How many gallons in an 18×36 inground pool?
An 18 × 36 ft inground pool going from 3.5 ft to 8 ft deep holds about 27,872 gallons (average depth 5.75 ft).
How to work it out
- Rectangle: 36 ft × 18 ft = 648 sq ft
- Average depth: (3.5 ft + 8 ft) ÷ 2 = 5.75 ft
- 648 sq ft × 5.75 ft = 3,726 cu ft × 7.48 = 27,872 gal
An 18 × 36 is the step up from a 16 × 32: 648 sq ft of water instead of 512, and very often a diving end. It is a common vinyl-liner kit size, so a lot of them share the same 3.5 ft to 8 ft profile.
Adjust for your project
- Water surface area
- 648 sq ft
- Average depth
- 5.75 ft
- Water in each inch of level
- 404 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.
18×36 ft pool by deep-end depth (3.5 ft shallow end)
| Deep end | Pool volume (gallons) | Average depth |
|---|---|---|
| 5 ft | 20,601 gal | 4.25 ft |
| 6 ft | 23,025 gal | 4.75 ft |
| 7 ft | 25,449 gal | 5.25 ft |
| 8 ft | 27,872 gal | 5.75 ft |
| 9 ft | 30,296 gal | 6.25 ft |
Reading the 18×36 number
At about 27,900 gallons this pool sits between the standard 20,000-gallon and 30,000-gallon label rates, so read the label per 10,000 gallons and multiply by 2.8, not 2 or 3.
- Deep end makes the difference. A sport version that only reaches 6 ft holds about 23,000 gallons; a 9 ft diving well pushes it past 30,000.
- Hopper floors. Vinyl diving pools usually have a hopper with sloped sides rather than a straight ramp, so the real figure can be 5–10% under the straight-slope average. That is within the error of most test kits.
- Per inch. 648 sq ft holds about 404 gallons per inch — useful when you top up or lower the water for a winter cover.
- Chlorine. 1 ppm in 27,900 gallons is about 28 fl oz of 12.5% liquid chlorine; a 3 ppm daily top-up is roughly 2/3 of a gallon.
Compare a slightly smaller pool on how many gallons in a 16x32 inground pool or the next size up on how many gallons in a 20x40 inground pool. For salt, the closest worked example is how much salt for a 20,000 gallon pool, scaled by 1.4.
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) - FAS-DPD drop test kit (chlorine, pH, TA, CH, CYA)Mid-range price
A drop-count kit reads free chlorine to 0.5 ppm (or 0.2 ppm), plus pH, alkalinity, calcium and stabilizer. Every dose on this site starts from a test, so this is the one thing worth buying first.
See options on Amazon (opens in a new tab) - Solar cover (bubble cover)Mid-range price
The cheapest heating upgrade there is: cuts evaporation and holds heat in overnight. Buy the size up and trim it.
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 an 18×36 pool with a 6 ft deep end?
About 23,025 gallons (average depth 4.75 ft).
How many gallons in an 18×36 pool with a 9 ft deep end?
About 30,296 gallons from a 3.5 ft shallow end.
Other common sizes
- How many gallons in a 16x32 inground pool — 18,193 gal
- How many gallons in a 20x40 inground pool — 34,410 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.