Figure-8 Pool Volume Calculator

How to Use This Figure-8 Pool Volume Calculator

What Is Figure-8 Pool Volume?

Figure-8 pool volume is the total amount of water your figure-8 pool holds, measured in US gallons, litres, or cubic feet. This number is the starting point for every maintenance decision — from how much chlorine to add to how long a full refill takes.

A figure-8 pool is not a single smooth oval. It has two distinct basins connected by a narrowed waist, so a simple length-times-width formula does not apply. This calculator treats each basin as an independent circle (or ellipse), computes the lens-shaped area where the two basins overlap at the waist, and subtracts that overlap so the shared zone is not counted twice. The result is a basin-by-basin calculation that matches the pool's actual geometry.

Typical Figure-8 Pool Sizes and Water Capacity

Residential figure-8 pools range from compact 12 ft × 12 ft equal-basin layouts up to 22 ft × 18 ft unequal-basin designs. A symmetric 16 ft × 16 ft figure-8 pool at 5 ft average depth holds 13,955 gallons (52,827 litres), while a larger 20 ft × 20 ft pool at the same depth holds 21,805 gallons (82,542 litres).

Two figure-8 pools with the same overall length can hold very different volumes if the basin diameters or the waist width differ. A 20 ft × 16 ft unequal-basin pool holds 18,124 gallons (68,607 litres) — about 17% less than a symmetric 20 ft × 20 ft pool — because the smaller basin contributes less water and the waist deduction changes too.

Internal water dimensions matter more than deck-edge or coping measurements. Coping overhangs typically extend 2–4 inches beyond the waterline on each side, and entering the outer edge instead of the water surface inflates each basin diameter enough to skew the result by hundreds of gallons.

Figure-8 pools are easier to misjudge than simple round or oval shapes because the waist creates a visible pinch that a single measurement cannot capture. Measuring only the overall length and overall width produces a bounding box, not the actual water area.

How to Calculate Figure-8 Pool Volume

A figure-8 pool is best treated as two joined basins, so each basin is measured separately and then combined. This differs from an oval pool formula, which treats the pool as one continuous elliptical shape. Each basin in a figure-8 pool behaves like a round pool (or an ellipse, if the lobe is oval), and the waist overlap is subtracted to avoid double-counting the shared zone.

The Figure-8 Pool Volume Formula

Area₁ = D1² × π/4 (circular basin) or D1 × W1 × π/4 (oval basin)
Area₂ = D2² × π/4 (circular basin) or D2 × W2 × π/4 (oval basin)
Lens Overlap = exact two-circle intersection area
Total Area = Area₁ + Area₂ − Lens Overlap
Volume = Total Area × Average Depth
US Gallons = Volume (ft³) × 7.48052 | Litres = Volume (ft³) × 28.3168

D1 is the diameter (or length, if oval) of the larger basin. D2 is the diameter of the smaller basin. W1 and W2 are only needed if a basin is oval rather than circular. The calculator accepts unequal basins — D1 and D2 can be any two positive values.

The waist overlap is the lens-shaped zone where the two basins intersect. The calculator auto-estimates the overlap width (OW) as 25% of the smaller basin diameter, which is a reliable default for standard residential figure-8 pools. If you have builder plans with the exact waist width, check the manual override box and enter it directly. The lens area is then computed using the exact two-circle intersection formula — not an approximation.

A figure-8 pool is not the same as an oval pool. An oval formula treats the pool as a single ellipse (π/4 × Length × Width), which does not account for the pinched waist. The figure-8 formula works basin by basin and subtracts the shared zone, producing a result that reflects the actual water shape.

Unit consistency matters: if you measure in feet, the result is cubic feet. Multiply by 7.48052 for US gallons or 28.3168 for litres. If you measure in metres, multiply cubic metres by 1,000 for litres or by 264.172 for US gallons.

Worked Example: Figure-8 Pool Volume

An unequal-basin figure-8 pool has a larger basin diameter of 18 ft, a smaller basin diameter of 14 ft, a shallow end of 3.5 ft, and a deep end of 6.5 ft. Both basins are circular (no W1 or W2 needed). Overlap width is auto-estimated.

An 18 ft × 14 ft unequal-basin layout is a common residential figure-8 pool size, and 14,392 gallons (54,480 litres) falls in the mid-range of typical pool volumes — enough water for a standard filtration pump rated at 1.5 HP or higher.

How to Measure a Figure-8 Pool

Measuring Figure-8 Pool Dimensions

Each basin should be measured independently at its widest internal point. Stretch a tape measure across the centre of the larger basin from inner wall to inner wall at the waterline — that is D1. Do the same for the smaller basin to get D2. Do not measure from the far edge of one basin to the far edge of the other; that gives you the overall pool length, which is not enough for a two-basin formula.

If your builder plans show only the overall length and maximum width, you can sometimes estimate D1 and D2 by dividing the length into two basin zones. However, a direct measurement of each basin is always more accurate — especially on pools where one basin is noticeably larger or where the waist is offset from centre.

Calculating Average Depth

Figure-8 pools can have a single consistent depth or a sloped floor that runs from a shallow end in one basin to a deep end in the other. The calculator uses: Average Depth = (Shallow End + Deep End) ÷ 2. For a flat-bottom pool, enter the same depth in both fields — the calculator confirms the floor is flat and the result is exact. If only one basin has a deep section, measure the shallowest and deepest points across the entire pool, not within a single basin.

Figure-8 Pool Volume by Size — Reference Table

Calculated using the figure-8 pool formula shown above. Each basin is calculated separately and then combined, with the waist overlap subtracted using the exact two-circle intersection formula. Overlap width auto-estimated at 25% of the smaller basin diameter. Average depth = 5.0 ft based on 3.5 ft shallow and 6.5 ft deep water. Use actual internal water measurements for the most reliable result.

This range covers compact equal-basin layouts through large unequal-basin designs. A 2 ft increase in basin diameter adds roughly 1,500–3,500 gallons (5,700–13,200 litres), depending on whether one or both basins grow. For more shapes and sizes, see the pool volume by size reference.

Figure-8 Pool vs Oval Pool — Volume Comparison

Figure-8 and oval pools are often confused because both are elongated and curved. However, an oval pool has a smooth, continuous elliptical outline. A figure-8 pool pinches inward at the waist, creating two distinct basins that share a narrow connection.

An oval formula (π/4 × Length × Width) treats the pool as one unbroken ellipse. A figure-8 pool formula calculates each basin independently and subtracts the lens-shaped overlap where the two basins share space. The oval approach can misstate volume when the waist is narrower than the smooth oval outline would suggest, because it does not account for the water area lost at the pinch.

For example, an oval pool measuring 18 ft long by 14 ft wide at 5.0 ft average depth holds 7,403 gallons (28,022 litres) using the standard oval formula. A figure-8 pool with D1 = 18 ft and D2 = 14 ft at the same depth holds 14,392 gallons (54,480 litres) — nearly twice as much — because the figure-8 consists of two full basins, not a single ellipse bounded by 18 × 14 ft. The two shapes occupy fundamentally different footprints even when they share the same length and width extremes.

Whether a figure-8 pool holds more or less water than an oval of similar exterior dimensions depends entirely on how the basins and waist are proportioned. Using the wrong calculator skews chemical dosing, pump sizing, and refill estimates. Match the formula to the actual pool shape.

Figure-8 Pool vs Kidney Pool — Why the Formulas Differ

Both figure-8 and kidney pools pinch inward, but the geometry differs. A kidney pool has one continuous curved outline — the indentation is asymmetric and gradual, creating a bean or kidney shape. A figure-8 pool has a symmetric or near-symmetric pinch that creates two clearly defined basins.

Kidney pool calculators typically use a compactness factor (often 0.45 or similar) applied to overall length × width, because the single-curve shape does not break cleanly into independent geometric parts. The figure-8 formula works basin by basin — each basin is a circle or ellipse, and only the waist overlap needs correction.

Using a kidney calculator on a figure-8 pool, or vice versa, can skew the result by 10–20%. That gap is large enough to throw off chlorine dosing by more than 1 ppm at a 2 ppm target, or to undersize a replacement pump. Always match the calculator to your pool's actual shape.

Why Figure-8 Pool Volume Matters for Chemicals, Pump Sizing, and Refills

Chemical dosing depends directly on true water volume. Chlorine, pH adjusters, and algaecide labels specify amounts per gallon or per litre. Over-dosing a figure-8 pool because the volume was overstated can irritate skin and damage surfaces; under-dosing allows algae and bacteria to grow. Always follow the product label and your equipment specifications for your exact pool setup. For a detailed look at dosing by volume, see pool volume and chemical dosing.

<strong>Pump sizing and circulation</strong> depend on total gallons or litres. Most residential pools need the full volume to turn over at least once every 8–12 hours. A figure-8 pool with misestimated volume may end up with an under-sized pump that cannot filter the water fast enough, shortening equipment life and reducing water quality.

<strong>Refill planning</strong> depends on accurate volume. A standard garden hose delivers about 9 gallons (34 litres) per minute. Filling a 14,392-gallon (54,480-litre) figure-8 pool from empty takes roughly 26.5 hours of continuous flow — useful to know before you start a drain-and-clean.

Figure-8 pools are particularly prone to volume misestimates because outer dimensions hide the waist effect. Two pools with identical overall length and width can hold different volumes if their basin proportions or waist widths differ. Measure each basin independently for the most reliable number.

Common Mistakes When Estimating Figure-8 Pool Volume

1. Measuring only the overall length and width. The overall length spans from the far edge of one basin to the far edge of the other, and it does not tell you where one basin ends and the other begins. The calculator needs each basin diameter separately (D1 and D2). Measure across the centre of each basin at the waterline.
2. Assuming both basins are equal when they are not. Many figure-8 pools have one basin noticeably larger than the other. Entering the same diameter for both basins when the actual sizes differ by even 2 ft can shift the result by 1,500–2,500 gallons (5,700–9,500 litres). Measure each basin independently.
3. Ignoring the waist overlap. The calculator subtracts the lens-shaped area where the two basins overlap at the waist. Skipping this deduction — or using a formula that simply adds two full circles — overstates the total area and inflates the volume by 5–12%, depending on waist width.
4. Using an oval or kidney formula instead of a figure-8 formula. An oval formula treats the pool as one smooth ellipse, and a kidney formula uses a single compactness factor. Neither accounts for the two-basin geometry of a figure-8 pool. The volume errors range from 10% to over 50%, depending on the pool proportions.
5. Using a single depth instead of the shallow/deep average. A figure-8 pool with a 3.5 ft shallow end and a 6.5 ft deep end has a 5.0 ft average depth. Using the deep end alone overstates volume by 30%; using the shallow end alone understates it by the same amount. Enter both values and let the calculator average them.
6. Measuring to coping or outer edge instead of the internal waterline. Coping and decorative edges extend 2–4 inches beyond the waterline on each side. On a 16 ft basin, that adds up to 8 inches of false width, inflating the basin area by roughly 8% and the volume by several hundred gallons. Measure from the inner wall at the water surface.

Frequently Asked Questions