Chlorine Tablet Calculator: 3 Inch Tabs Per Week
How many 3 inch chlorine tablets a week a pool actually needs, from the registered label rate or your measured chlorine loss, plus the stabilizer they stack up over a season.
How this is calculated
One tablet is converted to the free chlorine it delivers to your pool, and everything else follows from that single number.
pool_multiple = volume_gal / 10000
lb_per_tab = (tablet_oz / 16) x (available_chlorine_pct / 100)
ppm_fc_per_tab = lb_per_tab x 11.9827 / pool_multiple
From the label rate:
tabs_per_week = tabs_per_10k_per_week x pool_multiple
weekly_fc_ppm = tabs_per_week x ppm_fc_per_tab
From measured demand:
weekly_fc_ppm = daily_fc_loss_ppm x 7
tabs_per_week = weekly_fc_ppm / ppm_fc_per_tab
Both routes:
weekly_cya_ppm = weekly_fc_ppm x 0.6
season_cya_ppm = weekly_cya_ppm x season_weeks
projected_cya = cya_now + season_cya_ppm
weeks_to_ceiling = ceil( (ceiling − cya_now) / weekly_cya_ppm )Worked example. A 20,000 gallon pool at the label rate, with 8 oz tablets at 90% available chlorine. One tablet carries 0.45 lb of available chlorine, which in this pool is 2.70 ppm of free chlorine. The label rate puts 2 tablets in the feeder each week, so the pool receives 5.39 ppm of free chlorine and 3.24 ppm of cyanuric acid every week. Over a 26 week season that is 52 tablets and 84.1 ppm of cyanuric acid. A pool that started the season at 30 ppm finishes at 114 ppm, and crosses 50 ppm in the seventh week.
Where the tablet constants come from. EPA Reg. No. 45458-18, Trichlor 3" Tablets, the master label in the EPA Pesticide Product Label System, notification accepted 23 January 2014. Active ingredient trichloro-s-triazinetrione 99.0%, available chlorine 90%, tablets that dissolve over 3 to 7 days. Its routine chlorination direction reads: "Add one 7 ounce [8 ounce] tablet for each 10,000 gallons of pool water to maintain chlorine residual at 1-3 ppm", under the label heading "Every week (or as needed)". So the label rate is one tablet per 10,000 gallons per week, not the one-tab-per-5,000-gallons rule that circulates in shop copy, and both 7 oz and 8 oz are label tablet weights. Every one of those figures is an editable field above, because the bucket you bought is the authority on its own contents.
Where the cyanuric acid figure comes from. 0.6 ppm of cyanuric acid per 1 ppm of free chlorine follows from molar composition rather than from formulation: one cyanurate ring rides in with every three chlorines, and free chlorine is measured as Cl₂ equivalent, so 129.07 / (3 × 70.906) = 0.607. APSP, via Aquatics International, publishes the matching figure from the other direction: 10 ppm of available chlorine from trichlor raises cyanuric acid by 6 ppm. The engine carries the rounded 0.6, the same constant the rest of the pool tools on this site use, which puts a season projection about 1% below the unrounded 0.607.
Why one pound is 11.98 ppm. Water weighs 8.3454 lb per US gallon, so 10,000 gallons weigh 83,454 lb and one pound of pure product in it is 1 / 83,454, which is 11.98 parts per million. That is the reciprocal of the 0.0000083454 lb per gallon per ppm constant the chlorine dose calculator runs on, so the two pages cannot drift apart.
What the projection deliberately leaves out. Splash-out, backwashing and rain overflow all dilute cyanuric acid, and none of them is modelled here, because none of them is a number you can predict for a given pool. The season figure is therefore a ceiling, not a forecast: it is what the tablets put in, before whatever the pool happens to lose. Test rather than trust it, and treat the week count as the moment to start planning the water change.
Sources: U.S. EPA Pesticide Product Label System, EPA Reg. No. 45458-18 Trichlor 3" Tablets master label, accepted 23 January 2014 (tablet weights, 90% available chlorine, the weekly per-10,000-gallon rate, the 3 to 7 day dissolve time, and the requirement to keep pH at 7.2 to 7.6 and total alkalinity at 90 to 125 ppm). Manufacturer safety data sheet for trichloroisocyanuric acid (pH 3.0 to 3.5 in a 1% solution at 25 °C, molecular mass 232.41 g/mol), which is why a tablet feeder pulls pH down. PubChem for the molar masses behind the 0.6 ratio. Aquatics International, carrying the APSP figure of 6 ppm of cyanuric acid per 10 ppm of available chlorine from trichlor. Volume converts with 1 US gallon = 3.785411784 L.
How many chlorine tablets does a pool need per week?
The registered label answer is one 3 inch tablet per 10,000 gallons per week. The EPA master label for trichlor 3 inch tabs (EPA Reg. No. 45458-18, accepted 23 January 2014) reads: “Add one 7 ounce [8 ounce] tablet for each 10,000 gallons of pool water to maintain chlorine residual at 1-3 ppm”, under the label heading “Every week (or as needed)”. So a 20,000 gallon pool starts at two tablets a week, and the tool above turns that into the tablet count, the case count for the season, and the number the label does not print: the cyanuric acid those tablets leave behind.
Everything on this page is a pure function of what you typed this session. Nothing is saved, there is no pool profile and no account, and the label on the bucket you actually bought is the authority on everything the page does not compute.
The label rate, and the rule of thumb that competes with it
Most pool writing online repeats a different number: one tablet per 5,000 gallons. It is not a fabrication, it is a different claim. A tablet is a rate of supply, and the pool sets the rate of demand, so a shaded pool with two swimmers a week and a sun-blasted pool with a pool party every Saturday cannot share a tablet count. What the label gives you is the published starting point, and what settles it is your own measurement: test free chlorine, wait 24 hours with the pump on its normal schedule, test again. Type that loss into the measured demand mode above and the tablet count reflects your pool rather than an average of everyone’s.
The gap between the two is worth seeing. A 20,000 gallon pool at the label rate takes two tablets a week. The same pool losing 2 ppm of free chlorine a day needs 14 ppm a week, which is 5.19 tablets, so six go in the feeder. That pool is not being over-dosed; it is being dosed for what it actually uses. What changes underneath is the stabilizer.
The number that ends the route in a drain
Trichlor is stabilized chlorine. Every tablet carries cyanuric acid in with the chlorine, at a fixed ratio of about 0.6 ppm of cyanuric acid per 1 ppm of free chlorine, because one cyanurate ring rides in with every three chlorines and free chlorine is measured as Cl₂ equivalent. Work that through and one 8 ounce tablet at 90% available chlorine is 5.39 ppm of free chlorine and 3.24 ppm of cyanuric acid per 10,000 gallons.
Now scale it to a season, which is the part nobody types out:
| Route on a 20,000 gallon pool | Tablets per week | Cyanuric acid per week | Over 26 weeks |
|---|---|---|---|
| Label rate, one tab per 10,000 gal | 2 | +3.24 ppm | +84 ppm |
| Measured demand, 2 ppm per day | 5.19 | +8.40 ppm | +218 ppm |
A pool that opened at 30 ppm of cyanuric acid and ran the label rate finishes the season near 114 ppm. On the measured-demand route it is past 50 ppm inside three weeks. There is no chemical that lowers cyanuric acid, which is why the only honest ending to that arithmetic is a partial water change, sized by the CYA drain and dilution calculator. If your problem is the opposite one, a stabilizer reading that is too low rather than too high, the cyanuric acid calculator sizes the granular dose instead.
The tool prints the week your projection crosses the ceiling you set, because that is the number you can act on. Plan a drain in week seven and it is maintenance; discover it in August and it is an emergency in the middle of the season.
The second coupled effect: pH and alkalinity
Tablets are acidic. A trichlor safety data sheet puts a 1% solution at pH 3.0 to 3.5, so a feeder running all season pulls pH and total alkalinity down with it, steadily and invisibly. The tablet label says to hold pH at 7.2 to 7.6 and total alkalinity at 90 to 125 ppm and to “make any necessary adjustments promptly”, which is the label’s way of saying the tablets will move both. A tablet-fed pool usually needs base rather than acid, which surprises operators who arrive from a liquid-chlorine route where the drift runs the other way.
What this page is not
It is not a single-dose calculator. If you are standing at a pool with a free chlorine reading and a target and you want the amount to add today, that is the pool chlorine dose calculator, which prints whole tabs among its pourable units. This page answers the weekly and seasonal question instead.
It is also not a cost page. What tablets cost per ppm against liquid chlorine, cal-hypo or dichlor is a real question with a real answer, and it belongs on the chlorine type cost comparator, which ranks them properly. Publishing a second, slightly different cost table here would only give you two numbers to choose between.
Reading it in litres and grams
Switch the volume unit to L and the tablet weight unit to g and both convert at the form edge; the engine still runs on gallons and ounces because that is how every published tablet rate is written. One US gallon is 3.785411784 L and one ounce is 28.3495 g. A 75,000 litre pool is 19,813 gallons, so the label rate puts it at two tablets a week, the same answer from the other set of units.
What it is not
It is a quantity estimate, not a water-safety record and not a substitute for the product label. The bucket you bought carries the strengths, the directions and the warnings that govern its use, and they win over any figure on this page. Handling sits under the label’s own METHOD OF APPLICATION heading, which directs the tablets to a floating dispenser, automatic chlorinator or feeder designed for the product, and states that they are not to be thrown directly into the pool, nor used in any device that has held another chlorinating compound. That is the text to work from, not this page. Print the week’s tablet count as a route sheet, or export CSV, PDF or copy as text, all ungated and without an account.
Frequently asked questions
How many chlorine tablets do I need for a 10,000 gallon pool?
Is it one chlorine tablet per 5,000 gallons or per 10,000 gallons?
How much cyanuric acid do chlorine tablets add?
Do chlorine tablets lower pH?
When do the tablets stop working?
Sources
- EPA Reg. No. 45458-18, Trichlor 3" Tablets master label: trichloro-s-triazinetrione 99.0%, available chlorine 90%, tablets dissolving over 3 to 7 days, and the routine chlorination direction to add one 7 ounce (8 ounce) tablet for each 10,000 gallons every week to hold a 1 to 3 ppm residual. U.S. EPA Pesticide Product Label System (notification accepted 23 January 2014). Retrieved .
- Trichloroisocyanuric acid safety data sheet: pH 3.0 to 3.5 in a 1% solution at 25 C, molecular mass 232.41 g/mol, which is why a tablet feeder pulls pH and total alkalinity down. Qingdao Pima Industrial (manufacturer SDS). Retrieved .
- Comparing the operating costs of calcium hypochlorite and trichlor: trichlor raises cyanuric acid 6 ppm per 10 ppm of available chlorine, per APSP. Aquatics International. Retrieved .
- Cyanuric acid, CID 7956 (molar mass 129.07 g/mol, the basis of the CYA-per-ppm-FC derivation). PubChem, National Library of Medicine. Retrieved .
- Trichloroisocyanuric acid, CID 6909 (molar mass 232.41 g/mol). PubChem, National Library of Medicine. Retrieved .