Octagon Pool Volume Calculator

Select Imperial (feet) or Metric (metres) using the toggle at the top of the calculator. Imperial mode returns US gallons as the primary unit. Metric mode returns litres and cubic metres — choose Metric if you dose chemicals in litres or live outside the United States.

Results appear instantly in gallons, litres, and cubic feet. Use the volume to dose chemicals, estimate fill time, or size a pool heater and pump.

Pool builders quote octagon pool size as the overall flat-to-flat width — this is not the same as the side length, so do not measure one side of the octagon and enter that value as the pool diameter.

Octagon pool volume is the total amount of water the pool holds when filled to the waterline. It is expressed in US gallons, litres, or cubic feet — the same units printed on chemical labels, heater spec sheets, and pump sizing charts.

A regular octagon pool has eight equal sides and eight equal 135° angles. Its surface area is calculated exactly from the flat-to-flat diameter using the factor 2(√2 − 1) ≈ 0.8284 — no approximation or shape factor is needed. Multiply that exact area by average depth to get volume.

Knowing your octagon pool's volume matters every time you add chlorine, estimate fill time, or check whether your heater is sized correctly. An 18 ft octagon pool at 5 ft average depth holds 10,039 gallons (38,003 litres) — but dropping the depth by just one foot cuts that by roughly 2,008 gallons.

A regular octagon inscribed in a square of side D fills exactly 82.84% of that square. The four corner triangles that are "cut" to form the octagon have a combined area of 17.16% of D². This ratio — 2(√2 − 1) — is mathematically exact, not an estimate.

This is different from freeform, kidney, or lagoon pools that need approximation factors because their curves do not follow a single formula. A regular octagon is a well-defined geometric shape, so its area comes directly from one measurement: the flat-to-flat diameter.

If your pool has eight sides but they are not all the same length, it is an irregular octagon. The 2(√2 − 1) factor does not apply. In that case, use the "I know my pool's surface area" panel in the calculator above — enter the measured surface area from your builder's plans or a satellite image and let the calculator multiply by average depth.

A regular octagon pool has eight equal sides and a known geometric area — it is not a round pool and not a hexagon pool. The octagon area formula is exact, unlike approximation factors used for freeform shapes. This produces a precise surface area from a single measurement: the flat-to-flat diameter.

D = flat-to-flat diameter of the regular octagon pool, measured from the centre of one flat side to the centre of the opposite flat side. 2(√2 − 1) ≈ 0.8284 is the octagon area factor. It represents the fraction of the bounding square that the octagon fills — a regular octagon inscribed in a square of side D covers exactly 82.84% of that square's area. The remaining 17.16% consists of four identical right-angled triangles cut from the corners of the bounding square.

All dimensions must use the same unit — all feet or all metres — before multiplying. Mixing inches with feet is the most common unit error in octagon pool volume calculations. See our complete list of pool volume formulas for other pool shapes.

Pool: 18 ft octagon pool (flat-to-flat diameter), Shallow End = 3.5 ft, Deep End = 6.5 ft.

An 18 ft octagon pool at 5 ft average depth is a mid-size residential pool, comparable in volume to a 15 ft round pool at the same depth. For chemical dosing tables matched to your exact volume, see pool volume chemical dosing.

Flat-to-flat diameter (D) is the perpendicular distance from the centre of one flat side to the centre of the directly opposite flat side. This is the measurement the octagon pool volume calculator requires. Stretch the tape across the water surface through the centre of the pool — do not follow the pool edge.

Corner-to-corner diagonal connects two opposite vertices and is always about 8.2% longer than the flat-to-flat width on a regular octagon pool. Entering the diagonal instead of the flat-to-flat width overstates the surface area by about 17% and the calculated volume by the same proportion. For an 18 ft octagon pool, the corner-to-corner diagonal measures approximately 19.5 ft — a 1.5 ft difference that would add roughly 1,700 gallons (6,434 litres) of phantom volume.

Side length (s) is the length of one of the eight equal edges. The calculator does not use side length directly — it uses the flat-to-flat diameter. For reference, the side length of a regular octagon equals D × (√2 − 1), or about 41.4% of the flat-to-flat width. An 18 ft octagon pool has a side length of approximately 7.46 ft.

Average Depth = (Shallow End + Deep End) ÷ 2. Example: (3.5 + 6.5) ÷ 2 = 5.0 ft. Measure depth from the waterline straight down to the octagon pool floor — not along a sloped bottom.

Octagon pools with variable depth typically slope from a 3–4 ft shallow end to a 5–7 ft deep end. The two-point average approximates total water volume within ±3% for gradual slopes. For an octagon pool with a flat shelf that drops sharply to a deep section, measure depth at 3 points along the pool and average all 3 for a tighter result.

The table below shows octagon pool volume at 8 standard sizes. All rows use 5.0 ft average depth (shallow 3.5 ft + deep 6.5 ft). Volumes are calculated using the exact regular octagon formula — D² × 2(√2 − 1) × average depth — not a shape-factor approximation.

Calculated using the regular octagon formula: Area = D² × 2(√2 − 1), then Volume = Area × Average Depth. 1 cu ft = 7.48052 US gal = 28.3168 L. Average depth = 5.0 ft (shallow 3.5 ft + deep 6.5 ft) ÷ 2. Measure the flat-to-flat width — not corner-to-corner diagonal.

Residential octagon pools typically range from 12 ft (4,462 gallons / 16,890 litres) up to 24 ft (17,847 gallons / 67,560 litres). Commercial and custom-built octagon pools can reach 28 ft or larger. Volume scales with the square of the diameter — doubling the width quadruples the water volume. For benchmarks across other pool shapes, see pool volume by size. Enter your exact octagon pool diameter in the calculator above for depths other than 5 ft.

Switch the unit toggle to <strong>Metric</strong> to enter your octagon pool's flat-to-flat diameter in metres and read the primary result in litres. The calculator displays volume in litres, cubic metres (m³), and US gallons simultaneously. Surface area appears in square metres (m²) in the live preview chip above the input fields.

For the 18 ft worked example above: 10,039 US gallons × 3.78541 = 38,003 litres. You can also convert directly from cubic feet: 1,342.05 ft³ × 28.3168 = 38,003 L. Both paths produce the same result because 7.48052 × 3.78541 ≈ 28.3168.

Pool owners in Australia, the United Kingdom, the EU, and New Zealand buy chemicals dosed per litre, making litres the primary unit for those markets. A 6 m octagon pool (flat-to-flat) at 1.5 m average depth has a surface area of 29.82 square metres and holds 44,735 litres (11,818 US gallons). For a full guide to litre-based pool calculations across all shapes, see pool volume in litres.

Chemical dosing. Chlorine, pH adjusters, and algaecides are dosed per gallon or per litre. Overestimate volume and you under-dose — risking algae. Underestimate and you over-dose — wasting chemicals and irritating skin. See pool volume chemical dosing for dose tables matched to your exact volume.

Fill and drain planning. A standard garden hose delivers about 500 gallons per hour. A 10,039-gallon octagon pool takes roughly 20 hours to fill — knowing the volume lets you plan around water utility schedules and avoid overflow.

Heater sizing. Pool heaters are rated by BTU output per gallon. A heater sized for a 7,000-gallon pool will take noticeably longer to warm a 10,039-gallon octagon pool, increasing energy costs and delaying swim time.

Pump and turnover rate. Pumps are sized to turn over the full pool volume in 8–12 hours. An inaccurate volume means the pump runs too short (poor filtration) or too long (wasted electricity). Accurate octagon pool volume keeps turnover on target. For calculators covering other pool shapes, visit Pool Volume.

These six mistakes cause the largest errors in octagon pool volume calculations: