I've heard 1 out of a million tossed around. I vaguely recall it being in a valid source (and not mere heresay), but I can't find it.

A more "solid" number is found in the Deathwatch rule book that states there are "millions" of psykers out of a "billion billion" (1 QUINTILLION!!!)humans in the Imperium. If we estimate by "millions" there were 5 million psykers...

That means there's one psyker for every 200,000,000,000 people...or one every two hundred billion if my math is right.

Even if we took "millions" to the absolute maximum of 9,999,999 limit (otherwise ten million would be used) it's still 1/100billion. I'm still not satisfied with that number. There's clearly something lopsided with these numbers.

For one thing, there wouldn't be enough psykers to man the Grey Knights! We know from *The Emperor's Gift* that only one out of a million psykers that are brought to Titan are able to past their tests to become a Grey Knight! If a Grey knight lives, on average, say 200 years, that means the Imperium is probably running at least 800million psykers every 200 years...or 4 million psykers, a year!

And since Grey Knights get the absolute worst missions in the Imperium, I could imagine their average lifespan is significantly below this.

~~~~~~~

Going back, I think a billion billion humans is way too high. If the Imperium averages a million planets, that puts 1,000,000,000,000 people on each planet (or one trillion). This is clearly way too high. Probably a few orders of magnitude too high.

I did some calculations (I can dig them up if you'd like, but basically assumed the ~30,000 Hiveworlds we know exist had 30 billionish people and the rest of the planets had a 1 billion average population) and came up with something along the lines of 2 quadtrillion people in the Imperium.

5 million psykers would mean one out of four hundred million (400,000,000) are psykers. Clearly too low.

I'll go for a conservative amount of psykers for the the Imperium.

If we look at the Grey Knights and assume they get 5% of the eligible psykers (IE male) born in the Imperium, that means at a rate of 4 million psykers a year going through Titan, a total of 80 million male psykers are born a year. Double that to account for female psykers, and that's 160 million a year.

That means there's about 160 million psykers out of a 2 quadrillion population.

That leaves us with with one psyker out of a 12 million people.

That's probably a decent low end figure. I couldn't hazard a real guess what a mid or high end could be.

Edit: Screwed up my numbers, I forgot to add a few zeros to the Imperium's population!