What a 20x40 inground pool needs (26,930 gallons, 3 to 6 ft deep)
| What | Amount | Assumes |
|---|---|---|
| Salt for a salt chlorinator | 720 lb (18 bags of 40 lb) | fresh water to 3,200 ppm |
| Chlorine to raise FC 1 ppm | 28.8 fl oz | 12.5 % liquid chlorine |
| Pump flow | 56 gpm | whole pool through the filter in 8 hours |
| Fill time | about 50 hours | one garden hose at 9 gpm |
| Evaporation | about 125 gal a day | 800 sq ft of water, 0.25 in a day |
Gallons by depth profile
| Shallow to deep end | Average depth | Gallons | Litres | Salt to 3,200 ppm |
|---|---|---|---|---|
| 3 to 5 ft | 4 ft | 23,940 | 90,600 | 640 lb |
| 3 to 6 ft | 4.5 ft | 26,930 | 101,900 | 720 lb |
| 3.5 to 6 ft | 4.75 ft | 28,430 | 107,600 | 760 lb |
| 3 to 8 ft | 5.5 ft | 32,910 | 124,600 | 880 lb |
| 4 to 8 ft | 6 ft | 35,910 | 135,900 | 960 lb |
Rectangular pool: length × width × average depth × 7.48 gallons per cubic foot. The average depth is the shallow end plus the deep end, divided by 2. For a 20x40 inground pool 3 to 6 ft deep: 20 × 40 × 4.5 × 7.4805 = 26,930 gallons. That assumes the floor slopes evenly; a pool with a long flat shallow area and a short, steep deep end holds less, so for those measure the depth every few feet along the pool and average the readings.
Gallons by average depth
| Average depth | Gallons | Litres |
|---|---|---|
| 3.5 ft | 20,950 | 79,300 |
| 4 ft | 23,940 | 90,600 |
| 4.5 ft | 26,930 | 101,900 |
| 5 ft | 29,920 | 113,300 |
| 5.5 ft | 32,910 | 124,600 |
| 6 ft | 35,910 | 135,900 |
Your depths are not in the tables? Open this pool in the pool volume calculator and type your shallow and deep end; the pool gallon calculator also works out a pool with a sloped floor.
Other sizes
The pool sizes in gallons charts list every common round, oval and inground size by average depth, and the pool volume calculator works out any other shape from its dimensions.
Other sizes with their own page, each with its salt, chlorine, pump flow and fill time: 24 ft round pool, 18 ft round pool, 27 ft round pool, 18x33 oval pool, 16x32 inground pool, 15x30 oval pool, 16 ft round pool, 30 ft round pool, 15 ft round pool, 21 ft round pool and 12x24 oval pool.
Frequently asked questions
How many gallons does a 20x40 inground pool hold?
26,930 gallons with a 3 ft shallow end and a 6 ft deep end (4.5 ft average), 23,940 at 3 to 5 ft and 32,910 at 3 to 8 ft with a diving end. Steps and benches take a little of that water.
How many gallons is a 20x40 pool that is 3 to 8 ft deep?
32,910 gallons, about 124,600 litres: 20 × 40 ft × 5.5 ft average depth × 7.48 gallons per cubic foot. The average is (shallow end + deep end) ÷ 2, which assumes the floor slopes evenly from end to end.
How much chlorine does a 20x40 inground pool need?
Raising free chlorine by 1 ppm in 26,930 gallons (3 to 6 ft deep) takes 28.8 fl oz of 12.5 % liquid chlorine; 3 ppm takes 86 fl oz. How high to go depends on your stabilizer (CYA) level, which the pool chlorine calculator works out.
How much salt does a 20x40 inground pool need?
From fresh water to 3,200 ppm: 720 lb at 3 to 6 ft deep (18 bags of 40 lb), 640 lb at 3 to 5 ft and 880 lb at 3 to 8 ft. Check your chlorinator's target first and add in steps.
How many litres is a 20x40 inground pool?
90,600 litres at 3 to 5 ft deep, 101,900 at 3 to 6 ft and 124,600 at 3 to 8 ft (average depth = shallow plus deep end ÷ 2). One US gallon is 3.785 litres.
Gallons: length × width × average depth × 7.48 gallons per cubic foot (231 cubic inches per US gallon). Depth profiles are examples of common inground pools, not a standard: use your own shallow and deep end. Salt: 1 lb per 10,000 gal ≈ 12 ppm and liquid chlorine 10.7 fl oz per ppm per 10,000 gal (Trouble Free Pool, Pool School; product labels), read October 18, 2026. Hose flow, turnover hours and evaporation rate are example inputs: the linked calculators take your own. Gallons rounded to the nearest 10, litres to the nearest 100.