How do I calculate the right pump run time for my pool?

Divide your pool’s gallon capacity by your pump’s actual gallons-per-hour flow rate (assume 75% of the rated flow due to friction and filter resistance). That gives you the hours for one full water turnover, which is the gold standard for filtration. For example, a 20,000-gallon pool with a pump rated at 60 GPM (3,600 GPH) at 75% efficiency gives 2,700 GPH, so run it about 8 hours in summer and half that in winter.