How many bags of shock for a 20,000 gallon pool?
Shocking a 20,000 gallon pool from 1 ppm to 10 ppm takes about 2.06 lb (32.9 oz) of 73% cal-hypo, so buy 3 bags of 1 lb.
How to work it out
- In short: raise chlorine from where it is to the shock level, then work out how much of your product holds that much chlorine.
- Shock level: 10 ppm
- 10 − 1 = 9 ppm × 75,708 L = 681 g of chlorine
- 681 g ÷ 73% = 2.06 lb (32.9 oz) = 2.06 bags → 3 bags
At 20,000 gallons the bag count roughly doubles, and so does the case for checking the strength on the bag: three bags of 73% do what about three and a third bags of 65% do. For a pool this size the price per pound of chlorine is worth comparing between bags and liquid.
Adjust for your project
More options: Bag size
- Bags
- 3 bags · 1 lb bag
- Shock level
- 10 ppm
- Then
- Retest after an hour of circulation. Before anyone swims, let it fall to the level your product’s label allows — many say 4 ppm or less.
- Calcium hardness this adds
- 6.4 ppm
1 lb bags of 73% cal-hypo for 20,000 gallons, by shock level (from 1 ppm)
| Shock level | Add this much shock | Bags | Calcium hardness this adds |
|---|---|---|---|
| 8 ppm | 1.6 lb (25.6 oz) | 2 bags | 4.9 ppm |
| 10 ppm | 2.06 lb (32.9 oz) | 3 bags | 6.4 ppm |
| 12 ppm | 2.52 lb (40.2 oz) | 3 bags | 7.8 ppm |
| 16 ppm | 3.43 lb (54.9 oz) | 4 bags | 10.6 ppm |
| 20 ppm | 4.34 lb (69.5 oz) | 5 bags | 13.4 ppm |
Bags for 20,000 gallons
A 1 lb bag of 73% cal-hypo adds about 4.4 ppm to 20,000 gallons. From 1 ppm to 10 ppm you need about 2.1 bags — call it three, and keep the leftover sealed and dry. With 65% bags it is about 2.3 bags; a 2 lb bag is just a double dose.
- Green pool at 40 ppm CYA: the SLAM level is 16 ppm, about 4 bags of 73% for the first dose, then more every day to hold it. Over a week that can be a lot of calcium — about 0.7 ppm of hardness for each ppm of chlorine — which is why liquid chlorine is usually used for SLAM.
- Opening the pool: if CYA is near zero after a winter drain, one dose of dichlor can build stabilizer while it shocks; at 10 ppm in 20,000 gallons it adds about 9 ppm of CYA.
- Dissolving three bags: do one bucket at a time, each in fresh pool water, and pour around the deep end.
- Never store or mix with trichlor tablets — the CDC’s storage poster warns against mixing different types of chlorine.
Liquid comparison: how much liquid chlorine to shock a 10,000 gallon pool doubled is about 1.44 gallons of 12.5% for this pool. Smaller pool: how much shock for a 10,000 gallon pool.
Different size? Use the full Pool Shock Calculator.
What you’ll need
- Cal-hypo shock (calcium hypochlorite)Budget price
Granular shock in 1 lb bags, typically 65–73% available chlorine. Adds a little calcium with every dose. Pre-dissolve in a bucket of water if the label says to.
See options on Amazon (opens in a new tab) - Liquid pool chlorine (sodium hypochlorite)Budget price
Adds chlorine without stabilizer or calcium. Strength is printed on the label — commonly 10% or 12.5% — and it weakens with age and heat, so buy it fresh.
See options on Amazon (opens in a new tab) - 5-gallon bucket (one per chemical)Budget price
For pre-dissolving granular chemicals in pool water. Label each one so chlorine and acid never share a bucket.
See options on Amazon (opens in a new tab)
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Frequently asked questions
How many bags of 65% shock for a 20,000 gallon pool?
About 2.31 lb (37 oz), which is 3 bags of 1 lb, to go from 1 to 10 ppm.
How much shock for a green 20,000 gallon pool with 40 ppm CYA?
A first dose of about 3.43 lb (54.9 oz) of 73% cal-hypo (4 bags) to reach 16 ppm, then daily top-ups to hold it.
How many bags of shock for a 15,000 gallon pool?
About 1.54 lb (24.7 oz) of 73% cal-hypo, so 2 bags of 1 lb, to reach 10 ppm.
Other common sizes
- How much shock for a 10,000 gallon pool — 1.16 lb (18.5 oz)
- How much liquid chlorine to shock a 10,000 gallon pool — 92.2 fl oz (2.9 qt)
- How much liquid chlorine to SLAM a pool — 1.92 gal (7.7 qt)
Sources
- OxyChem — Soda Bleach Solutions Handbook (re-hosted copy) (opens in a new tab) — Trade percent = grams per litre of available chlorine ÷ 10; weight % NaOCl ≈ 1.05 × weight % available chlorine; strength and specific gravity table.
- HASA — Hasachlor sodium hypochlorite SDS (2026) (opens in a new tab) — Pool liquid chlorine: sodium hypochlorite 12.5% by weight, about 1.2 g/ml.
- Hill Brothers — Sodium hypochlorite 12.5% specifications (opens in a new tab) — 12.5% by weight = 142 g/L available chlorine (14.2 trade percent), 11.9% available chlorine by weight.
- Pool Essentials chlorinating liquid (retail listing) (opens in a new tab) — A 10% sodium hypochlorite pool chlorine.
- US EPA label 57787-18 — calcium hypochlorite 65% (opens in a new tab) — 65% available chlorine; 2 oz per 10,000 gallons raises chlorine about 1 ppm; superchlorinate with 10 oz per 10,000 gallons.
- US EPA label 1258-161 — HTH calcium hypochlorite 73% (opens in a new tab) — Calcium hypochlorite 73% (70% minimum available chlorine); a 1 lb bag shock-treats 15,000 gallons.
- US EPA reg. 42177-80 — sodium dichlor label (ProPools copy) (opens in a new tab) — Dihydrate, 56% available chlorine; 2–3 oz per 10,000 gallons daily; 1 lb per 10,000 gallons to superchlorinate.
- ICC Building Safety Journal (PHTA Recreational Water Quality Committee) — Trichlor: the dependable pool performer (opens in a new tab) — Trichlor is 90% available chlorine and adds 0.6 ppm cyanuric acid for each ppm of chlorine.
- Trouble Free Pool (hobbyist forum wiki) — CYA/chlorine relationship (opens in a new tab) — A widely used hobbyist convention, not an official standard: minimum FC 7.5% of CYA, SLAM (shock) level 40% of CYA, salt-water pools 5%.
- CDC — Fecal incident response recommendations for aquatic staff (opens in a new tab) — Formed stool: 2 ppm for 25–30 min at pH ≤ 7.5. Diarrhea: CT 15,300 (20 ppm for 12.75 h); with CYA lowered to 15 ppm or less, 20 ppm for 28 h, 30 ppm for 18 h or 40 ppm for 8.5 h.
- CDC — Model Aquatic Health Code, 5th edition (2024), code (opens in a new tab) — Free chlorine minimum 1.0 ppm (2.0 with CYA, 3.0 in spas); pH 7.0–7.8; CYA at or below 90 ppm; CYA:FC above 45:1 is an imminent hazard; TA 60–180 ppm; turnover tables.
- CDC Healthy Swimming — Home pool and hot tub water treatment and testing (opens in a new tab) — pH 7.0–7.8; free chlorine at least 1 ppm in pools (2 ppm with cyanuric acid) and 3 ppm in hot tubs; no CYA in hot tubs; test twice a day.
- US EPA label 82289-2 — sodium hypochlorite pool chlorine (opens in a new tab) — Superchlorinate to 5–10 ppm; no pool re-entry above 4 ppm chlorine.
- US EPA — Use of the term “shock” on pool and spa products (opens in a new tab) — “Shock” is a pesticide claim; shock products must be registered and used as labelled.
- CDC — Pool chemical injuries press release (2014) (opens in a new tab) — Never mix pool chemicals, especially chlorine products with acid; add chemical to water, never water to chemical.
- CDC — Pool chemical safety: storage poster (opens in a new tab) — Separate acid and chlorine; don’t mix different types of chlorine; keep chemicals cool, dry and in original containers.
- US EPA — Introduction to pesticide labels (opens in a new tab) — Pool sanitizers are registered pesticides; the label is the law.