Oval Pool Volume Calculator (Gallons)

Find the gallons in an oval pool from its length, width and water depth. Common for oval above-ground and semi-inground pools such as 15×30 and 18×33.

Estimate: results come from your inputs and standard values (8.34 lb/gal, pool geometry). Measure your pool and verify before relying on a number.

Calculator

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ft
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Volume13,958 gallons
Volume (liters)52,839 L
Average depth4.00 ft
Surface area467 ft²

An oval pool 18.0 × 33.0 ft at 4.0 ft average depth holds ≈ 13,958 gallons.

An oval pool is a stretched circle: an ellipse. Its volume uses the same π-based math as a round pool, but with two different axes — the long length and the short width. Oval shapes are popular for above-ground and semi-inground pools because they fit a rectangular yard while holding less water (and therefore fewer chemicals) than a true rectangle of the same footprint.

As with every shape here, the volume depends on the average of the shallow and deep depths, so enter both and let the tool do the averaging.

Formula

An ellipse of length L and width W has an area of π × (L ÷ 2) × (W ÷ 2). Multiply by the average depth and by 7.48 gallons per cubic foot:

gallons = π × (length ÷ 2) × (width ÷ 2) × avg depth × 7.48

where avg depth = (shallow + deep) ÷ 2. Note that (L ÷ 2) and (W ÷ 2) are the two semi-axes; multiplied by π they give the ellipse area, exactly π/4 ≈ 0.785 of the enclosing rectangle.

Worked example

An oval pool 18 ft on one axis by 33 ft on the other, at a 4 ft average depth:

  1. Semi-axes = 18 ÷ 2 = 9 ft and 33 ÷ 2 = 16.5 ft.
  2. Surface area = π × 9 × 16.5 ≈ 467 ft².
  3. Volume = 467 × 4 ≈ 1,867 ft³.
  4. Gallons = 1,867 × 7.48 ≈ 13,957 gallons.

A rectangle of the same 18 × 33 ft footprint at 4 ft would hold 18 × 33 × 4 × 7.48 ≈ 17,772 gallons — about 27% more — which shows how much the rounded ends reduce the volume.

Length, width and axis labels

The oval formula is symmetric, so it does not matter which axis you call the length and which the width — the product is the same. Just measure the pool across its widest points on each axis, at the waterline.

  • Uniform-depth oval above-ground pools: enter the same depth in both fields.
  • Semi-inground ovals with a deep end: enter the real shallow and deep depths so the average is correct.
  • Buttressed / yardless ovals: measure the water surface, not the outer frame or the support straps, which extend beyond the water.

With the gallons in hand, size your salt, shock and heater. If your pool is a true circle, use the round calculator; if it is a rectangle, use the rectangular calculator.

Frequently asked questions

How many gallons is an 18 by 33 ft oval pool?
An 18 × 33 ft oval pool at a 4 ft average depth holds about 13,957 gallons (π × 9 × 16.5 × 4 × 7.48). Adjust the depth above for your exact fill level.
Does it matter which side I call length or width?
No. The oval formula multiplies the two half-axes together, so swapping length and width gives the same volume. Just measure each axis at its widest point along the waterline.
Why does an oval hold less than a rectangle of the same size?
Because an ellipse covers only π/4 ≈ 78.5% of the rectangle that encloses it. The rounded ends cut out the corners, so an oval holds roughly 21% less water than a rectangle of equal length and width.
How do I measure an oval above-ground pool?
Measure the water surface across the long axis and the short axis, at the waterline, and enter the actual water depth. Ignore the outer frame and support straps, which sit outside the water.

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