Pool shock calculator: how much shock to add
Pick why you’re shocking and what you’re shocking with. The calculator sets the chlorine level, then works out the bags or gallons to get there.
Enter your measurements
More options: Bag size
- Bags
- 2 bags · 1 lb bag
- Shock level
- 10 ppm
- Then
- Let it fall back to your normal level before swimming — many labels say 4 ppm or less.
- Calcium hardness this adds
- 6.4 ppm
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 × 37,854 L = 341 g of chlorine
341 g ÷ 65% = 1.16 lb (18.5 oz) = 1.16 bags → 2 bags
Quick answers for common sizes
| Example | Answer |
|---|---|
| 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 many bags of shock for a 20,000 gallon pool | 3 bags of 73% cal-hypo (1 lb bags) |
| How much liquid chlorine to SLAM a pool | 1.92 gal (7.7 qt) |
Shock for this pool by product (same shock level)
| Product | Add this much shock | Stabilizer (CYA) this adds | Calcium hardness this adds |
|---|---|---|---|
| liquid chlorine | 92.2 fl oz (2.9 qt) | 0 ppm | 0 ppm |
| bleach | 1.5 gal (6 qt) | 0 ppm | 0 ppm |
| cal-hypo | 1.16 lb (18.5 oz) | 0 ppm | 6.4 ppm |
| dichlor | 1.34 lb (21.5 oz) | 8.2 ppm | 0 ppm |
“Shocking” just means raising free chlorine well above its everyday level for a while, to oxidize what has built up or to kill algae and germs. How high depends on why you are doing it — and with stabilizer in the water, on your CYA.
How high to shock
| Why | Free chlorine | Source |
|---|---|---|
| Routine shock or superchlorination | 5–10 ppm | Pool chlorine labels (e.g. EPA reg. 82289-2) |
| Algae, a green or cloudy pool (SLAM) | 40% of CYA — 16 ppm at 40 CYA | Trouble Free Pool (hobbyist method) |
| Formed-stool accident | 2 ppm for 25–30 min, pH ≤ 7.5 | CDC fecal incident guidance |
| Diarrheal accident, no CYA | 20 ppm for 12.75 hours, pH ≤ 7.5 | CDC (CT 15,300) |
| Diarrheal accident, CYA 1–15 ppm | 20 ppm for 28 h, 30 ppm for 18 h or 40 ppm for 8.5 h | CDC / Model Aquatic Health Code |
How the shock dose is worked out
rise = shock level − current free chlorine grams of chlorine = rise × pool litres ÷ 1,000 liquid (L) = grams ÷ (label % × 10) granular (g) = grams ÷ (available chlorine % ÷ 100) SLAM level = CYA × 0.40
One 1 lb bag of 73% cal-hypo holds about 331 g of chlorine — about 8.7 ppm in 10,000 gallons, which is why a 73% label says a bag shock-treats 15,000 gallons (about 5.8 ppm there). A 65% bag gives about 7.8 ppm in 10,000 gallons.
Worked examples
- How much shock for a 10,000 gallon pool — bags of cal-hypo for a routine shock.
- How much liquid chlorine to shock a 10,000 gallon pool — the same shock from a jug.
- How many bags of shock for a 20,000 gallon pool — and when a case of liquid is cheaper.
- How much liquid chlorine to SLAM a pool — clearing algae at 40% of CYA.
Which shock to use
- Cal-hypo: concentrated and easy to store, but each dose adds calcium (about 0.7 ppm per ppm of chlorine). Many labels say to pre-dissolve it.
- Liquid chlorine or bleach: adds nothing that builds up. Bulky — a 20 ppm shock in a big pool is several gallons.
- Dichlor: dissolves fast but adds about 0.9 ppm of stabilizer per ppm of chlorine — a 10 ppm shock adds about 9 ppm of CYA.
- Non-chlorine shock (MPS): oxidizes swimmer waste but does not kill algae or raise chlorine.
- Never trichlor. Tablets are for slow feeding, not shocking — and trichlor must never be mixed with cal-hypo.
Shocking step by step
- Test pH first; chlorine works better near 7.2–7.5. Adjust pH and let it circulate before you shock (never add acid and chlorine together).
- Shock at dusk so the sun doesn’t burn the chlorine off.
- Run the pump, brush walls and floor, and add the shock in front of a return (pre-dissolve granular if the label says so).
- Retest after an hour. Keep everyone out until chlorine is back to the level your product’s label allows for swimming.
For the whole process, including green-to-clean, see how to shock a pool.
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) - FAS-DPD drop test kit (chlorine, pH, TA, CH, CYA)Mid-range price
A drop-count kit reads free chlorine to 0.5 ppm (or 0.2 ppm), plus pH, alkalinity, calcium and stabilizer. Every dose on this site starts from a test, so this is the one thing worth buying first.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How many bags of shock do I need?
For a routine shock to about 10 ppm, roughly one 1 lb bag of 65–73% cal-hypo per 10,000 gallons gets you most of the way, and a second bag finishes it if chlorine started near zero. A green pool needs far more — see how many bags of shock for a 20,000 gallon pool.
What is SLAM in pool care?
SLAM stands for Shock Level And Maintain, a Trouble Free Pool method: raise free chlorine to 40% of your CYA and hold it there until the water is clear and chlorine stops dropping overnight. It is a hobbyist method, not a label instruction. See how much liquid chlorine to SLAM a pool.
When can I swim after shocking?
When free chlorine is back down to the level your chlorine product’s label allows for swimming — many say 4 ppm or less — and pH is in range. That often means the next day.
Should I shock with liquid chlorine or granular?
Either raises chlorine. Liquid adds nothing that builds up; cal-hypo adds calcium and dichlor adds stabilizer. For repeated shocks (a green pool), liquid is usually the better choice; for storage and convenience, granular.
How much shock for a green pool?
Much more than a routine shock: at the SLAM level of 40% of your CYA, held for days. At 40 ppm CYA that is 16 ppm in a pool that may be consuming most of it every day. Vacuum out debris first, brush daily and test often.
Related guides
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.