Perk Rate

Perk Rate is a derived statistic. It affects how often the character is allowed to pick a perk. A character's perk rate depends on their race and on certain traits such as Skilled (which increases the rate) or Fear the Reaper (which decreases it). Some perks, like Divine Favor, may decrease the perk rate as well.

Fallout and Fallout 2
The perk rate in both games is 3, which means that it is possible to pick a new perk every three levels. They cannot be saved up for use at future perk acquiring levels. Any unselected perk will be lost forever at the next perk level.

Fallout Tactics
Perks can be saved for later use, and the perk rate depends on the character's race:

Fallout: Brotherhood of Steel
The game uses skills which work the same as perks in the other. A new skill can be selected at each level up.

Fallout 3, Fallout 4 and Fallout: The Roleplaying Game
The perk rate in all three games is always 1, meaning that a new perk can be picked at every level up. Additionally, perk points can be saved in Fallout 4, allowing for them to be distributed at a later time.

Fallout: New Vegas
The perk rate is set at 2, granting a new perk every two levels, and is governed by the game setting iLevelsPerPerk. As in Fallout 3, perks cannot be saved for later use.