How many chlorine tablets does your pool need?
The number of tablets depends on your daily chlorine loss: at 2 ppm a day, a 15,000-gallon pool needs about 4.4 three-inch tablets a week.
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.
On this page (11 sections)
That example assumes 200 g trichlor tablets with 90% available chlorine. Those tablets also add about 8.4 ppm of stabilizer (cyanuric acid, CYA) every week, so a pool that starts at 30 ppm passes 50 ppm in its third week.
Calculator: gallons, daily chlorine loss, tablet size
Pool size alone doesn't answer this question, which is why charts and quick answers disagree. The input most tablet charts skip is daily chlorine loss. Measure it: test free chlorine (FC) at dusk, test again at the next dusk without adding anything in between, and note the drop. Sun, heat, swimmers, rain, and CYA all change it, so repeat on a few typical days.
Then enter the tablet weight and available chlorine from your bucket's label. Three-inch tablets are often sold at 7–8 oz (about 200–225 g) and 90% available chlorine; check your label. The result is tablets per week plus a 12-week CYA forecast.
| Pool size | Loses 1 ppm/day | Loses 2 ppm/day | Loses 3 ppm/day |
|---|---|---|---|
| 5,000 gal | 0.7 | 1.5 | 2.2 |
| 10,000 gal | 1.5 | 2.9 | 4.4 |
| 15,000 gal | 2.2 | 4.4 | 6.6 |
| 20,000 gal | 2.9 | 5.9 | 8.8 |
| 25,000 gal | 3.7 | 7.4 | 11.0 |
A floater or feeder can't dissolve an exact fraction, so round to what your feeder setting or number of tablets allows and fine-tune by testing.
A single 200 g tablet at 90% contains about 180 g of available chlorine. Dissolved all at once, that would raise 10,000 gallons by about 4.8 ppm, or 15,000 gallons by about 3.2 ppm. Tablets release it over several days, which is what they are designed to do.
CYA build-up projection (weeks until 60/80/100 ppm)
Trichlor is stabilized chlorine: every 1 ppm of chlorine it delivers leaves about 0.6 ppm of cyanuric acid (CYA) behind (PHTA). Unlike chlorine, CYA isn't used up; it leaves only when water leaves the pool. The weekly rise depends on your chlorine loss, not your pool size:
| Chlorine loss | CYA added per week | Reaches 50 ppm | 60 ppm | 80 ppm | 100 ppm |
|---|---|---|---|---|---|
| 1 ppm/day | 4.2 ppm | week 5 | week 8 | week 12 | week 17 |
| 1.5 ppm/day | 6.3 ppm | week 4 | week 5 | week 8 | week 12 |
| 2 ppm/day | 8.4 ppm | week 3 | week 4 | week 6 | week 9 |
| 3 ppm/day | 12.6 ppm | week 2 | week 3 | week 4 | week 6 |
Weeks rounded up. Real pools lose some CYA to backwashing, splash-out, and rain overflow, so the rise may be slower. Test CYA monthly to see yours.
Why it matters: the chlorine level your pool needs rises with CYA. At CYA 40, the minimum FC is about 3 ppm; at CYA 80 it's about 6 ppm, above the 4 ppm swimming limit printed on many chlorine labels. Pools on tablets all season often show plenty of "chlorine" on the test and still grow algae. The fix at that point is a partial drain (how to lower CYA). A better plan: once CYA reaches 30–50 ppm, switch your daily chlorine to liquid chlorine or bleach, and save tablets for vacations. More reading: what cyanuric acid does, the liquid chlorine calculator for dosing liquid instead, and every chlorine product side by side on how much chlorine to add to a pool.
How long does one chlorine tablet last?
Measure it in chlorine, not in days. One 200 g tablet at 90% holds about 180 g of available chlorine. The 15,000-gallon pool above, losing 2 ppm a day, uses that much every 1.6 days, so one tablet covers about a day and a half of its chlorine need. How long a tablet takes to dissolve is a different number: it depends on water flow, temperature, and the feeder setting, and we could not find a published figure we trust. If FC falls between tests, add tablets or open the feeder. If it climbs, do the opposite.
Tablets for a 10 ft pool
A 10 ft round pool holds about 800–1,200 gallons (Intex rates its 10 ft × 30 in pools at 1,018 and 1,185 gallons). At 2 ppm of chlorine loss a day, 1,018 gallons needs about 60 g (2.1 oz) of 90% trichlor a week. That is less than a third of one 3-inch tablet, which is why a full-size tablet is hard to control in a small pool. See tablets for small pools below for better options.
Skimmer, floater, or feeder: where to put tablets
- Floater: cheap and simple, but output varies with where it drifts and how warm the water is. A floater parked in a corner or on the steps can leave concentrated, acidic water against a vinyl liner and bleach or weaken it. Tether it or check it daily.
- Inline or offline feeder (chlorinator): a closed canister plumbed into the return line with a dial for flow. Output is steadier and adjustable. Install it after the heater and any other equipment, so acidic, chlorine-rich water doesn't pass through them.
- Skimmer basket: common, but when the pump is off, the skimmer fills with highly concentrated acidic water. That water then goes through the pump, filter, and heater when the pump restarts. Check your pump and heater manuals before using the skimmer for tablets.
Whatever you use, the dissolving rate changes with water temperature and flow, so the number of tablets that holds FC steady in June won't do it in August. Test at least a few times a week.
Can you put chlorine tablets directly in the pool?
No. A tablet resting on the floor or a step dissolves into a small pocket of very strong, acidic chlorine. On vinyl it can bleach and weaken the liner; on plaster it can etch the surface. Use a floater or a feeder, as the product label directs, and keep tablets out of the pool itself.
pH drop from trichlor
Trichlor is acidic. As its chlorine is used, alkalinity drops by roughly 0.7 ppm for each 1 ppm of FC (from the reaction chemistry). At 2 ppm a day, that's about 10 ppm of total alkalinity (TA) lost per week, and pH drifts down with it. Signs: pH creeping below 7.2, stinging eyes, corrosion on ladders and heaters. Keep TA at 80–120 ppm when you rely on tablets and top it up with baking soda as needed. Low pH and metal corrosion are also a common source of stains.
Tablets for small pools
In a 1,000-gallon pool, one 200 g tablet contains enough chlorine for about 48 ppm. Even though it dissolves over days, that's hard to control, and its CYA adds up fast. For small above-ground and inflatable pools, use 1-inch tablets in a small floater or, better, measured doses of liquid chlorine or plain bleach; see kiddie pool chlorine and the bleach calculator. Compare what each option costs per ppm with the chemical cost calculator, and see how tablets stack up against other forms on types of pool chlorine.
- Never mix trichlor with calcium hypochlorite (cal-hypo) or other chlorine products, in a feeder, bucket, or scoop: the combination can react violently, ignite, or explode. Flush a feeder thoroughly before switching products.
- Don't open a feeder or bucket over your face; trichlor releases irritating gas. Wear gloves, and don't touch tablets with wet hands.
- Store tablets in a dry, cool place, away from acid and other chemicals. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
- Exposure: Poison Control, 1-800-222-1222.
Details for pool technicians
- Tablets per week = daily FC loss (ppm) × 7 × liters ÷ 1,000 ÷ available chlorine fraction ÷ tablet weight (g). 15,000 gallons (56,781 L), 2 ppm/day, 90%, 200 g: 883 g a week, 4.4 tablets.
- CYA rises by 0.6 ppm for each 1 ppm of FC delivered by trichlor (PHTA). No loss from splash-out, backwash, or rain is assumed, so the forecast is an upper bound.
- Not yet in our reference tests. The CYA projection is new in our engine and is not yet covered by the reference tests on the methodology page. The dose math it uses is covered.
FAQ
How much chlorine is in a chlorine tablet?
Multiply tablet weight by available chlorine. A 200 g (about 7 oz) trichlor tablet at 90% holds about 180 g of available chlorine, enough for about 4.8 ppm in 10,000 gallons if it all dissolved at once. Check your bucket: weights and percentages vary by brand.
Does trichlor raise CYA?
Yes. Every 1 ppm of chlorine from trichlor adds about 0.6 ppm of CYA (PHTA). A pool using 2 ppm a day on tablets gains about 36 ppm of CYA a month. Only draining and refilling lowers it.
Sources
- PHTA fact sheet, Dichlor (CYA per ppm from trichlor and dichlor): phta.org
- CDC, Model Aquatic Health Code 2024, Annex (chlorine and CYA): cdc.gov (PDF)
- Taylor Technologies, K-1005 instruction manual (trichlor dose table): PDF
- CDC, Pool chemical injuries 2008–2017: archive.cdc.gov