How many gallons in a 20×40 inground pool?
A 20 × 40 ft inground pool with a 3.5 ft shallow end and an 8 ft deep end holds about 34,410 gallons (average depth 5.75 ft).
How to work it out
- Rectangle: 40 ft × 20 ft = 800 sq ft
- Average depth: (3.5 ft + 8 ft) ÷ 2 = 5.75 ft
- 800 sq ft × 5.75 ft = 4,600 cu ft × 7.48 = 34,410 gal
A 20 × 40 is a big residential pool — 800 sq ft of water, often with a diving end. At well over 30,000 gallons, a guessed volume costs real money: every chemical dose is large, and an error of a few thousand gallons is a gallon of chlorine either way.
Adjust for your project
- Water surface area
- 800 sq ft
- Average depth
- 5.75 ft
- Water in each inch of level
- 499 gal
- A deep diving end usually has a hopper (a steeper bowl) rather than one long, even slope. If yours does, this average slightly overstates the volume — a meter on the fill hose is the only exact answer.
- Doses on this site use gallons. Round to the nearest few hundred gallons; your test kit is less precise than that.
20×40 ft pool by deep-end depth (3.5 ft shallow end)
| Deep end | Pool volume (gallons) | Average depth |
|---|---|---|
| 5 ft | 25,434 gal | 4.25 ft |
| 6 ft | 28,426 gal | 4.75 ft |
| 7 ft | 31,418 gal | 5.25 ft |
| 8 ft | 34,410 gal | 5.75 ft |
| 9 ft | 37,403 gal | 6.25 ft |
| 10 ft | 40,395 gal | 6.75 ft |
Living with a 34,000-gallon pool
- Chemical doses scale up. Raising free chlorine by 3 ppm in 34,400 gallons takes about 2.8 gal of 12.5% liquid chlorine. Buying chlorine by the case, or a salt chlorine generator, starts to make sense at this size.
- Salt from scratch to 3,200 ppm would be around 900 lb — about 23 bags of 40 lb.
- Gallons per inch. 800 sq ft is about 499 gallons per inch, so a 1/4 in-a-day evaporation loss in summer is roughly 125 gallons a day.
- Pump and heater. Turning 34,000 gallons over in 8 hours needs about 72 gpm — see what size pump for a 20,000 gallon pool for how the sizing works at a smaller volume.
If the deep end is shallower — many 20×40 sport pools stop at 6 ft — the volume drops to about 28,400 gallons. For a smaller rectangle with the same proportions, see how many gallons in a 16x32 inground pool; for a size in between, how many gallons in an 18x36 inground pool.
Different size? Use the full Pool Volume Calculator.
What you’ll need
- Hose water flow meterBudget price
Screws onto the hose and counts gallons, so you know exactly how much went in — and your real fill rate.
See options on Amazon (opens in a new tab) - Liquid pool chlorine (sodium hypochlorite)Budget price
Adds chlorine without stabilizer or calcium. Strength is printed on the label — commonly 10% or 12.5% — and it weakens with age and heat, so buy it fresh.
See options on Amazon (opens in a new tab) - Solar cover reelMid-range price
A cover that is a pain to take off doesn’t go back on. A reel makes it a one-person, two-minute job.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How many gallons in a 20×40 pool with a 6 ft deep end?
About 28,426 gallons — an average depth of 4.75 ft.
How many gallons in a 20×40 pool that is 5 ft deep all over?
About 29,922 gallons.
Other common sizes
- How many gallons in a 16x32 inground pool — 18,193 gal
- How many gallons in an 18x36 inground pool — 27,872 gal
- How many gallons in a kidney-shaped pool — 13,431 gal
Sources
- NIST Handbook 44, Appendix C — General tables of units of measurement (opens in a new tab) — 1 US gallon = 231 cubic inches, so 1 cubic foot = 7.48052 gallons; 1 gallon = 3.785 litres.
- CDC Healthy Swimming — Home pool and hot tub water treatment and testing (opens in a new tab) — Chemical doses on product labels are given per volume of water, so the volume has to be known first.