Balance Tank / Infinity Edge Sizing Calculator

What Is a Balance Tank and Why Infinity Pools Need One

A balance tank (also called a surge tank or compensation tank) is a reservoir that collects water displaced from an overflow pool and holds it until the circulation system can return it to the pool. Every infinity edge, vanishing edge, and perimeter overflow pool requires a balance tank to function properly.

When swimmers enter the pool, their body mass displaces water over the edge. When waves form from wind, diving, or play, additional water flows over the weir. Without a properly sized balance tank, this displaced water has nowhere to go, causing flooding, air locks in pumps, and potential structural damage to the surrounding deck and landscape.

The balance tank also serves as the suction source for the main circulation pump. If the tank runs dry because it is undersized, the pump will lose prime, cavitate, and potentially suffer permanent damage. Conversely, an oversized tank wastes construction budget and takes up valuable mechanical room space.

How to Calculate Balance Tank Size

Proper balance tank sizing accounts for three primary factors that contribute to water displacement:

1. Bather surge displacement: The volume of water displaced when the maximum number of swimmers enters the pool simultaneously. Each person displaces water equal to their body volume, calculated from their weight and the density of water (62.4 lbs per cubic foot).
2. Evaporation allowance: Overflow edges dramatically increase the exposed water surface area, accelerating evaporation. The tank must hold enough reserve to compensate for evaporation between auto-fill cycles or manual top-offs.
3. Wave action reserve: Wind, water features, and swimmer activity create waves that push water over the edge. A standard allowance of 1% of total pool volume accounts for this ongoing displacement.

The final recommended size includes a 25% safety factor above the calculated minimum to account for unusual conditions, simultaneous peak loads, and design tolerances.

The Surge Capacity Formula

Surge volume is the most critical component of balance tank sizing. The formula is straightforward:

Surge Volume = Max Bathers x (Avg Weight / 62.4) x 7.48

Where 62.4 is the weight of water in pounds per cubic foot, and 7.48 converts cubic feet to gallons. For a pool with a maximum bather load of 20 people at an average weight of 170 lbs, the surge volume would be approximately 407 gallons. This is the volume of water that gets pushed over the edge when every swimmer enters at once.

Evaporation Factors for Different Edge Types

Not all overflow edges evaporate at the same rate. The type of edge design significantly affects how quickly water is lost:

• Vanishing / Infinity edges: These have the highest evaporation rates because water flows in a thin sheet over the edge, dramatically increasing the exposed surface area. In hot, arid climates, expect up to 15 gallons per linear foot per day. Moderate climates see about 8 gallons per linear foot per day, and cool, humid climates around 4 gallons per linear foot per day.
• Perimeter overflow / Slot drain: These edges contain the water within a narrow channel, reducing the exposed area compared to vanishing edges. Rates are roughly two-thirds of vanishing edge values: 10, 5, and 3 gallons per linear foot per day for hot, moderate, and cool climates respectively.
• Partial overflow: Pools where only a portion of the perimeter overflows have the lowest evaporation rates, approximately half that of a full vanishing edge.

Common Sizing Mistakes

Balance tank sizing errors are among the most expensive mistakes in infinity pool construction. Here are the most common pitfalls:

Undersized Balance Tanks

The most frequent and most damaging error. An undersized balance tank causes the pump to lose prime during peak usage, leads to water spilling over the tank onto the mechanical room floor, and forces the auto-fill system to run constantly. In severe cases, the pump can cavitate and destroy its impeller and mechanical seal within weeks. Retrofitting a larger tank after construction is extremely costly, often requiring jackhammering of finished concrete and rerouting of plumbing.

Ignoring Evaporation in Hot Climates

Builders in Arizona, Nevada, Texas, and Southern California often underestimate evaporation losses. A 60-foot vanishing edge in Phoenix can lose over 900 gallons per day to evaporation alone. Without adequate reserve capacity, the auto-fill system cannot keep up during peak summer months, especially during water restriction periods.

Not Accounting for No Auto-Fill

Pools without automatic water leveling systems need significantly more balance tank capacity to compensate for evaporation between manual refills. Our calculator adds an extra full day of evaporation reserve when no auto-fill is present.

Forgetting Wave Action

Even calm pools experience some wave action from wind and water returns. Active pools with slides, diving boards, or heavy swimmer loads generate substantial waves. The 1% of pool volume reserve is a conservative starting point; pools with water features may need more.

Using the Wrong Edge Length

Measure the actual overflow weir length, not the pool perimeter. An L-shaped infinity pool might have a 40-foot perimeter but only a 25-foot vanishing edge. Conversely, a full perimeter overflow measures the entire circumference.

Retrofit Considerations

If you are adding or resizing a balance tank on an existing pool, keep these factors in mind:

• Location: The balance tank must be below the overflow edge water level for gravity feed. It should be as close to the pool as possible to minimize pipe friction losses.
• Material: Concrete tanks are most common for permanent installations. Fiberglass and polyethylene tanks work for smaller applications and retrofits where space is limited.
• Access: Ensure the tank has adequate access for cleaning, inspection, and pump maintenance. A tank that cannot be serviced will become a liability.
• Ventilation: Balance tanks in enclosed spaces need proper ventilation to prevent mold, humidity damage, and chloramine buildup.
• Structural engineering: Large balance tanks hold significant weight. A 2,000-gallon tank weighs over 16,000 lbs when full. Verify that the supporting structure can handle the load.

Frequently Asked Questions

What is the difference between a balance tank and a surge tank?

The terms are often used interchangeably. Technically, a surge tank specifically handles the sudden displacement from bathers entering the pool, while a balance tank serves the broader role of maintaining water level equilibrium including evaporation compensation and wave action. In practice, the same vessel serves both functions in residential and commercial infinity pools.

Can I use the pool equipment vault as a balance tank?

This is sometimes done in residential projects to save space, but it is generally not recommended. Equipment rooms are difficult to waterproof completely, and flooding can damage electrical components, heaters, and automation systems. A dedicated, purpose-built balance tank is always the better approach.

How deep should a balance tank be?

Balance tanks are typically 3 to 5 feet deep. Deeper tanks take up less floor area for the same volume. The minimum water depth above the pump suction line should be at least 12 inches to prevent vortexing and air entrainment. Many engineers specify 18 inches minimum above the suction inlet.

Does a balance tank need its own filtration?

No. The balance tank is part of the main circulation loop. Water flows from the pool over the edge into a catch basin, then by gravity to the balance tank, where it is drawn by the main pump through the filter and back to the pool. Some installations add a basket strainer at the tank inlet to catch large debris before it reaches the pump.

What happens if my balance tank is too large?

An oversized balance tank is far less problematic than an undersized one. The main downsides are increased construction cost, additional space required, and slightly more water and chemicals needed to fill the system. Some designers intentionally oversize by 50% or more as a conservative approach, especially for commercial pools with high bather loads.

When should I consult an engineer for balance tank sizing?

Consult a licensed hydraulic or pool engineer for commercial pools, pools larger than 50,000 gallons, installations with multiple overflow edges at different elevations, projects in extreme climate zones, or any pool with complex water features that affect displacement. This calculator provides a solid starting estimate, but complex projects benefit from professional engineering review.