How many gallons is a 20x40 pool?
A 20 × 40 ft inground pool holds about 28,400 gallons at 3.5 to 6 ft deep.
On this page (7 sections)
With a steady slope from 3.5 to 8 ft, it holds about 34,400 gallons. With a flat 4 ft floor, it holds about 23,900 gallons.
Quick answer for a 20x40
A 40 × 20 and a 20 × 40 are the same pool: 800 square feet of water. At that area, every half foot of average depth adds 2,992 gallons, so the depth you enter matters more than on any smaller pool. The data block gives average depths of 4 to 6.5 ft, with liters.
| Average depth | Gallons | Liters |
|---|---|---|
| 4 ft | 23,938 gal | 90,614 L |
| 4.5 ft | 26,930 gal | 101,941 L |
| 5 ft | 29,922 gal | 113,267 L |
| 5.5 ft | 32,914 gal | 124,594 L |
| 6 ft | 35,906 gal | 135,921 L |
| 6.5 ft | 38,899 gal | 147,248 L |
Average depth = (shallow end + deep end) ÷ 2 for a steady slope. Example: 3.5 ft to 8 ft averages 5.75 ft.
| Change | Amount |
|---|---|
| +1 ppm FC (12.5% liquid chlorine) | 31 fl oz906 mL |
| +10 ppm TA (baking soda) | 4.2 lb1.90 kg |
| +10 ppm CYA (stabilizer) | 2.5 lb1.13 kg |
Closest full chart: 30,000 gal dose chart.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
Depth profiles: flat, sloped, with deep end
| Floor profile | Average depth | Gallons |
|---|---|---|
| Flat, 4 ft throughout | 4.0 ft | 23,938 gal |
| Steady slope, 3.5 to 6 ft | 4.75 ft | 28,426 gal |
| Shallow end plus deep end: 20 ft at 3.5 ft, 8 ft slope, 12 ft at 8 ft | 5.30 ft | 31,717 gal |
| Steady slope, 3.5 to 8 ft | 5.75 ft | 34,410 gal |
Gallons = 40 × 20 × average depth × 7.48. With a separate deep end, weight each section's depth by its length.
A pool with half its length at 3.5 ft and a deep end of 8 ft holds about 31,700 gallons, not the 34,400 you'd get by averaging the two end depths. Some deep ends are shaped like a bowl or hopper rather than a flat floor, which lowers the real number further. If yours is shaped this way, the salt or stabilizer rise method on the pool volume calculator page gives a measured volume instead of a guess.
Starting doses for this volume
| Change | 3.5 to 6 ft (28,426 gal) | 3.5 to 8 ft (34,410 gal) |
|---|---|---|
| +1 ppm free chlorine (FC), 12.5% liquid chlorine | 29 fl oz | 35 fl oz |
| +1 ppm FC, 10% liquid chlorine | 36 fl oz | 44 fl oz |
| +10 ppm total alkalinity (TA), baking soda | 4.0 lb | 4.8 lb |
| Stabilizer (CYA), 0 to 30 ppm | 7.1 lb | 8.6 lb |
| Salt to 3,200 ppm (new salt pool) | 759 lb (19 bags of 40 lb) | 919 lb (23 bags) |
Bag counts round up; add the exact weight, test after 24 hours, then top up. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
At this size a 2 ppm daily chlorine loss means about 3.2 gallons of 12.5% liquid chlorine a week for 28,426 gallons. Measure your own loss before you commit to a supply, and compare per-ppm prices in the chemical cost calculator. Full sheets: 30,000 gallons and, for a shallower 20 × 40, 25,000 gallons. Targets and product choice: how much chlorine to add.
Calculator: your exact depth
The calculator opens at 40 × 20 ft, 3.5 to 8 ft. Enter the water depth at the shallow and deep ends (not the depth markers on the coping, which may be rounded).
Fill time and water cost
For 28,426 gallons: about 95 hours at 5 gallons per minute and 47 hours at 10. For 34,410 gallons: about 115 and 57 hours. For every $1 per 1,000 gallons on your water bill, that's about $28.43 or $34.41; enter your rate in the fill cost calculator. For a fill this large, ask your water utility about bulk delivery or whether sewer charges apply to pool water. Filled to 3.5 to 8 ft, the water weighs roughly 287,000 lb.
How many swimmers? Backyard pools have no legal limit. For public pools, the CDC's Model Aquatic Health Code uses 20 square feet per bather in flat water, which would cap 800 square feet at 40 people; for a home pool, a comfortable number is far lower.
Safety. Add large doses of liquid chlorine and acid separately, at least 30 minutes apart. Pour slowly in front of a return jet with the pump on. Never mix them, and keep swimmers out until you retest. Poison Control: 1-800-222-1222.
FAQ
How many gallons is a 40x20 pool?
About 23,938 gallons at a flat 4 ft, 28,426 at 3.5 to 6 ft, and 34,410 at 3.5 to 8 ft with a steady slope. With a flat shallow half and a short deep end, about 31,700 gallons.
How much chlorine for a 20x40 pool?
Per 1 ppm: about 29 fl oz of 12.5% liquid chlorine at 3.5 to 6 ft, or 35 fl oz at 3.5 to 8 ft. A 3 ppm top-up is 87 to 106 fl oz, most of a gallon jug.
Sources
- Model Aquatic Health Code (CDC): cdc.gov; density factors summarized by Counsilman-Hunsaker
- Weight of water, 8.34 lb per gallon: USGS Water Science School
- Hayward, salt dosing: hayward.com
- Volume formulas and dose math: methodology