Swimming Pool Water Volume: Calculate Gallons and Liters

Calculate swimming pool water volume from the pool’s shape and measured dimensions. Enter dimensions in feet or meters and average depth in the matching unit; the calculator returns an estimated volume in US gallons and liters.

Formula

For a rectangular pool, volume = length × width × average depth. For a circular pool, volume = π × (diameter ÷ 2)² × average depth. For an oval pool, this calculator estimates the footprint as an ellipse: volume = length × width × 0.785398 × average depth. In every case, US gallons = cubic feet × 7.48052, and liters = US gallons × 3.78541.

Variables: length (l), width (w), shallow depth (d1), and deep depth (d2) are entered in feet. Average depth is (d1 + d2) ÷ 2, in feet. For a circular pool, enter its diameter as both length and width. The shape selector (s) chooses the footprint formula. The outputs are US gallons, liters, and cubic feet. Select a shape that matches the pool; the oval option assumes a reasonably regular ellipse.

Worked Example

Using the default values—rectangular shape, length 20 ft, width 10 ft, shallow depth 3 ft, and deep depth 6 ft:

1. Average depth = (3 + 6) ÷ 2 = 4.5 ft.
2. Footprint area = 20 × 10 = 200 ft².
3. Volume = 200 × 4.5 = 900 ft³.
4. US gallons = 900 × 7.48052 = 6,732.468, which rounds to 6,732 US gal.
5. Liters = 6,732.468 × 3.78541 = 25,489.42, which rounds to 25,489 L.

The calculator also reports 900.0 ft³. Its gallons and liters are rounded to whole units.

What You’ll Need

• A tape measure or other suitable measuring tool.
• A way to record length, width or diameter, and water depth.
• The pool’s basic shape and, for a sloped floor, shallow- and deep-end depths.
• A calculator, or the calculator above. Measure in feet for this calculator; convert metric measurements before entering them (1 m = 3.28084 ft).

Step-by-Step

1. Choose the pool shape. Use rectangular for a rectangular footprint, circular for a round pool, and oval for an oval footprint. Irregular pools need to be split into simpler sections for a better estimate.
2. Measure the inside water length and width. Measure along the pool’s interior, rather than including coping or decking. For a round pool, measure straight across its widest point through the center; enter that diameter for both length and width.
3. Measure depth at the waterline. Record the vertical water depth, not the outside wall height. If the bottom slopes between shallow and deep ends, measure both ends. Enter these as shallow and deep depth. If the pool floor is level, enter the same depth twice.
4. Use consistent units. The fields are in feet, so convert dimensions measured in inches or meters. Divide inches by 12 to get feet; multiply meters by 3.28084. Do not enter length in feet and depth in inches.
5. Enter the measurements and select the shape. The calculator averages the two depths, estimates the footprint area from the selected shape, then converts cubic feet to US gallons and liters.
6. Read the volume as an estimate. Use the result as an approximate capacity, not an exact water-meter reading. For an irregular outline, attached spa, steps, or varying floor, calculate sections separately where possible and add their volumes.

Common Mistakes

• Mixing units: A depth entered in inches alongside length in feet makes the result far too small. Convert every measurement to feet first.
• Using outside dimensions: Coping, walls, and decorative edges are not part of the water footprint. Measure the interior.
• Entering radius instead of diameter: A circular pool requires the full distance across. Entering the radius halves the diameter and substantially understates volume.
• Assuming an oval is rectangular: Multiplying its full length by width treats the corners as filled. The oval estimate uses an elliptical footprint; a freeform pool may differ considerably.
• Using one depth for a sloped pool: The shallow and deep measurements help represent the sloping floor. The simple average is an estimate, especially if the slope changes sharply or there is a deep hopper.
• Confusing US gallons with imperial gallons: The calculator reports US gallons. An imperial gallon is larger, so do not use the figures interchangeably.

Tips

• Take each measurement twice and use the water’s current level, since a pool filled below its normal line holds less than its nominal capacity.
• For a rectangular pool with a sloped floor, average the depths at the ends. If the floor has distinct flat sections or a deep end, calculate each section’s volume separately for improved accuracy.
• For an L-shaped pool, divide the footprint into non-overlapping rectangles, calculate each section with its own average depth, and add the results.
• Keep unrounded measurements through the calculation; round only the displayed final volume. The calculator rounds gallons and liters to whole units.
• Record the measured dimensions and date. Reuse the estimate as a reference, but recalculate if the pool’s water level or configuration changes.

Troubleshooting

The result seems much too high or low: Check that every input is in feet, that the shape selection is correct, and that a circular pool’s diameter—not radius—was entered. Confirm depth is measured vertically from the water surface to the floor.

The pool has a sloped or uneven bottom: The average of shallow and deep depth works best for a fairly consistent slope. For a steep hopper or several floor levels, divide the pool into sections, estimate each section’s average depth, and add the section volumes. Avoid counting overlapping areas twice.

The pool is kidney-shaped or freeform: The oval formula may not match its footprint. Divide the outline into rectangles, circles, or other manageable shapes; estimate each separately, then sum the volumes. Treat the answer as approximate.

Your pool’s measured fill differs from the estimate: Check the water meter or filling record and consider water lost to splashing, evaporation, backwashing, or leakage. Such differences do not necessarily mean the geometric calculation is wrong.

Frequently Asked Questions

How do I calculate pool volume in gallons?
Find volume in cubic feet, then multiply by 7.48052 for US gallons. For a rectangle, multiply length, width, and average depth.

How do I calculate a round pool’s volume?
Use π × radius² × average depth to get cubic feet, then multiply by 7.48052 for US gallons. Radius is half the measured diameter.

How do I account for a sloping pool floor?
For a steady slope, average shallow and deep depths and multiply by the footprint area. For substantial level changes or a hopper, calculate sections separately.

How many liters are in a US gallon?
One US gallon is approximately 3.78541 liters. This differs from an imperial gallon.

Is the calculated volume exact?
No. It is an estimate based on measured dimensions and a simplified shape. Curves, steps, benches, slopes, and water level can affect actual capacity.

Can I use metric measurements?
Yes. Convert meters to feet by multiplying by 3.28084 before entering them. The calculator then provides both US gallons and liters.

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in l | Length | ft | 20 | 0
in w | Width | ft | 10 | 0
in d1 | Shallow depth | ft | 3 | 0
in d2 | Deep depth | ft | 6 | 0
sel s | Shape | Rectangular=1, Circular=2, Oval=3 | 1
out gal | Pool volume | US gal | (s=1)*l*w*((d1+d2)/2)*7.48052+(s=2)*pi*(l/2)^2*((d1+d2)/2)*7.48052+(s=3)*l*w*0.785398*((d1+d2)/2)*7.48052 | 0
out lit | Pool volume | L | gal*3.78541 | 0
out ft3 | Pool volume | ft³ | (s=1)*l*w*((d1+d2)/2)+(s=2)*pi*(l/2)^2*((d1+d2)/2)+(s=3)*l*w*0.785398*((d1+d2)/2) | 1