Round Pool Water Volume Calculator

This calculator works out the water volume of a round above-ground or inground pool in gallons and liters. You enter the pool’s diameter and depth (or water depth), and it returns the total volume, plus the surface area and approximate fill time from a typical garden hose. The calculation uses the standard formula for a cylinder: volume = Ï€ × radius² × depth.

Formula

The volume of a round pool is the same as the volume of a cylinder:

V = π × r² × h

Where:

• V = volume in cubic feet (then converted to gallons or liters)
• Ï€ ≈ 3.14159
• r = radius of the pool in feet (diameter ÷ 2)
• h = water depth in feet (not the pool wall height, unless the pool is filled to the brim)

One cubic foot of water equals 7.48052 US gallons, or 28.3168 liters. If you measure in meters, the volume in cubic meters is simply π × r² × h, and 1 cubic meter = 1000 liters = 264.172 US gallons.

Worked Example

Using the default values from the calculator: an 18-foot diameter pool with 4 feet of water.

1. Radius = 18 ÷ 2 = 9 ft
2. r² = 9 × 9 = 81 sq ft
3. π × r² = 3.14159 × 81 = 254.47 sq ft (surface area)
4. Volume in cubic feet = 254.47 × 4 = 1017.88 cu ft
5. Gallons = 1017.88 × 7.48052 = 7614.4 US gallons
6. Liters = 1017.88 × 28.3168 = 28,823 L

The calculator rounds the gallons to 7614 and liters to 28,823, matching the above.

Why Water Depth Matters, Not Wall Height

A common mistake is entering the pool’s advertised height (e.g., a 52-inch wall) instead of the actual water depth. Most pools are filled 6–8 inches below the top of the wall. For a 52-inch wall, typical water depth is about 44–46 inches (3.7–3.8 ft). Using wall height instead of water depth can overestimate volume by 10–15%, which matters when dosing chemicals or estimating fill time.

To measure water depth accurately, lower a weighted tape measure or a pool ruler to the bottom at the deepest point (for a flat-bottom above-ground pool, it’s uniform). For an inground pool with a sloped floor, you need a more complex calculation that accounts for the slope – the simple cylinder formula only works for flat-bottom pools.

How to Read the Result

The volume number tells you:

• Chemical dosing: Most pool chemical instructions give doses per 10,000 gallons. For example, if your pool is 7,600 gallons, you’d use 76% of the dose listed for 10,000 gallons.
• Fill time: A typical garden hose delivers 5–10 gallons per minute. At 7.5 gallons per minute, filling 7,600 gallons takes about 17 hours. The calculator’s surface area output helps estimate evaporation losses (roughly 1/4 inch per day in hot weather).
• Heater sizing: Larger volumes require more BTUs to raise temperature. A 7,600-gallon pool needs roughly 76,000 BTUs to heat 1°F per hour (a rule of thumb: 10 BTUs per gallon per hour for 1°F rise).

Common Mistakes and How to Avoid Them

• Mixing units: If you measure diameter in feet and depth in inches, you must convert depth to feet (divide by 12). The calculator expects both in feet.
• Using wall height instead of water depth: As noted, this overestimates volume.
• Forgetting the pool isn’t a perfect cylinder: Some round pools have a slight taper or a deeper center. For a typical flat-bottom above-ground pool, the cylinder formula is accurate to within 2–3%.
• Ignoring the offset for a skimmer or overflow: If your pool has an overflow line, the actual water level may be lower than your tape measure suggests – measure at the deepest point.
• Rounding too early: If you round Ï€ to 3.14 and radius to the nearest foot, you can introduce a 5% error. Keep at least 3 significant figures until the final step.

Round vs. Oval vs. Rectangular Pools

This calculator is for round pools only. For other shapes:

• Oval pools: Volume ≈ Ï€ × (long diameter/2) × (short diameter/2) × depth × 7.48. This treats the oval as an ellipse.
• Rectangular pools: Volume = length × width × average depth × 7.48. For sloped floors, use the average of the shallow and deep ends.
• Kidney/freeform pools: Use a water displacement method or a professional measurement. A rough estimate is length × width × average depth × 7.0, but this can be off by 10%.

If your pool has a deep end, the cylinder formula underestimates volume. For a round pool with a sloped floor, you’d need to integrate the depth profile – beyond the scope of this simple calculator.

Safety and Accuracy Notes

Always double-check your measurements with a second person for large pools. When filling, never leave a hose unattended – overfilling can damage the filter or cause water to enter the skimmer incorrectly. For chemical dosing, always follow the manufacturer’s instructions on the product label, which are based on the actual water volume. If you’re unsure about your pool’s volume, you can do a salt test: add a known amount of salt (e.g., 10 lbs), wait 24 hours, test the salt level, and calculate volume from the dilution. But for most routine purposes, the geometric formula with accurate depth is sufficient.

Frequently Asked Questions

How many gallons is a 12-foot round pool?

Assuming a water depth of 3 feet (a typical 12×30-inch pool filled to 36 inches), the volume is Ï€ × (6)² × 3 × 7.48 = 2,542 gallons. If the water depth is only 2.5 feet, it’s about 2,118 gallons. Always measure your actual water depth.

Do I need to include the pool’s filter and pipes in the volume?

No. The filter system holds only a small fraction of the total volume (usually 10–50 gallons for a residential pool). For chemical dosing, you can ignore it. For total water needed to fill from empty, include it, but it’s negligible for most calculations.

What if my pool has a deep end?

The cylinder formula is only accurate for flat-bottom pools. For a round pool with a sloped floor, you can approximate by using the average depth (shallow end depth + deep end depth, divided by 2) in the formula. This is accurate to within 5–10% for typical slopes. For precise volume, use a pool volume calculator that asks for multiple depth measurements.

How long will it take to fill my pool with a garden hose?

Measure your hose flow rate by timing how long it takes to fill a 5-gallon bucket. If it takes 1 minute, you have 5 gallons per minute. Divide your pool volume in gallons by that rate. For example, a 7,600-gallon pool at 5 gpm takes 1,520 minutes, or about 25 hours. At 10 gpm, it’s 12.7 hours.

Does the calculator work for metric measurements?

The calculator as shown uses feet and gallons. If you measure in meters, you can convert: 1 meter = 3.28084 feet. Or use the liters output by entering your diameter and depth in feet (you can convert meters to feet first). For a meter-based calculation, multiply the volume in cubic meters by 1000 to get liters.

Why does my pool’s label say a different volume than this calculator?

Manufacturer labels often use the wall height (not water depth) and may round up. For example, an 18-foot pool with 52-inch walls is often labeled as 7,600 gallons, but with a typical water depth of 44 inches, the real volume is closer to 6,900 gallons. The label is a maximum, not the actual operating volume.

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in d | Pool diameter | ft | 18 | 0
in h | Water depth | ft | 4 | 0
out vol_gal | Water volume | US gal | pi * (d/2)^2 * h * 7.48052 | 0
out vol_l | Water volume | L | pi * (d/2)^2 * h * 28.3168 | 0
out area | Surface area | sq ft | pi * (d/2)^2 | 1