How the volume is measured
Measured, not multiplied
Most calculators multiply length by width by depth by a number for the shape: 0.45 to 0.585 for a kidney, say. But a kidney is not one outline. The factor is a guess for the whole family, and a pool with a deeper waist or fuller lobes fills a different share of its bounding rectangle. This calculator uses the outline itself: the shape is sampled into 140 points, cut into triangles and the water added up over the floor, which is how the designer reports the volume of the same pool.
The result is a calculation, not a measurement on site. Read it as “about”.
Where the water stands
Depths are measured from the coping down to the floor, as in the designer, and the water stands lower: about 5 7/8″ (15 cm) below the coping in a skimmer pool, about 1 5/8″ (4 cm) in a high-level one, level with it in an overflow pool. That is why the same 8 × 4 m pool holds about 44 m³ with skimmers and about 48 m³ as an overflow pool. The edge type moves the water line and nothing else; the floor stays where it is.
What it takes into account
Steps, a tanning ledge and a bench take the place of water: switch them on under “Take steps, ledge and bench out of the water” and they are subtracted, where the designer puts them by default. The fill time assumes a garden tap at about 1.5 m³/h (change the flow if yours is faster); the heat pump is about 0.35 kW per m³ of water, a rule of thumb; wall skimmers are one for every 25 m² of surface, three at most.
Common sizes and the water they hold
| Pool | Water volume | Fill time (garden tap) | Heat pump size |
|---|---|---|---|
| 4 × 2.5 m Rectangular | 3,091 gal | 8 hours | 17,000 BTU/h |
| Ø 4 m Round | 4,120 gal | 10 hours | 20,000 BTU/h |
| 6 × 3 m Roman | 5,357 gal | 14 hours | 27,000 BTU/h |
| 8 × 4 m Rectangular | 11,571 gal | 29 hours | 55,000 BTU/h |
| 8 × 4 m Oval | 9,050 gal | 23 hours | 41,000 BTU/h |
| 8.5 × 4.8 m Kidney | 8,490 gal | 21 hours | 41,000 BTU/h |
| 9 × 5.5 m L-shape | 14,257 gal | 36 hours | 65,000 BTU/h |
| 9 × 4.6 m Figure 8 | 8,460 gal | 21 hours | 41,000 BTU/h |
| 10 × 5 m Rectangular | 18,080 gal | 46 hours | 82,000 BTU/h |
| 12 × 6 m Rectangular | 27,930 gal | 3 days | 130,000 BTU/h |
Frequently asked questions
- How do I calculate the volume of a kidney-shaped pool?
- Not with length × width × depth: a kidney fills only a little over half of its bounding rectangle, and exactly how much depends on its outline. Choose Kidney above, enter the length, width and depths, and the calculator measures the outline itself. In the example, the designer's kidney of 28 × 16 ft (8.5 × 4.8 m) fills 57% of its rectangle and holds about 8,454 gal (32 m³); a rectangle of the same size and depth would hold about 15,058 gal (57 m³).
- How many litres or gallons is that?
- One cubic metre is 1,000 litres or about 264 US gallons. The calculator prints cubic metres, litres and US gallons together; in the example, 32 m³ is about 32,000 litres or 8,500 US gallons.
- Why is the volume smaller than length × width × depth?
- Three reasons: the shape (corners, curves and lobes), the floor (it slopes from the shallow end to the deep end, so the average depth is not the deep end's) and the water line, which stands below the coping. The calculator takes all three into account.
- How long does it take to fill a pool?
- The volume divided by the flow. A garden tap delivers about 6.6 GPM (1.5 m³/h): an 8 × 4 m pool holding 11,624 gal (44 m³) is full in about 29 hours. A bigger supply or a tanker is faster; enter your own flow in the calculator.
- What size heat pump does my pool need?
- As a rule of thumb about 0.35 kW for every cubic metre: 11,624 gal (44 m³) calls for around 55,000 BTU/h (16 kW). It is a starting point, not a sizing. Exposure, a cover and how warm you want the water change it, and your installer should size the unit.
