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Pool Closing Chemicals
Balance, then a last strong dose of chlorine before the cover goes on.
1 of 2: your gallons
Your dose appears here after the last question.
- 7.4 to 7.6
- pH before closing
- Under 65°F
- water is cool enough to close
- Clean
- brush and vacuum first
Pool closing chemicals start with a water test: use the readings to decide which adjustments your water needs before the cover goes on. Here's what to aim for and how much to add. ppm stands for parts per million. It tells you how much of a chemical is dissolved in the water, and it is what your test strip or kit measures.
Finding your gallons
Every dose depends on how much water your pool holds, so start here. Find your size in this table, or multiply the surface area in square feet by the average depth in feet, then by 7.48 (use 5.9 for round and oval pools). Our dose calculators remember your gallons in your browser and fill them in for you.
Here are some typical volumes, assuming a 4.5 ft average depth:
| Pool size | Roughly |
|---|---|
| 12 x 24 ft | 9,690 gallons |
| 16 x 32 ft | 17,200 gallons |
| 20 x 40 ft | 26,900 gallons |
Balancing the water
Start with a test, then work on one chemical at a time.
| Reading | Target | How to correct it |
|---|---|---|
| Free chlorine | 3 to 5 ppm at 30 ppm stabilizer, or 4 to 6 at 40 | To raise it 1 ppm, add 10 fl oz of liquid chlorine at 12.5%. |
| pH | Somewhere between 7.4 and 7.6, though anything from 7.2 up to 7.8 works | At normal alkalinity, 9 fl oz of 31.45% muriatic acid per 10,000 US gallons lowers pH from 7.8 to 7.5. To bring 7.2 up to 7.5, use 1 lb 1 oz of soda ash. |
| Total alkalinity | Between 80 and 120 ppm | To raise it 10 ppm, add 1 lb 6 oz of baking soda. To lower it 10 ppm, add 26 fl oz of 31.45% muriatic acid. |
| Calcium hardness | Between 200 and 400 ppm | To raise it 10 ppm, add 1 lb 3 oz of calcium chloride 77%. |
| Stabilizer | Between 30 and 50 ppm, or 70 to 80 if you have a salt cell | To raise it 10 ppm, add 13 oz of cyanuric acid. |
All of these amounts are per 10,000 gallons. Got 20,000 gallons? Double them. Got 5,000? Cut them in half. The doses are for US gallons.
Stabilizer, also called cyanuric acid, works differently from the other chemicals. It doesn't break down or filter out, so test it before you add more. If it's too high, the only fix is to replace some of the water. Dropping from 100 ppm to 50 ppm means swapping out roughly half the water.
Adding the closing chlorine
If you are shocking the pool, a dose that raises free chlorine by 10 ppm is 102 fl oz of 12.5% liquid chlorine for every 10,000 US gallons. Free chlorine means the chlorine that is still working in the water. A 20,000 gallon pool takes twice that.
Keep the pump running while you add it, and pour it in front of a return jet. Wait until free chlorine is back in its normal range, 10 ppm or less, before anyone swims.
How to add chemicals safely
- 1. Keep the pump running so each chemical gets carried around the pool.
- 2. Add one chemical at a time, pouring it in front of a return jet with the pump running.
- 3. Acid is added to water, never water to acid.
- 4. Never mix acid with chlorine.
- 5. Test pH again 30 minutes after adding acid or soda ash.