How to Calculate Pool Volume: Formulas for Every Shape

Knowing your pool's volume in gallons is essential for proper chemical dosing, equipment sizing, and turnover rate calculations. An incorrect volume estimate means over- or under-dosing chemicals, which leads to water balance problems and wasted money.

Why Pool Volume Matters

Pool volume is the foundation for nearly every pool maintenance calculation:

• Chemical dosing: Chlorine, acid, and other chemicals are dosed based on gallons
• Pump sizing: Flow rate requirements depend on pool volume
• Turnover rate: Health codes often require 6-8 hour turnover times
• Heater sizing: BTU requirements scale with water volume
• Salt calculations: Salt pools need precise ppm adjustments

A 10% error in volume estimation means a 10% error in every chemical addition. Over time, this compounds into serious water balance issues.

The Universal Multiplier: 7.5

The number 7.5 appears in pool volume calculations because one cubic foot of water equals 7.48 gallons. We round to 7.5 for simpler math.

All pool volume formulas follow the same pattern:

The only difference between pool shapes is how you calculate the surface area.

Rectangular Pool Volume

Most residential and many commercial pools are rectangular, making this the most common calculation.

Calculating Average Depth

For pools with a consistent slope from shallow to deep end:

For pools with a hopper bottom (flat shallow, then drops to deep end), you'll get more accurate results by calculating each section separately and adding them together.

Circular Pool Volume

Round pools use a modified formula that accounts for the circular surface area (π × radius²).

Oval Pool Volume

Oval pools combine elements of rectangular and circular calculations.

Irregular and Freeform Pools

Kidney-shaped, lagoon-style, and custom freeform pools require a different approach. You have two options:

Method 1: Surface Area Estimation

Estimate the surface area by breaking the pool into simpler shapes (rectangles and half-circles), then use:

Method 2: The 80% Rule

For a kidney-shaped pool, calculate as if it were a rectangle using the longest length and widest width, then multiply by 0.80:

This accounts for the curved sides that reduce total volume compared to a full rectangle.

Spa and Hot Tub Volume

Spas typically use the same formulas as pools but on a smaller scale. Most residential spas hold 300-500 gallons.

Spa Size	Typical Volume
2-3 person	150-250 gallons
4-5 person	250-350 gallons
6-7 person	350-500 gallons
8+ person	500-800 gallons

Manufacturer specifications are more accurate for spas since the molded shell shape makes manual calculation difficult.

Common Pool Volumes by Size

Use this table for quick estimates:

Pool Size	Average Depth	Approximate Gallons
12' × 24' Rectangle	5 ft	10,800
16' × 32' Rectangle	5 ft	19,200
18' × 36' Rectangle	5.5 ft	26,730
20' × 40' Rectangle	5.5 ft	33,000
15' Round	4 ft	5,310
18' Round	4 ft	7,646
24' Round	4 ft	13,594
27' Round	4 ft	17,205

Using Volume for Chemical Dosing

Once you know your pool volume, you can calculate chemical additions. Most dosing charts assume 10,000 gallons, so you'll need to adjust proportionally.

For more precise calculations, use our chemical dosage calculator which factors in your current and target levels.

Turnover Rate Calculations

Health departments often require pools to "turn over" (filter all the water) within 6-8 hours. Knowing your volume lets you verify your pump is adequate.

Need help sizing a pump? Check our pump run time calculator and DOE pump regulations guide.

Measurement Tips for Accuracy

Accurate measurements lead to accurate volume calculations:

• Measure at water level, not the coping or tile line
• Take multiple depth readings for irregular bottoms
• Account for steps and benches by subtracting their volume
• Measure to the water line, not the top of the wall
• Use a tape measure, not pacing or estimation

For existing pools, check the builder's specifications or permit documents—they often include calculated volume.