a table choice, though the math is the same.
The Observant feat (2014 vs 2024)
- •2014 Player's Handbook: +5 to your passive Wisdom (Perception) and passive Intelligence (Investigation) scores, plus +1 Intelligence or Wisdom.
- •2024 Player's Handbook: the +5 is gone. Observant now gives +1 Intelligence or Wisdom, proficiency in Insight, Investigation or Perception (Expertise if you are already proficient), and lets you Search as a bonus action. To model it, set that skill to Proficient or Expertise instead of ticking the 2014 box.
Things that lower it
Anything that gives disadvantage on Perception checks costs 5 points. In both editions, a lightly obscured area such as dim light gives disadvantage on Perception checks that rely on sight. A fast travel pace costs the same 5: the 2014 rules give −5 to passive Perception, and the 2024 rules give disadvantage on Perception checks. The travel time calculator shows the pace penalty.
When a DM uses passive checks
A passive score is the DM's way of checking a character without asking for a roll. The usual uses are noticing a hiding creature (compare its Stealth check to each passive Perception), spotting a trap the party walks past, and seeing who notices an ambush before combat. Asking for a roll tells the players something is there; comparing passive scores doesn't. Players should write the number on their character sheet so the DM can ask for it.