Roman Pool Volume Calculator

Select Imperial (feet) or Metric (metres) using the toggle at the top of the calculator. Imperial mode shows US gallons as the primary result. Metric mode shows litres first.

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 measure the Roman end diameter at the widest point of the semicircle — this equals the pool width, not the radius, so do not halve the width before entering it into the calculator.

Roman 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 you see on chemical labels, heater manuals, and pump spec sheets.

Unlike kidney or lagoon pools that rely on approximation factors, a Roman pool is a rectangle with one or two semicircular ends — a shape that breaks down into exact geometric components. That means the surface area calculation is mathematically precise, and the only real source of error is depth measurement.

Knowing your Roman pool's volume matters every time you add chlorine, estimate how long a garden hose takes to fill the pool, or check whether your heater is sized correctly. A 40 × 20 ft double Roman pool at 5 ft average depth holds 26,711 gallons (101,114 litres) — but dropping the depth by just one foot cuts that by roughly 5,300 gallons.

A Roman pool is a rectangle plus one or two semicircles. Both shapes have exact area formulas, so the total surface area is mathematically precise — no shape factor or compactness estimate is needed. This is what separates Roman pool volume from freeform, lagoon, or kidney pool calculations.

For a double Roman pool, the two semicircles together form one full circle with a diameter equal to the pool width. The rectangle section spans the remaining length. For a single Roman pool, only one end is semicircular — the other is a flat wall — so the rectangular section is longer. Both decompositions produce exact areas when the total length and width are measured correctly.

Because the geometry is exact, the formula's accuracy depends almost entirely on how well you measure depth. A 2-inch depth error on a 40 × 20 ft double Roman pool shifts the result by about 890 gallons (3,370 litres). Measuring length and width accurately is straightforward — the straight edges and predictable curves of a Roman pool make it one of the easiest pool shapes to measure.

A Roman pool combines a rectangular centre with one or two semicircular ends — it is not a pure rectangle and not a true oval. The semicircular ends have a known geometric area, so the formula uses exact decomposition rather than an approximation factor. This produces a precise surface area from just two measurements: total length and width.

L = total length of the Roman pool, measured tip to tip. W = width across the straight midsection, which also equals the diameter of each semicircular end. π/4 (≈ 0.7854) appears because a semicircle fills exactly half of a circle, and a circle inscribed in a square fills π/4 of that square — two semicircles together fill π/4 × W² of their combined bounding square. For a double Roman pool, the rectangular section length is L − W (because each semicircle extends W/2 from the rectangle edge). For a single Roman pool, only one semicircle extends W/2, so the rectangle length is L − W/2.

All dimensions must use the same unit — all feet or all metres — before multiplying. Mixing inches with feet is the most common unit error on Roman pool calculations.

Pool: 40 × 20 ft double Roman pool, Shallow End = 3.5 ft, Deep End = 6.5 ft.

A 40 × 20 ft double Roman pool is a mid-to-large residential size, comparable in volume to a 20 × 40 ft rectangular pool with rounded ends. For chemical dosing tables matched to your exact volume, see pool volume chemical dosing.

Total Length (L) is the full distance from the outermost tip of one semicircular end to the other. For a double Roman pool, this is tip to tip. For a single Roman pool, measure from the tip of the semicircle to the flat wall. Stretch the tape in a straight line across the water surface — do not follow the curved edge. The straight-section length of the rectangle is Total Length minus one full Width (double Roman) or minus half the Width (single Roman).

Width (W) is measured across the straight midsection at the widest point, perpendicular to the length line. This measurement is also the diameter of each semicircular end. The most common Roman pool measurement error is measuring on the diagonal from one corner of the rectangle to the curve — this inflates the width and over-estimates volume by 10–15%.

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 Roman pool floor — not along a sloped bottom or a curved wall.

Roman pools typically slope from a 3–4 ft shallow end to a 6–8 ft deep end. The two-point average approximates total water volume within ±3% for gradual slopes. For a Roman pool with a flat shallow shelf that drops sharply to a deep section, measure at 3 points along the length and average all 3 readings for a tighter result.

The table below shows Roman 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 exact stadium geometry — rectangle area plus semicircle area — not a shape-factor approximation.

Calculated using exact Roman pool geometry: rectangle area [(L − W) × W] plus circle end-pair area [π/4 × W²] for Double Roman, or rectangle [(L − W/2) × W] plus semicircle [π/8 × W²] for Single Roman. 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 your own Roman pool for precision.

Residential Roman pools typically range from 30 × 15 ft (15,025 gallons / 56,876 litres) up to 50 × 25 ft (41,737 gallons / 157,990 litres). The single Roman configuration holds 5–7% more water than a double Roman at the same overall dimensions because replacing one semicircle with a flat wall adds rectangular area. For volume benchmarks across other pool shapes, see pool volume by size. Enter your exact Roman pool dimensions in the calculator above for depths other than 5 ft.

A double Roman pool has a semicircular end on both short sides of the rectangular centre, forming a stadium (discorectangle) shape. A single Roman pool has a semicircle on one end and a flat wall on the other. The double Roman is the more common residential design, but both configurations are fully supported by the calculator above.

Two semicircles of diameter W combine into one full circle of area π/4 × W². A single semicircle contributes half that: π/8 × W². Replacing one semicircle with a flat wall extends the rectangular section by W/2 — and a rectangle of width W × length W/2 has a larger area than the semicircle it replaces. That is why a single Roman pool holds more water than a double Roman at the same overall length and width.

The single Roman pool holds 1,300 more gallons (4,922 L) — about 6% more — because the flat wall adds more rectangular area than the semicircle it replaces. Choosing the wrong configuration in the calculator shifts the result by this margin, so always confirm whether your Roman pool has one or two semicircular ends before calculating.

For the 40 × 20 ft double Roman worked example above: 3,570.80 cu ft × 28.3168 = 101,114 litres. You can also convert from gallons directly: 26,711 gal × 3.78541 = 101,114 L. Both paths produce the same result because the conversion constants are mathematically equivalent.

If you need litres for an Australian, UK, or European context, switch the toggle to Metric and enter Roman pool dimensions in metres. The calculator multiplies m³ by 1,000 for litres and by 264.172 for US gallons. A 12 × 6 m double Roman pool at 1.5 m average depth holds approximately 75,398 litres (19,919 gallons). For the underlying conversion formulas, see pool volume formulas.

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 26,711-gallon double Roman pool takes roughly 53 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 20,000-gallon pool will struggle to warm a 26,711-gallon Roman pool, adding hours to heat-up time and increasing energy costs.

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

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