Calculating the correct pool pump size involves determining the flow rate needed to properly filter your pool water based on its volume and desired turnover rate, and then selecting a pump that can provide this flow rate under the specific resistance of your plumbing system.
Understanding the Basics
Choosing the right pump is essential for maintaining clean, clear pool water and ensuring your filtration system works efficiently. A pump that's too small won't filter effectively, while one that's too large wastes energy and can even damage your filter. The key is to match the pump's capacity to your pool's requirements.
The core calculation, as referenced, is:
Pool Volume ÷ Desired Turnover Rate (in hours) = Required Flow Rate
Let's break this down.
Step 1: Determine Your Pool's Volume
The first step is to find out how much water your pool holds. This volume is typically measured in gallons or liters.
- For rectangular pools: Length x Width x Average Depth = Volume
- For round pools: π (3.14) x Radius² x Average Depth = Volume
- For oval pools: Length x Width x Average Depth x 0.89 = Volume (approximate)
(Ensure all measurements are in the same unit, e.g., feet for length/width/depth to get cubic feet, then convert to gallons by multiplying cubic feet by 7.48. Or use meters for length/width/depth to get cubic meters, which equals liters).
Step 2: Choose Your Desired Turnover Rate
The turnover rate is the amount of time it takes for the pump and filter to circulate and clean the entire volume of water in your pool.
- Most residential pools aim for an 8-hour turnover rate, meaning the entire pool volume is filtered at least three times per day (24 hours / 8 hours = 3 turnovers).
- Some sources recommend 10 or 12 hours, but 8 hours is a common standard for good water clarity.
- Commercial or heavily used pools may require faster turnover rates (e.g., 6 hours or less).
For residential purposes, an 8-hour turnover is generally a good target.
Step 3: Calculate the Required Flow Rate
Using the formula derived from the reference, divide your pool's volume by your desired turnover rate in hours.
Required Flow Rate (Volume per Hour) = Pool Volume / Desired Turnover Time (Hours)
The reference specifies calculating the flow rate in liters per minute for pump selection. If you calculated volume in gallons and time in hours, you'll need to convert units.
Here's how you can perform the calculation and unit conversion:
- Calculate Volume per Hour:
- If using Gallons: Calculate Volume in Gallons / Turnover Time in Hours = Gallons per Hour (GPH)
- If using Liters: Calculate Volume in Liters / Turnover Time in Hours = Liters per Hour (LPH)
- Convert to Flow Rate per Minute:
- Convert GPH to Gallons per Minute (GPM): GPH / 60 minutes/hour = GPM
- Convert LPH to Liters per Minute (L/min): LPH / 60 minutes/hour = L/min
Example Calculation:
Let's say you have a rectangular pool with the following dimensions:
- Length: 30 feet
- Width: 15 feet
- Average Depth: 5 feet
- Desired Turnover Rate: 8 hours
- Calculate Volume in Cubic Feet: 30 ft 15 ft 5 ft = 2250 cubic feet
- Convert Volume to Gallons: 2250 cubic feet * 7.48 gallons/cubic foot = 16,830 gallons
- Calculate Required Flow Rate in Gallons per Hour (GPH): 16,830 gallons / 8 hours = 2103.75 GPH
- Convert GPH to Gallons per Minute (GPM): 2103.75 GPH / 60 minutes/hour ≈ 35.06 GPM
To follow the reference strictly and calculate in Liters per Minute:
- Convert Volume to Liters: 16,830 gallons * 3.785 liters/gallon ≈ 63,680 liters
- Calculate Required Flow Rate in Liters per Hour (LPH): 63,680 liters / 8 hours = 7960 LPH
- Convert LPH to Liters per Minute (L/min): 7960 LPH / 60 minutes/hour ≈ 132.67 L/min
Based on this example, your filtration system needs to be capable of processing about 35 GPM or 133 L/min.
Step 4: Account for System Resistance (Total Dynamic Head)
This is a critical step mentioned in the reference ("taking into consideration the back pressure in the system") that many people miss. Pumps don't operate at their maximum flow rate when connected to a real pool system. Pipes, fittings, the filter, heater, sanitizer, and elevation changes all create resistance to water flow. This resistance is measured as Total Dynamic Head (TDH).
A pump's performance is shown on a curve graph, illustrating the flow rate (GPM or L/min) it can deliver at different levels of TDH (measured in feet or meters of head).
- You need to estimate or calculate your system's TDH. This involves factors like pipe diameter and length, number of elbows and fittings, filter type, and elevation difference between the pump and the highest point the water is pushed.
- Most residential systems have a TDH between 30 and 60 feet (or roughly 9-18 meters). Your pool professional can help determine this more accurately.
- Once you know your required flow rate (from Step 3) and your system's estimated TDH, you look at pump performance curves. You need a pump whose curve shows it can deliver at least your required flow rate at your system's TDH.
Selecting the Pump:
- Calculate: Follow Steps 1-3 to get your required flow rate (e.g., 35 GPM or 133 L/min).
- Estimate TDH: Get an estimate for your pool's system resistance (e.g., 50 feet of head).
- Look at Pump Curves: Find a pump whose performance curve shows it delivers at least 35 GPM when the system resistance is 50 feet of head.
| Calculation Step | Input/Formula | Example (Gallons) | Example (Liters) |
|---|---|---|---|
| 1. Pool Volume | L x W x Avg.D (converted to vol.) | 16,830 Gallons | 63,680 Liters |
| 2. Turnover Time | Desired Hours | 8 Hours | 8 Hours |
| 3. Required Flow Rate | Volume / Time / 60 (for per min) | ≈ 35 GPM | ≈ 133 L/min |
| 4. System Resistance | Estimated/Calculated TDH | e.g., 50 ft Head | e.g., 15 m Head |
| 5. Pump Selection | Choose pump matching flow @ TDH | Pump rated ≥ 35 GPM @ 50ft | Pump rated ≥ 133 L/min @ 15m |
By calculating your required flow rate and considering your system's resistance, you can select a pump that is appropriately sized for efficient and effective pool filtration.
