20,000 gallon pool: how much of each chemical
To raise free chlorine (FC) by 1 ppm in a 20,000-gallon pool, add about 20 fl oz of 12.5% liquid chlorine, 26 fl oz of 10%, or 41 fl oz of 6% bleach.
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.
| Product | +1 ppm FC | +2 ppm FC | +5 ppm FC | +10 ppm FC |
|---|---|---|---|---|
| Pool liquid chlorine 12.5% | 20 fl oz606 mL | 41 fl oz1.21 L | 102 fl oz3.03 L | 1.6 gal (205 fl oz)6.06 L |
| Pool liquid chlorine 10% | 26 fl oz757 mL | 51 fl oz1.51 L | 1.0 gal (128 fl oz)3.79 L | 2.0 gal (256 fl oz)7.57 L |
| Household bleach 6% | 41 fl oz1.22 L | 82 fl oz2.44 L | 1.6 gal (206 fl oz)6.09 L | 3.2 gal (412 fl oz)12.19 L |
| Household bleach 8.25% | 29 fl oz858 mL | 58 fl oz1.72 L | 1.15 gal (145 fl oz)4.29 L | 2.25 gal (290 fl oz)8.58 L |
| Cal-hypo 65% | 4.1 oz116 g | 8.2 oz233 g | 1.3 lb582 g | 2.55 lb1.16 kg |
| Dichlor 56% | 4.8 oz135 g | 9.5 oz270 g | 1.5 lb676 g | 3.0 lb1.35 kg |
| Trichlor 90% | 3.0 oz84 g | 5.9 oz168 g | 14.8 oz421 g | 1.85 lb841 g |
Liquid chlorine percent is the trade percent on pool chlorine jugs; bleach percent is by weight. Cal-hypo adds calcium. Dichlor and trichlor add stabilizer (CYA). To shock, follow the directions on your product’s label.
| Product | Change | Amount |
|---|---|---|
| Baking soda | TA +10 ppm | 2.8 lb1.27 kg |
| Muriatic acid 31.45% | TA −10 ppm | 51 fl oz1.51 L |
| Dry acid 93.2% | TA −10 ppm | 4.25 lb1.94 kg |
| Muriatic acid 31.45% | pH 7.8 → 7.5 (TA 100, CYA 40) | 19 fl oz569 mL |
| Dry acid 93.2% | pH 7.8 → 7.5 (TA 100, CYA 40) | 1.6 lb731 g |
| Soda ash | pH 7.2 → 7.5 (TA 80, CYA 30) | 1.85 lb841 g |
pH doses depend on your TA and CYA, so use the calculator for your numbers. Acid and pH-up amounts are rounded down.
| Product | Change | Amount |
|---|---|---|
| Stabilizer (CYA) | +10 ppm | 1.65 lb757 g |
| Stabilizer (CYA) | 0 → 40 ppm (new fill) | 6.7 lb3.03 kg |
| Calcium chloride 77% | CH +10 ppm | 2.4 lb1.09 kg |
| Calcium chloride 77% | CH +50 ppm (one step) | 12.0 lb5.45 kg |
| Pool salt | 0 → 3,200 ppm (new fill) | 534 lb242.27 kg · 14 × 40 lb bags |
| Pool salt | +500 ppm | 83.5 lb37.85 kg |
On this page (9 sections)
Quick answer: chlorine for a 20,000 gallon pool (depends on product)
A routine top-up from FC 1 to 4 ppm takes about 61 fl oz of 12.5% liquid chlorine (just under half a gallon), 77 fl oz of 10%, or 124 fl oz of 6% bleach, almost a full gallon. At this size, bleach from the grocery store gets heavy to carry and store. Pool chlorine at 10–12.5% needs roughly half the volume for the same result.
A quick check you can do in your head: one gallon of 10% chlorine raises 20,000 gallons by 5 ppm, and one gallon of 12.5% by 6.25 ppm. If your target is 3–5 ppm with stabilizer (CYA) at 40 ppm, a gallon is a big correction, not a daily top-up.
Dose table: chlorine by product for +1/+2/+5/+10 ppm
The +10 ppm column reaches 1.6 gallons of 12.5% and 2 gallons of 10%, so plan jugs before a big correction. Dry products per 1 ppm: about 4.1 oz of 65% cal-hypo (calcium hypochlorite), 4.8 oz of 56% dichlor, and 3 oz of 90% trichlor. In an inground plaster pool, cal-hypo's added calcium is sometimes welcome; in vinyl, liquid chlorine adds no calcium or stabilizer. See how much chlorine to add for how the products compare.
How much shock for 20,000 gallons: double the label
A shock dose comes from your product's label, and most labels are written per 10,000 gallons. This pool is twice that, so double the printed amount. A liquid chlorine label that says 1 gallon per 10,000 gallons means 2 gallons here; a cal-hypo label that says 1 lb means 2 lb.
To check what a jug or bag will do at this size: a gallon of 12.5% liquid chlorine adds about 6.25 ppm, and a 1 lb bag of 65% cal-hypo about 3.9 ppm, plus about 2.7 ppm of calcium. Buy enough for the doubled amount before you start. Don't swim until chlorine is back at or below the label's swim limit (often 4 ppm). See how to shock a pool for the steps and the swim-again rule.
pH and alkalinity: acid and baking soda
If your pH tends to creep up, acid is the chemical you'll measure most, and at this size the amounts are no longer small. For pH 7.8 to 7.5, our model gives about 19 fl oz of 31.45% muriatic acid at total alkalinity (TA) 100 and CYA 40. If your readings differ, so does the dose:
- TA 80: about 16 fl oz; TA 120: about 22 fl oz (same pH change, CYA 40).
- CYA 0: about 15 fl oz; CYA 80: about 23 fl oz (TA 100).
- Starting from pH 8.0 instead: about 26 fl oz, which the calculator splits into two steps.
For alkalinity: about 51 fl oz of muriatic acid lowers TA by 10 ppm, and about 2.8 lb of baking soda raises it by 10 ppm. Run your own readings through the muriatic acid calculator; for raising TA, see baking soda in a pool.
Stabilizer, calcium, and salt
- Stabilizer: 1.65 lb per 10 ppm and 6.7 lb from 0 to 40 ppm. Because only draining removes it, test first and add only the difference. The stabilizer calculator works from your current reading.
- Calcium chloride (77%): 2.4 lb per 10 ppm. The 12 lb in the table is one +50 ppm step; larger increases go in several steps.
- Salt: about 534 lb from 0 to 3,200 ppm, which is fourteen 40 lb bags with the last one partly used. A +500 ppm top-up is about 83.5 lb. Use the pool salt calculator for your cell's target.
Pools that hold about 20,000 gallons
None of the above-ground models in our list is within 12% of 20,000 gallons, so a pool this size is usually inground. A 16 × 32 ft pool holds about 19,150 gallons at a 5 ft average depth and 21,065 at 5.5 ft. If yours has a flat shallow end and a separate deep end, the length-weighted average matters: one example layout (16 ft of floor at 3.5 ft, an 8 ft slope, 8 ft of floor at 8 ft) works out to about 19,868 gallons, where the simple average would say 22,023. The 16 × 32 pool page shows the options, and the pool volume calculator handles sloped floors.
Calculator preset to this volume
The calculator at the top is set to 20,000 gallons. Enter your test and products to see the steps in order, with large acid doses split into smaller steps. Nearby sizes: 15,000 gallons and 25,000 gallons.
Safety with acid. Doses of 20 fl oz or more are common at this size. Add acid to a bucket of pool water (never water to acid), pour it slowly in front of a return with the pump running, and keep it away from chlorine. Stay out of the water for 30 minutes after acid. Always follow the product label. Poison Control: 1-800-222-1222.
FAQ
How much chlorine do I put in a 20,000 gallon pool?
Per 1 ppm: about 20 fl oz of 12.5% liquid chlorine, 26 fl oz of 10%, or 41 fl oz of 6% bleach. From FC 1 to 4, about 61 fl oz of 12.5%. The liquid chlorine calculator corrects for jugs that have lost strength.
How much shock for a 20,000 gallon pool?
Twice what the label on your shock product says for 10,000 gallons. For example, where a liquid chlorine label says 1 gallon per 10,000 gallons, add 2 gallons. Stay out of the pool until chlorine returns to the label's swim limit.
How much muriatic acid for a 20,000 gallon pool?
About 51 fl oz of 31.45% acid lowers total alkalinity by 10 ppm. For pH, it depends on your water: about 19 fl oz takes pH from 7.8 to 7.5 at TA 100 and CYA 40.
How much muriatic acid to add to a 20,000 gallon pool to lower pH?
For 7.8 to 7.5 with CYA 40: about 16 fl oz at TA 80, 19 fl oz at TA 100, 22 fl oz at TA 120. From 7.9 to 7.5 at TA 100, about 23 fl oz. The calculator never lowers pH by more than 0.4 in one step or targets below 7.2; retest 30 minutes after each dose.
Sources
- Taylor Technologies, K-1005 treatment tables (alkalinity and acid demand): PDF
- Hayward, How much salt to add: hayward.com
- Our pH model, volume formulas and reference checks: methodology