Readiness

Fleet Readiness (FR):

Fleet readiness is a measure of how ready your forces are for another combat. The highest it can be is 100%, which means that your army is good to fight another battle. But as you keep on attacking you fleet readiness will decrease, the amount you lose each attack depends on the size of faction you attack compared to your own size. The smaller the other faction is compared to you, the more FR you will lose. Also exploring empty planets will take down your FR, and the more planets you have the more FR it will take to explore every next planet. Sooner or later your FR will become negative and you will not be able to explore anymore, but you will still be able to attack, though your forces will be a lot less effective in combat, the lower your FR goes the less effective your forces will be. If you FR passes -100% mark not only would you not be able to attack anymore but some of units will start to desert, so try not to make your FR go that low. Here is the calculator to calculate how much FR you will lose according to number of your planets, your targets planets and respective empires, if you want to see the formulas themselves go here.

Note: it takes maximum 20% fr to attack an artifact.

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Psychic/Agent Readiness:

This is a measure of how ready your agents, psychics or ghost ships are to perform another spell. This is analogous to Fleet Readiness, the only difference is that this affects your fleet in no way, but rather your agents and psychics. The formula for this is somewhat similar in parts to the one of Fleet Readiness and can be found here.
(Note: Ghost ships and Psychics both use Psychic readiness)