Dichlor Calculator: Dose and CYA Added
In 10,000 gallons, about 2.4 oz of 56% dichlor (or 2.15 oz of 62%) raises free chlorine by 1 ppm. Every 1 ppm also adds about 0.9 ppm of stabilizer (CYA), so your CYA climbs each time you use it.
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 (7 sections)
Dichlor is a granular chlorine with stabilizer built in. It dissolves fast and is close to pH-neutral, which is why people use it in small pools and hot tubs. The catch is the stabilizer. Enter your gallons, your free chlorine now, and the level you want, and the calculator below shows the ounces and the CYA you will add.
How much dichlor per ppm
Dose scales with pool size and with the percent on your label. These figures come from the calculator's engine for a 10,000-gallon pool. For another size, scale up or down, or just use the calculator above, which does it for you. The two strengths sold are 56% and 62% available chlorine (PHTA).
| Product | For +1 ppm free chlorine | For +10 ppm free chlorine | CYA added for +1 ppm |
|---|---|---|---|
| Dichlor 56% | 2.4 oz | 24 oz (1.5 lb) | +0.9 ppm |
| Dichlor 62% | 2.15 oz | 21.5 oz | +0.9 ppm |
The 62% product is stronger, so you use less of it. The CYA added is the same, because it depends on the chlorine you add, not on the product's strength. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling. If your label's directions and the calculator differ, the label wins.
The hidden cost: stabilizer
Cyanuric acid (CYA) protects chlorine from sunlight, which helps outdoors. But CYA does not leave the water on its own. Rain, splash-out, and backwashing dilute it a little, and the only real way to lower it is to replace water. Per PHTA, dichlor adds 0.9 ppm of CYA for every 1 ppm of chlorine. Trichlor adds 0.6 ppm, and liquid chlorine and cal-hypo add none.
So the question is not whether dichlor is good. It is how many weeks you can use it before CYA gets too high. The table shows the weekly math: weekly chlorine added times 0.9 gives CYA added per week. It assumes you start at 0 ppm CYA and that nothing dilutes the water. Real pools lose some CYA to rain and splash-out, so treat these as the fastest case.
| Chlorine added per week | CYA added per week | Weeks to reach 80 ppm | Months, about |
|---|---|---|---|
| 2 ppm | 1.8 ppm | about 44 | 10 |
| 5 ppm | 4.5 ppm | about 18 | 4 |
| 10 ppm | 9 ppm | about 9 | 2 |
Why 80? In chlorine pools, we suggest keeping CYA around 30–50 ppm. At 50 ppm, the free chlorine minimum alone reaches 4 ppm, which is also the swim limit on many chlorine labels. See pool chlorine levels for the full table. If you start at 40 ppm CYA, you reach 80 in about half the time shown. Test CYA every month or two while you use dichlor; how to test cyanuric acid shows how. If it gets too high, see how to lower CYA.
Dichlor vs cal-hypo vs liquid chlorine
| Form | Adds CYA? | Adds calcium? | pH effect |
|---|---|---|---|
| Dichlor (56% or 62%) | Yes, 0.9 ppm per ppm | No | Close to neutral |
| Cal-hypo (calcium hypochlorite) | No | Yes, about 0.7 ppm per ppm | Raises pH |
| Liquid chlorine or bleach | No | No | Raises pH |
Pick the one whose side effect your pool can afford. A pool with low CYA can use dichlor for a while. A pool with high CYA should use liquid chlorine or cal-hypo instead. Never mix dichlor with cal-hypo or other chlorine products. All the forms are compared on types of pool chlorine, and the liquid alternative has its own tool: the liquid chlorine calculator.
Dichlor as shock
"Shock" is a high dose of chlorine, not a different chemical. Many shock bags are dichlor or cal-hypo. If your CYA is already above 50 ppm, a dichlor shock of 10 ppm adds about 9 ppm of CYA more, and that makes your next shock harder. The calculator's number for +10 ppm is in the table above. For when and how to shock, use how to shock a pool. Do not swim until free chlorine is at or below the swim limit on your label.
Dichlor in a hot tub
Dichlor is popular in hot tubs because it dissolves quickly and the doses are small. The CDC, though, recommends not using cyanuric acid or chlorine products with cyanuric acid in hot tubs. Both can be true, because hot tub water is changed often. A common approach is dichlor for the first days after a fill, then liquid chlorine or bleach. In a 400-gallon tub, about 2.7 g of 56% dichlor raises free chlorine 1 ppm (see hot tub chlorine, which also has the table for other tub sizes). Use a scale; spoon measures of granules vary.
Safety. Never mix dichlor with acid, with other chlorine products, or with anything else. Add chemicals to the water, never water to the chemicals. Wear gloves and eye protection. Keep granules dry and in the original container, away from acid and other chemicals. Don't swim while chlorine is above the limit on your label. Poison Control: 1-800-222-1222.
FAQ
How much dichlor do I add to raise chlorine 1 ppm?
In 10,000 gallons, about 2.4 oz of 56% dichlor or 2.15 oz of 62%. Multiply by the ppm you need, and scale for your gallons. Use the calculator above for your pool.
How much CYA does dichlor add?
About 0.9 ppm of CYA for every 1 ppm of chlorine (PHTA). A 10 ppm shock adds about 9 ppm CYA, whatever the pool size.
Dichlor or cal-hypo: which is better?
Neither is better in general. Dichlor adds stabilizer and leaves calcium alone, and cal-hypo adds calcium and no stabilizer. Choose by what your water can take.
Can I use dichlor in a hot tub?
Many people do for a short time, but the CDC recommends against CYA in hot tubs. Stabilizer builds with each dose, so switch to liquid chlorine or bleach after the first days, or drain and refill when chlorine stops holding.
How often can I use dichlor before CYA gets too high?
It depends on how much chlorine you add. At 5 ppm a week from zero, about 18 weeks to reach 80 ppm. Test CYA monthly.
Sources
- PHTA fact sheet, Dichlor (56% and 62%, CYA added per ppm of chlorine): phta.org
- CDC, Home pool and hot tub water treatment and testing (no CYA in hot tubs): cdc.gov
- CDC, Model Aquatic Health Code 2024, Annex (chlorine and CYA): cdc.gov (PDF)
- Doses and the weeks-to-80 table: from our engine's constants (0.9 ppm CYA per ppm FC); see methodology. The weeks table is simple multiplication of that constant, not a measured result.