The short answer: a 20×20 insert for an 18×18 cover. That is the standard advice and it is right for almost every insert you can buy, but it is worth knowing why, because the rule is a workaround rather than a law. Size is a proxy for fill: a 20×20 insert has more material in it than an 18×18, so it fills the corners and the cover looks plump instead of saggy. What actually decides the result is fill weight in grams, and almost no listing publishes it. Which is why one insert manufacturer states its 20×20 suits a 20×20 cover, while every cover shop tells you to size up. Both are describing what they sell honestly.
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