How many chlorine tablets for a 20,000 gallon pool?
A 20,000 gallon pool using 2 ppm of chlorine a day needs about 5.2 tablets a week (3 inch, 8 oz), or 63 tablets over twelve weeks.
How to work it out
- In short: how much chlorine one tablet adds to your pool, how much the pool uses in a week, and the stabilizer the tablets leave behind.
- One 8 oz tablet × 90% = 204 g of chlorine ÷ 75,708 L = 2.7 ppm
- 2 ppm a day × 7 = 14 ppm a week ÷ 2.7 ppm = 5.2 tablets
- CYA: 14 ppm × 0.61 = 8.5 ppm a week; 30 + 8.5 × 12 weeks = 132 ppm
A 20,000 gallon pool is where tablet buckets start to come in 50 lb sizes. The weekly count doubles from a 10,000 gallon pool, but the stabilizer build-up per week is the same in ppm — the bigger pool just takes twice the tablets to get there.
Adjust for your project
More options: CYA · Weeks
- Chlorine one tablet adds
- 2.7 ppm
- Stabilizer (CYA) added per week
- 8.5 ppm
- CYA at the end of that time
- 132 ppm
- Tablets over that time
- 63 tablets
- Weight of tablets over that time
- 31.2 lb
- At this rate CYA reaches about 132 ppm — above the 90 ppm the Model Aquatic Health Code allows in public pools, and high enough that chlorine has to run very high to stay effective. Many owners switch to liquid chlorine once CYA is in range, or plan a partial drain.
- Tablets supply chlorine slowly; they can’t raise chlorine quickly. For a fast rise or a shock, use liquid chlorine or cal-hypo — never in the same feeder or bucket as tablets.
- Trichlor is acidic, so pools on tablets tend to drift down in pH and alkalinity. Check both weekly.
Weekly 8 oz tablets at 2 ppm a day, by pool size
| Pool | Tablets per week | Tablets over that time | Weight of tablets over that time |
|---|---|---|---|
| 10,000 gal | 2.6 tablets | 32 tablets | 15.6 lb |
| 15,000 gal | 3.9 tablets | 47 tablets | 23.4 lb |
| 20,000 gal | 5.2 tablets | 63 tablets | 31.2 lb |
| 25,000 gal | 6.5 tablets | 78 tablets | 38.9 lb |
| 30,000 gal | 7.8 tablets | 94 tablets | 46.7 lb |
Five tablets a week, and what they leave behind
At 2 ppm a day the pool needs 14 ppm of chlorine a week. One 8 oz tablet adds about 2.7 ppm to 20,000 gallons, so that is five to six tablets — usually more than one floater handles evenly, which is why bigger pools often use an inline or offline feeder after the heater.
- Season budget. About 63 tablets over 12 weeks, roughly 31 lb — a 50 lb bucket covers a season with some to spare.
- CYA. Stabilizer rises about 8.5 ppm a week regardless of pool size, so from 30 ppm you reach roughly 130 ppm in twelve weeks. The cure is the same as in a small pool: stop tablets once CYA is in range, or replace water.
- Cost comparison. For the same 14 ppm a week, 12.5% liquid chlorine is about 1.1 gallons a week in 20,000 gallons. Compare that with the bucket price before choosing.
- Feeder safety. Only trichlor goes in a trichlor feeder — the CDC warns against mixing chlorine types.
Smaller pool: how many chlorine tablets for a 10,000 gallon pool. If CYA has already climbed, see how much water to drain to lower CYA.
Different size? Use the full Chlorine Tablet Calculator.
What you’ll need
- 3-inch trichlor tabletsMid-range price
Slow-dissolving stabilized chlorine for a floater or feeder. Convenient, but every tablet adds stabilizer (CYA) and lowers pH.
See options on Amazon (opens in a new tab) - Inline / offline tablet chlorinatorMid-range price
Plumbed after the filter and heater, it meters tablets more evenly than a floater. Never put any other chemical in it.
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)
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 chlorine tablets for a 20,000 gallon pool using 1 ppm a day?
About 2.6 tablets a week, adding about 4.2 ppm of CYA a week.
How many chlorine tablets for a 25,000 gallon pool?
About 6.5 tablets a week at 2 ppm a day.
Other common sizes
- How many chlorine tablets for a 10,000 gallon pool — 2.6 tablets
Sources
- US EPA label 36284-5 — 3 in trichlor tablet (7 oz) (opens in a new tab) — 90% available chlorine; one tablet gives 4.7 ppm in 10,000 gallons; 1 tablet per 8,000 gallons every 7–14 days to maintain.
- ICC Building Safety Journal (PHTA Recreational Water Quality Committee) — Trichlor: the dependable pool performer (opens in a new tab) — Trichlor is 90% available chlorine and adds 0.6 ppm cyanuric acid for each ppm of chlorine.
- Leslie’s — In The Swim 3 inch chlorine tablets (50 lb) (opens in a new tab) — 99% trichlor, 90% available chlorine, about 100 tablets per 50 lb; gives both “one tablet per 10,000 gallons” and “one 3 inch tablet for every 5,000 gallons” as weekly rates.
- CDC — Model Aquatic Health Code, 5th edition (2024), code (opens in a new tab) — Free chlorine minimum 1.0 ppm (2.0 with CYA, 3.0 in spas); pH 7.0–7.8; CYA at or below 90 ppm; CYA:FC above 45:1 is an imminent hazard; TA 60–180 ppm; turnover tables.
- CDC Healthy Swimming — Home pool and hot tub water treatment and testing (opens in a new tab) — pH 7.0–7.8; free chlorine at least 1 ppm in pools (2 ppm with cyanuric acid) and 3 ppm in hot tubs; no CYA in hot tubs; test twice a day.
- Trouble Free Pool (hobbyist forum wiki) — CYA/chlorine relationship (opens in a new tab) — A widely used hobbyist convention, not an official standard: minimum FC 7.5% of CYA, SLAM (shock) level 40% of CYA, salt-water pools 5%.
- CDC — Pool chemical safety: storage poster (opens in a new tab) — Separate acid and chlorine; don’t mix different types of chlorine; keep chemicals cool, dry and in original containers.