How to Calculate the Volume of Water in Your Pool

Knowing how much water your pool or spa holds is essential for tasks like chemical dosing, equipment sizing, or just planning a refill. The formulas below will help you estimate the water volume based on your pool’s shape.

 
Rectangular or Square Pools

Formula (in gallons):
Length × Width × Average Depth × 7.5 = Volume (gallons)

Why 7.5? Because there are 7.5 gallons in each cubic foot.

Example:
If your pool is 30 ft long, 15 ft wide, and has an average depth of 5 ft:
30 × 15 × 5 × 7.5 = 16,875 gallons

Metric Version (in liters):
Length × Width × Average Depth × 1000 = Volume (liters)

Circular Pools or Spas

Formula (in gallons):
3.14 × Radius² × Average Depth × 7.5 = Volume (gallons)

Measure the radius (half the diameter), square it, then apply the formula.

Example:

Radius = 5 ft, Depth = 4 ft
3.14 × 25 × 4 × 7.5 = 2,355 gallons

Metric Version (in liters):
3.14 × Radius² × Average Depth × 1000 = Volume (liters)

 
Kidney-Shaped or Irregular Pools

Use this simplified approach:

Formula (in gallons):
0.45 × (Width A + Width B) × Length × Average Depth × 7.5 = Volume (gallons)

A and B are the two widest points on each end.

Example:

A = 8 ft, B = 10 ft, L = 25 ft, Depth = 5 ft
0.45 × 18 × 25 × 5 × 7.5 = 7,593.75 gallons

Metric Version (in liters):
0.45 × (A + B) × L × Depth × 1000

Spas or Tiered Depth Pools

If the pool has different depths (like a shallow and deep end), find the average depth:

(Shallow Depth + Deep Depth) ÷ 2 = Average Depth

If the pool has very distinct depth zones, break it into sections and calculate each volume separately, then add them together.

 
Tips for Precision

1. Convert inches to feet by multiplying by 0.0833.
2. Always use water depth—not container depth—especially if the pool isn’t filled to the top.
3. Round off to the nearest decimal for easier calculation.
4. When using metric, remember 1 cubic meter = 1000 liters.
    

   Why It Matters

Getting an accurate volume helps you:

Dose pool chemicals correctly
Size pumps and heaters properly
Manage water costs and safety
Need help calculating a specific pool? Just share your measurements and shape!