How to Calculate Pool Volume in Gallons
Determining the exact water volume of your swimming pool in gallons is the foundation of pool maintenance. Without an accurate capacity figure, you cannot calculate correct chemical dosages (which can lead to under-treating or toxic chemical concentrations), size your filter pump cycles, or select a pool heater. While the calculation can feel daunting if your pool has a sloped floor or curved shape, it relies on basic geometric volume formulas. In this guide, we break down the math for rectangular, circular, oval, and kidney pools.
Table of Contents
1. Determining Average Pool Depth
Most swimming pools do not have a flat bottom. To calculate volume accurately, you must first determine the average depth.
Measure the depth of the water at the shallowest point and the deepest point, and calculate the average:
*Note: If your pool floor drops off sharply rather than sloping gradually, measure the depths of the individual shallow and deep zones separately and calculate a weighted average based on surface area.*
2. Rectangular Pool Volume Formula
Rectangular pools represent the simplest calculation because the walls are straight and meet at 90-degree angles:
*Example: A 30ft long by 15ft wide pool with a shallow end of 3ft and a deep end of 8ft (average depth = 5.5ft): Volume = 30 × 15 × 5.5 × 7.5 = 18,562.5 gallons.*
3. Circular & Oval Pool Volume Formulas
Curved pools require slightly modified math:
- Circular Pools: Calculate the area using $\pi r^2$, multiply by depth, and multiply by 7.5:
Volume (Gallons) = 3.14159 × Radius² (ft) × Average Depth (ft) × 7.5*(Radius is half of the diameter. For an 18ft round pool, the radius is 9ft).*
- Oval Pools: Because the corners are rounded, the standard rectangular multiplier of 7.5 is reduced to **5.9**:
Volume (Gallons) = Length (ft) × Width (ft) × Average Depth (ft) × 5.9
4. Kidney-Shaped Pool Calculations
Kidney-shaped pools feature an indentation along one side, creating two distinct lobes. To calculate volume:
- Measure the length of the pool.
- Measure the width of the larger lobe (A) and the smaller lobe (B).
- Calculate the average width: (A + B) / 2.
- Apply the kidney shape multiplier (**0.45** for surface area, then scale by 7.48 to convert cubic capacity):
5. The Magic Multipliers: Converting Cubic Feet
Why do these formulas contain multipliers like **7.5** or **5.9**?
Calculations of dimensions (Length × Width × Depth) yield the pool's volume in **cubic feet**. A single cubic foot of space holds exactly **7.48 gallons** of water. The standard rectangular multiplier rounds this to **7.5** for fast estimation. The oval multiplier (**5.9**) combines the 7.48 conversion factor with a geometric corner adjustment ($0.785$) to account for the curved shape of the footprint.
To calculate these values automatically and plan your chemical doses, use our interactive pool calculator on the home page. Choose your shape and dimensions, and get instant capacity and dosage requirements.