How to select (size) a pump for a swimming pool?
You will need to know how many gallons of water are in your pool. The general rule of thumb is your pool pump should be able to clean your pool in 8 hours so you'll divide your total gallons of water by 8 to get gallons filtered per hour. Then divide that number by 60 to get gallons filtered per minute because that's the figure most pool pumps are rated by. Now look for a pump motor that is close to the gallons per minute you figured out for your pool. You have to take into account what your pool filter is rated for too as it has to be very close to the pump output. You'll also need to know the pipe size of your existing pump so you get one similar to it. You also want to get a pump with the same electrical specs.

Often people are inclined to get the same size pump as the one they are replacing. That's not a good idea because efficiencies of motors and pumps have greatly improved over the years.

I've covered a lot and here's a web site I've used often for doing this. Good luck.

http://www.inyopools.com/HowToPage/how_to_size_a_pool_pump_for_your_in_ground_pool_.aspx?CommentPage=1