Why only about one third of a fleet may be ready for duty

How much of a navy is usually available for sea duty?

About one third is a useful baseline, with the rest in training or maintenance, though long refits and accidents can reduce availability further.

The one-third rule provides a rough fleet-planning baseline: one third on duty, one third training or working up, and one third in repair or refit. Long refits can remove a ship for years. Accidents and unplanned damage reduce the available share further. Nominal fleet totals therefore overstate the force a navy can normally employ at one time.