What size rain cover should I buy for my backpack?

Match the cover to your pack's liter rating, not its physical dimensions. Most makers list size bands like S (15-30L), M (30-50L), L (50-70L), and XL (70L+). Reviewers consistently warn that an oversized cover will balloon and catch wind, while an undersized one rides up and exposes the bottom. If you're between sizes, sizing down typically gives a tighter, more secure fit.