A practical guide with key formulas and concepts for choosing the right circulation equipment, without oversizing or undersizing

Properly sized pumps and filters are the foundation of a clean, safe, and efficient pool.

Choosing a pool pump or filter shouldn't be guesswork. It's common to select equipment based on its HP, what the neighbor's pool has, or simply by budget, and end up with a pump that can't circulate the water correctly, or a filter that clogs prematurely. The result: cloudy water, higher electricity consumption, premature equipment wear, and avoidable headaches.

The good news is that selecting the correct equipment follows a logical process, based on your pool's volume, the filtration time required by Costa Rican regulations, and the hydraulic formulas that determine the flow rate and pressure you need. In this guide, we review this process step-by-step.

First, understand the circulation system

Before calculating anything, it's important to understand how water moves within a pool. "Dirty" water is removed by the skimmers (which collect from the surface) and the main drain, passes through the pump and filter, and returns clean to the pool through the return inlets:

Typical circulation diagram: skimmers and main drain, self-priming pump, sand filter with multiport valve, and return inlets

The pump is the heart of the system: it generates the necessary pressure to move water through the pipes and the filter media. The filter, in turn, retains the particles that the pump pushes through it. Both pieces of equipment work as a unit: if one is undersized, the other cannot compensate.

Step 1: Calculate the pool volume

All sizing starts with a single piece of data: how many cubic meters of water the pool holds. The formula changes depending on the shape:

Volume formulas according to pool shape, and a calculation example

• Rectangular: V = Length × Width × Depth
• Circular: V = Diameter × Diameter × Depth × 0.785
• Oval: V = Length × Width × Depth × 0.785
• Freeform: V = Length × Width × Depth × 0.85
• For example, a pool 8 × 4 m with a depth of 1.5 m: V = 1.5 × 8 × 4 × 0.785 = 37.68 m³.

Step 2: Define the filtration time

"Turnover time" (or filtration time) is the time required for the circulation system to move the entire pool volume through the filter. In Costa Rica, the Pool Management Regulation (Decree DE-35309) establishes maximum times according to the type of pool:

Clarification: the indicated maximum time is the maximum time for the entire pool volume to pass through the filter once.

Why is a single turnover per day not enough?

Just because the pump moves the entire volume of the pool once doesn't mean that 100% of the water has passed through the filter: some of the water recirculates without being filtered due to the way it mixes within the pool. The actual percentage of filtered water increases with each additional turnover:

Therefore, to size the flow rate, it is common to aim for 3 or 4 turnovers within the available filtration time, instead of just one, if a high percentage of effectively filtered water is desired.

Step 3: Calculate the required flow rate

With the pool volume, the available filtration time, and the number of turnovers desired, the flow rate (Q) that the system must move is calculated:

Q = (Volume × Number of turnovers) ÷ Filtration time

Example: a rectangular pool 8 × 4 m with a depth of 0.9 m has a volume of 28.8 m³. If a filtration time of 6 hours with 4 turnovers is desired:

Q = 28.8 m³ × 4 ÷ 6 h = 19.2 m³/h (≈ 84.5 gpm)

This flow rate (19.2 m³/h) is the data that will be used to choose both the filter and the pump.

Step 4: Choose the correct filter

A sand filter is selected by its filtering area, not by its size itself. The relationship is:

Filter flow rate = Filter area × Filter media velocity

The standard velocity of the filter media (sand) is approximately 20 gpm/ft², equivalent to 49 m³/m²/h. From there, each filter model has a maximum nominal flow rate according to its diameter. You can validate this in the filter manufacturer's technical sheet. Below is the data for ESPA filters manufactured in Spain:

Model	Diameter	Filter area	Flow rate (m³/h)	Flow rate (gpm)
FKB 450	0.45 m	0.16 m²	8	35.23
FKP 520	0.52 m	0.21 m²	11	48.44
FKB 550	0.55 m	0.24 m²	12	52.84
FKP 620	0.62 m	0.30 m²	15	66.05
FKB 650	0.65 m	0.33 m²	17	Discover special DISCOUNTS for returning customers Especially aimed at installers, pool installers, builders, property managers and maintenance service companies Email Take advantage of our training and promotions Subscribe to our newsletter and be the first to learn about new products, exclusive offers, and training workshops. Email Support Typically replies within an hour Hi there 👋 How can I help you? × How can I help you?