Determine your pond's volume. Measure the average length, average width and average depth of your pond in feet. To measure the average each one, get several measurements, add them up and divide the sum by the number of times you took the measurement. For example, take four measurements from different sections of the length taking the shortest and longest points in consideration then sum them up and divide by four. Do the same with the width and depth. Jot the measurements in feet and multiply them to each other (LxWxD). Then, multiply the product by a factor of 7.48 to get the volume in gallons. For an average length of 6 feet multiply by average width of 4 feet multiply by average depth of 4 feet equals 96 multiply by 7.48 equals 718 gallons (6ft x 4ft x 4ft=96ft x 7.48=718 gallons).
Choose between submersible and external models. For smaller ponds of up to 1000 gallons of water, choosing a submersible pump is an economical and practical choice, which does not need additional attachments to install. The out-of-pond or external pump models, however, are good for larger ponds (over 1000 gallons) and would need extra attachments during installation.
Learn how to measure the size of pump to use for your pond by using the manufacturer's guidelines. Manufacturers measure size of pond pumps by gallons per water (GPH) output at one foot of lift or height. Some manufacturers rate larger capacity pumps by horsepower (HP).
Determine your submersible pump measurement based on the pond's volume to get the correct GPH. It is advisable to circulate the water every 2 hours, which means you should use a GPH rating of half of your pond size. Therefore, for 1000 gallons of pond you would need a 500 GPH pump.
Follow the GPH chart when choosing pumps. For 30-100 gallons, use 50 GPH pump, 100-200 gallons, use 100 GPH, 200-400 gallons, use 200 GPH, 400-700 gallons, use 350 GPH, 700-900 gallons, use 450 GPH, 900-1100 gallons, use 550 GPH, 1100-1500 gallons, use 750 GPH and 1500-2000 gallons, use 1000 GPH.
Take into account fish, vegetation and accessories, such as water fountain, when choosing a pump. Choose a pump one size larger if you have these extras.
Take into consideration the waterfall, if you have any, when choosing pumps. Estimate your waterfall's height by measuring the vertical height from the top of the pump to the top of the waterfall. Add a foot or lift height for every 10 feet of hosing to account for loss of volume due to hose resistance. To get the GPH calculation, for every inch of width of the channel of waterfall spillway you would need to add 100 GPH. Therefore, for a spillway of 8 inches wide you would need 800 GPH.