How Many Gallons of Water Does My Pool Hold?
The formula: length × width × average depth × 7.48. Here are gallons for common pool sizes and why getting this number right matters.
There's no single answer to this one because every pool is different. But there's one formula and one number you need to know:
Length × Width × Average Depth × 7.48 = Gallons
That 7.48 is gallons per cubic foot. It's the one constant that makes the whole calculation work. Average depth is just your shallow end plus your deep end divided by two.
Rectangle Pools
Most residential pools are rectangular (or close enough). Here's a quick reference at 5-foot average depth:
| Pool Size | Cubic Feet | Gallons | |-----------|-----------|---------| | 12 × 24 ft | 1,440 | 10,771 | | 15 × 30 ft | 2,250 | 16,830 | | 16 × 32 ft | 2,560 | 19,149 | | 18 × 36 ft | 3,240 | 24,235 | | 20 × 40 ft | 4,000 | 29,920 |
If your average depth is different, adjust proportionally. A 15×30 pool at 4 feet average holds 13,464 gallons instead of 16,830.
Round Pools
For round above-ground pools, swap in the circle formula:
3.14 × radius² × average depth × 7.48 = gallons
A 24-foot-round pool at 4 feet deep: 3.14 × 12² × 4 × 7.48 = 13,536 gallons.
Why Getting This Right Actually Matters
This isn't just a fun math exercise. Your pool volume determines everything about water chemistry:
Chlorine dosing. Most shock treatments are dosed per 10,000 gallons. If you think your pool is 15,000 gallons but it's actually 20,000, you're under-chlorinating by 25%. Your water stays cloudy, algae gets a foothold, and you end up dumping in more chemicals than you would have if you'd dosed correctly from the start.
pH adjustment. Muriatic acid and pH increaser are both volume-dependent. Overdosing acid because you underestimated your pool size can crash the pH and damage your heater, pump seals, and vinyl liner.
Salt systems. Salt chlorine generators need the salt level dialed in precisely — usually 2,700–3,400 ppm. You add salt by the bag based on gallons. Wrong volume = wrong salt level = the generator doesn't work properly or throws error codes.
Water balance. Calcium hardness, alkalinity, cyanuric acid — it's all dosed by volume. A pool maintenance professional's first question is always "how many gallons?" If you don't know, everything else is guesswork.
Measuring Tips
Measure the water, not the pool walls. Most pools aren't filled to the brim — the water line usually sits 4–6 inches below the coping. Use the actual water depth, not the wall height.
For irregular shapes, break the pool into rectangles and semicircles, calculate each section separately, and add them up. An L-shaped pool is just two rectangles. A kidney shape can be approximated as an oval: length × width × average depth × 5.9.
Ask your builder. If your pool was professionally installed, the builder should have the exact volume in their records. It's also sometimes listed on the original permit paperwork.
Use our pool volume calculator to get the exact gallon count for rectangular, round, and oval pools — including fill time estimates.
Frequently Asked Questions
- How many gallons are in a 12×24 pool?
- A 12×24-foot rectangular pool at an average depth of 5 feet holds approximately 10,800 gallons. At 4 feet average depth, that drops to about 8,600 gallons. The exact volume depends on whether the pool has a gradual slope from shallow to deep end or a constant depth.
- How do I calculate the volume of my pool?
- For a rectangular pool: Length × Width × Average Depth × 7.48 = gallons. For average depth with a slope, add shallow depth + deep depth and divide by 2. For a round pool: 3.14 × Radius² × Average Depth × 7.48. For an oval: 3.14 × (Length/2) × (Width/2) × Average Depth × 7.48.
- How long does it take to fill a pool with a garden hose?
- A standard garden hose flows at about 10 gallons per minute. A 10,000-gallon pool takes roughly 17 hours of continuous flow. A fire hose from the water company can fill the same pool in 1–2 hours but usually costs $150–$400 for delivery. Municipal water companies sometimes offer discounted rates for one-time pool fills.
- How much does it cost to fill a pool with water?
- At typical municipal water rates of $0.003–$0.008 per gallon, filling a 10,000-gallon pool costs $30–$80. Delivery by water truck typically costs $175–$400 for the first load. Many homeowners fill with municipal water for the first fill and water the lawn if they ever need to add more.
- How much water does a pool lose to evaporation?
- A typical residential pool in a warm climate loses 1/4 to 1/2 inch of water per day to evaporation — roughly 500–1,000 gallons per week in summer. Wind dramatically accelerates evaporation. A pool cover can reduce evaporation loss by 95%, saving thousands of gallons per season.