What size hydration vest do I need for marathon vs. ultra running?

Across the reviewers we read, a 5-8L vest with two 500ml soft flasks is the consensus pick for road marathons and trail runs up to about 25 miles. For ultras, mountain runs, or self-supported efforts of 4+ hours, specialist trail-running subreddits consistently recommend stepping up to a 10-12L vest so you can carry mandatory safety gear, layers, and extra nutrition.