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Pool Opening Chemicals
Clear, cloudy or green under the cover? Three answers give your opening shock.
1 of 3: your gallons
Your dose appears here after the last question.
- Day 1
- shock and run the filter non-stop
- Overnight
- chlorine should hold
- 7.4 to 7.6
- balance pH next
The chemicals for opening a pool mostly come down to one big dose of chlorine, called a shock, then a few small adjustments to pH, alkalinity, stabilizer and calcium. Test the water first, then use the results to decide which chemicals to add.
- Plan on roughly 102 fl oz of liquid chlorine at 12.5% for each 10,000 gallons
- Add chemicals with the pump running; for routine filtering, aim to move all the pool water through the filter once a day.
- After a shock, wait to swim until free chlorine falls back to the normal range for your stabilizer, no higher than 10 ppm.
- pH goal: 7.4 to 7.6
Checklist for opening day
- 1. Turn on the pump and filter. Add chemicals only while the pump is circulating.
- 2. Run your test, then dose in the shock.
- 3. Then fix alkalinity and pH.
How much shock to open a pool?
A shock that lifts free chlorine by 10 ppm takes roughly 3 liters of 12.5% liquid chlorine (about 102 fl oz) for every 10,000 gallons of water. Free chlorine is what is left over to keep cleaning the water. Your test strip or kit gives its reading in parts per million (ppm).
| Pool size | Liquid chlorine (12.5%) | Roughly in liters |
|---|---|---|
| 5,000 gallons | 51 fl oz | about 1.5 liters |
| 10,000 gallons | about 102 fl oz | roughly 3 liters |
| 20,000 gallons | 205 fl oz | roughly 6 liters |
Liquid chlorine comes in jugs at most stores. Buy enough jugs to cover the full dose for your pool's size.
For small daily top-ups once the season starts, roughly 10 fl oz of liquid chlorine at 12.5% nudges free chlorine up 1 ppm in a 10,000-gallon pool. To get the same lift with 65% cal-hypo, you would use 2.1 oz.
Working out your gallons
All doses here assume US gallons. For a rectangular pool, take the length times the width times the average depth in feet, then times 7.48. If the floor slopes, average the depth at the two ends. For round and oval pools, swap 7.48 for 5.9.
| Pool size (avg. depth 4.5 ft) | Holds roughly |
|---|---|
| 12 ft x 24 ft | 9,690 gallons |
| 16 ft x 32 ft | 17,200 gallons |
| 20 ft x 40 ft | 26,900 gallons |
So a 16 x 32 ft pool, about 17,200 gallons, needs a little under twice the 10,000-gallon dose of liquid chlorine for its opening shock.
Adding the shock safely
Add each chemical by itself while the pump runs, pouring near a return jet so it spreads through the whole pool fast. Acid and chlorine react with each other, so never mix them. Acid goes into the water first, never water onto acid.
Once you have shocked the pool, no one should swim until free chlorine drops back to 10 ppm or less and into its usual range. At normal daily levels, it is safe to swim about half an hour after you pour in liquid chlorine.
Balancing after the shock
After the shock, check your other numbers and nudge each one where it belongs.
| What to test | Where you want it |
|---|---|
| pH | 7.4 to 7.6 (anything from 7.2 up to 7.8 works) |
| Alkalinity (total) | keep it between 80 and 120 ppm |
| Calcium hardness | aim for 200 to 400 ppm |
| Stabilizer | keep it near 30 to 50 ppm, or 70 to 80 with a salt system |
| Salt | follow the number your cell manufacturer gives, usually somewhere near 3,000 to 3,500 ppm |
Stabilizer goes by another name too, cyanuric acid, and it shields chlorine from sunlight. The free chlorine you need shifts with it: 3 to 5 ppm when stabilizer reads 30, and 4 to 6 ppm when it reads 40.
Balancing doses, per 10,000 gallons
- To bring pH down from 7.8 to 7.5: pour about 9 fl oz of muriatic acid at 31.45% (with alkalinity in the normal range)
- To lift pH from 7.2 to 7.5: use 1 lb 1 oz of soda ash
- To raise alkalinity by 10 ppm: use 1 lb 6 oz of baking soda
- To lower alkalinity by 10 ppm: pour in about 26 fl oz of muriatic acid at 31.45%
- To nudge stabilizer up 10 ppm: 13 oz of cyanuric acid
- To raise calcium hardness by 10 ppm: add 1 lb 3 oz of calcium chloride at 77%
Use twice as much for 20,000 gallons and half as much for 5,000. Both acid and soda ash move pH fast, so retest 30 minutes after you add either one.
Stabilizer that is too high
Stabilizer never breaks down, and no filter catches it, so it can pile up over past seasons. Replacing some of the water is the only fix. To go from 100 ppm to 50 ppm, plan on swapping out roughly half the water.
Pump and filter time after opening
Shoot for one turnover a day. A turnover is one full pass of all the water through the filter, which usually takes 8 hours or so. Tack on another 1 to 2 hours when the weather is hot or the pool got heavy use.
Keep an eye on the pressure gauge while the filter handles the opening dirt. Backwash when it reads 8 to 10 psi over the clean figure.