What a 21 ft round pool needs (10,410 gallons, 52 in wall)
| What | Amount | Assumes |
|---|---|---|
| Salt for a salt chlorinator | 280 lb (7 bags of 40 lb) | fresh water to 3,200 ppm |
| Chlorine to raise FC 1 ppm | 11.1 fl oz | 12.5 % liquid chlorine |
| Pump flow | 22 gpm | whole pool through the filter in 8 hours |
| Fill time | about 19 hours | one garden hose at 9 gpm |
| Sand base | 2.1 cu yd (116 bags of 50 lb) | 2 in under the 21 ft footprint |
| Evaporation | about 54 gal a day | 346 sq ft of water, 0.25 in a day |
Gallons by wall height
| Wall | Water depth | Gallons | Litres | Salt to 3,200 ppm |
|---|---|---|---|---|
| 48 in | 3.5 ft | 9,110 | 34,500 | 240 lb |
| 52 in | 4 ft | 10,410 | 39,400 | 280 lb |
| 54 in | 4.5 ft | 11,710 | 44,300 | 310 lb |
Round pool: diameter × diameter × water depth × 5.9. For a 21 ft round pool with 4 ft of water: 21 × 21 × 4 × 5.9 = 10,408 gallons. 5.9 is π ÷ 4 × 7.48 gallons per cubic foot, rounded as the pool trade does, so these figures match the capacity charts pool stores print. The charts assume a water level for each wall height; if yours differs, use the water depth table below.
Gallons by water depth
| Water depth | Gallons | Litres |
|---|---|---|
| 36 in (3.00 ft) | 7,810 | 29,500 |
| 39 in (3.25 ft) | 8,460 | 32,000 |
| 42 in (3.50 ft) | 9,110 | 34,500 |
| 45 in (3.75 ft) | 9,760 | 36,900 |
| 48 in (4.00 ft) | 10,410 | 39,400 |
| 51 in (4.25 ft) | 11,060 | 41,900 |
| 54 in (4.50 ft) | 11,710 | 44,300 |
Measure from the floor to the waterline in a few places and use the average. Your level is not in the table? Open this pool in the pool volume calculator and type your depth; the round pool volume calculator also takes the depth in inches.
Other sizes
The pool sizes in gallons charts list every common round, oval and inground size, and the above ground pool volume calculator works out any other above-ground pool from its wall height.
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, 20x40 inground pool, 15x30 oval pool, 16 ft round pool, 30 ft round pool, 15 ft round pool and 12x24 oval pool.
Frequently asked questions
How many gallons does a 21 foot by 48 inch pool hold?
A 48 in wall is filled to about 3.5 ft of water, so the pool holds 9,110 gallons (about 34,500 litres). Filled to 4 ft, as with a 52 in wall, it holds 10,410 gallons.
How many gallons is a 21 ft round pool that is 4.5 ft deep?
11,710 gallons, about 44,300 litres. 4.5 ft is the water level the capacity charts assume for a 54 in wall. Every 3 inches of water adds or removes about 650 gallons.
How much chlorine does a 21 ft round pool need?
Raising free chlorine by 1 ppm in 10,410 gallons (52 in wall) takes 11.1 fl oz of 12.5 % liquid chlorine; 3 ppm takes 33 fl oz. How high to go depends on your stabilizer (CYA) level, which the pool chlorine calculator works out.
How much salt does a 21 ft round pool need?
From fresh water to 3,200 ppm: 280 lb at a 52 in wall (7 bags of 40 lb), 240 lb at 48 in and 310 lb at 54 in. Check your chlorinator's target first and add in steps.
How many litres is a 21 ft round pool?
34,500 litres with a 48 in wall, 39,400 with a 52 in wall and 44,300 with a 54 in wall, at the water levels the capacity charts use (3.5, 4 and 4.5 ft). One US gallon is 3.785 litres.
Water depth per wall height (48 in = 3.5 ft, 52 in = 4 ft, 54 in = 4.5 ft): pool-trade capacity charts (Swimming Pool Discounters, "Gallons of Water per Pool Size Chart"; Energy Kinetics, "Standard Pool Above Ground Pool Sizes"), read October 18, 2026. 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. Sand: 2 in base, 50 lb bag = 0.5 cu ft (Royal Swimming Pools; Quikrete All-Purpose Sand), read October 23, 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.