CYA Buildup Calculator: Trichlor and Dichlor Over Time

Projected stabilizer (CYA)
Week	CYA	Status
Week 1	38 ppm	OK
Week 2	47 ppm	OK
Week 3	55 ppm	Over
Week 4	64 ppm	Over
Week 5	72 ppm	Over
Week 6	80 ppm	Over
Week 7	89 ppm	Over
Week 8	97 ppm	Over
Week 9	106 ppm	Over
Week 10	114 ppm	Over
Week 11	122 ppm	Over
Week 12	131 ppm	Over
Week 13	139 ppm	Over
Week 14	148 ppm	Over
Week 15	156 ppm	Over
Week 16	164 ppm	Over

This calculator shows the week your pool crosses the CYA limit you choose, and how much the water you replace each week (rain overflow, backwashing, splash-out) slows the climb.

Calculator: CYA week by week

Enter your CYA now, how much chlorine (in ppm a day) comes from tablets or dichlor, which product you use, the CYA level you want to stop at, and the share of water you replace each week. With the example values (30 ppm now, 2 ppm a day from trichlor, stop at 50 ppm, no water replaced):

CYA rises 8.4 ppm a week.
Week 1: 38 ppm. Week 2: 47 ppm. Week 3: 55 ppm, over the 50 ppm limit.
Week 6: 80 ppm. Week 9: 106 ppm. Week 16: 164 ppm.

The formula: CYA next week = CYA this week × (1 − share of water replaced) + CYA per ppm of chlorine × ppm a day × 7. With no water replaced, the projection is the most CYA you could end up with. If you backwash often or get heavy rain that overflows the pool, enter a weekly share to see a slower rise.

To find your chlorine use, test free chlorine (FC) one evening and again the next evening without adding any. If tablets supply all of it, that drop is the "ppm a day from tablets".

How much CYA tablets and dichlor add

Trichlor (tablets and sticks): 0.6 ppm CYA for each 1 ppm of chlorine it delivers.
Dichlor (granules, often sold as shock or for hot tubs): 0.9 ppm CYA for each 1 ppm of chlorine.

Both figures come from the Pool & Hot Tub Alliance (PHTA) and are the same ones our dose calculators use. Liquid chlorine, bleach, cal hypo, and salt cells add no CYA. Dichlor shock adds up quickly: a large shock dose with dichlor can raise CYA more in one day than a week of tablets. For dichlor doses, see the dichlor calculator. For tablet counts, see the chlorine tablets calculator. For how each chlorine type compares, see types of pool chlorine.

Which limit to stop at

CYA protects chlorine from the sun, but it also holds part of the chlorine in reserve, so the free chlorine you need goes up as CYA goes up. Our engine's balance targets are:

Pools chlorinated with liquid chlorine, bleach, or tablets: 30–50 ppm CYA.
Salt pools: 60–80 ppm CYA.

Choose the top of your range as the limit. The pool chlorine levels page shows the free chlorine you need at each CYA level. At high CYA, the chlorine you need can be more than many product labels allow for swimming, which is why a pool on tablets all season can turn green with chlorine still showing on the test.

When to switch to liquid chlorine

Switch before the week the calculator flags. A simple plan:

Use tablets or dichlor until CYA reaches your target range.
Then move daily chlorine to liquid chlorine or bleach, which add no CYA. The liquid chlorine calculator gives the daily amount. Always follow the product label.
Keep tablets for times you can't add chlorine by hand, such as a week away. Check CYA when you return.
Test CYA every month during the season, even when you think nothing has changed.

Safety. Never mix trichlor or dichlor with cal hypo or other chlorine products, dry or wet, and never put different chlorine products in the same feeder, floater, or bucket. Never mix chlorine with acid. Store chemicals dry, apart, and in their original containers. See pool chemical safety. Always follow the product label.

Only water replacement lowers CYA

CYA does not get used up the way chlorine does, and it doesn't evaporate. It leaves only with water: splash-out, backwashing, rain overflow, or a partial drain. So once it is too high, the fix is to replace water. The how to lower CYA page has the drain percentage for your numbers, and how often to drain a pool covers CYA, calcium, and salt together.

To keep from getting there, use this calculator at the start of the season with your expected chlorine use, and set a reminder for the week before you reach your limit.

FAQ

How long until tablets make my CYA too high?

It depends on your chlorine use and where you start. At 2 ppm a day from trichlor, CYA rises about 8.4 ppm a week: from 30 ppm, it passes 50 ppm in week 3 and reaches 80 ppm by week 6. Enter your own numbers above.

Why does my CYA go up every week?

Trichlor tablets and dichlor contain stabilizer. Every 1 ppm of chlorine they deliver leaves 0.6 ppm (trichlor) or 0.9 ppm (dichlor) of CYA behind, and CYA leaves the pool only with water. Evaporation and top-ups do not lower it.

When should I switch from tablets to liquid chlorine?

When CYA reaches the top of your target range: about 50 ppm for most chlorine pools, or 80 ppm for salt pools, per our engine's targets. The calculator shows the week you get there.

Does rain lower CYA in a pool?

Only if the pool overflows or you drain the extra water. Rain itself has no CYA, so the share of pool water that leaves takes the same share of CYA with it. Enter that share as "water replaced per week" to see the effect.

Sources

PHTA fact sheet, Dichlor (available chlorine and CYA per ppm): phta.org
CDC, Model Aquatic Health Code 2024, Annex (chlorine and CYA): cdc.gov (PDF)
CYA per ppm constants and balance targets: methodology