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Reviewed · Prices checked October 2026 · How we calculate

Pool Shock Calculator: How Much Shock for Your Gallons

Enter your gallons and your test readings. You get the shock level your water needs, the amount of cal-hypo or liquid chlorine to reach it, and the bags or jugs to buy.

Pool and test readings

gal

Don't know your gallons? Use the pool volume calculator.

ppm

From your test kit.

ppm

Total minus free. 0 if unknown.

ppm

0 for an unstabilized pool.

Shock product

Bags labeled "shock" are usually cal-hypo; check the label for 65% or 73%. Liquid chlorine works as shock too.

How much to add

Updates as you type
3.5lb

of cal-hypo 65% to shock 18,000 gal from 1 ppm to 16 ppm free chlorine

Add 3.5 lb: 4 bags of cal-hypo 65% (1 lb each).

  • 4 bags of cal-hypo 65% (1 lb each)$14–$40
    3.5 lb needed; about 8.5 oz left over for next time.

    Typical price (estimate, checked Oct 2026): $3.50–$10.00 per 1 lb bag of cal-hypo shock

    4 bags × $3.50–$10.00 = $14–$40Estimate
Project total (estimate where no price is entered)$14–$40
Checked Oct 2026
  • Raises calcium hardness and pH a little. Add it the way the label says (some say broadcast, some say pre-dissolve); undissolved granules can bleach vinyl.
  • This dose adds about 10.6 ppm of calcium hardness.
  • Swim again once free chlorine is back in your normal range and within the product label's limit (some labels say 4 ppm or less; the CDC maximum is 10 ppm).

Safety: Estimate only. Add chemicals to water, never water to chemicals. Never mix products. Add one product at a time with the pump running, and retest before adding more.

Shock level 16 ppm. Targets: CDC Model Aquatic Health Code.

Show the math
Breakpoint: 10 × 0.5 CC = 5 ppmStabilizer: 40% of 40 CYA = 16 ppmShock level: the highest of those and 10 ppm = 16 ppmRaise: 16 − 1 = 15 ppm18,000 gal ÷ 10,000 × 15 ppm × 2.1 = 3.5 lb

How it works

Finding the shock level

shock level = the highest of: 10 × CC, 40% of CYA, 10 ppm (at most 40)
amount = gallons × (shock level − FC), at the product's strength
  • Breakpoint. Combined chlorine (chloramines) causes the "chlorine smell" and red eyes. Raising free chlorine to 10 times the CC breaks them down.
  • Stabilizer. CYA protects chlorine from sunlight but also slows it down. With stabilizer, shock needs free chlorine at about 40% of the CYA reading to kill algae (the level the Trouble Free Pool method uses).
  • Limits. We never go below 10 ppm or above 40 ppm. Above 100 ppm CYA, shocking barely works: drain some water first.
  • Big doses. If the dose is more than 15 ppm, add it in two halves and retest between them.

Worked example

18,000 gallon pool: FC 1, CC 0.5, CYA 40 ppm, cal-hypo 65%

  1. Breakpoint 10 × 0.5 = 5 ppm; stabilizer 40% × 40 = 16 ppm; minimum 10 ppm
  2. Shock level: 16 ppm, so raise by 16 − 1 = 15 ppm
  3. Cal-hypo 65%: 2.1 oz per 10,000 gal per ppm × 18,000 ÷ 10,000 × 15 = 3.5 lb
  4. Buy 4 bags of cal-hypo 65% (1 lb each): $14–$40 at typical prices

How much shock for common pool sizes

Cal-hypo 65% and 12.5% liquid chlorine for a 10 ppm shock
Pool sizeCal-hypo 65%1 lb bagsLiquid 12.5%
5,000 gal10.3 oz151.2 fl oz
10,000 gal1.3 lb2102.4 fl oz
15,000 gal1.9 lb21.2 gal (154 fl oz)
20,000 gal2.6 lb31.6 gal (205 fl oz)
25,000 gal3.2 lb42 gal (256 fl oz)
30,000 gal3.9 lb42.4 gal (307 fl oz)
40,000 gal5.1 lb63.2 gal (410 fl oz)

For a 10 ppm rise. A stabilized pool needs a higher shock level:

CYA0 ppm30 ppm40 ppm50 ppm60 ppm80 ppm
Shock level10 ppm12 ppm16 ppm20 ppm24 ppm32 ppm

Mistakes to avoid

  • One bag per 10,000 gallons, whatever the water. That raises free chlorine by about 8 ppm. Fine with no stabilizer, too little at 50 ppm CYA.
  • Shocking once for green water. Algae needs the shock level held: retest every few hours and top up until the water is clear, CC is 0.5 ppm or less, and the pool loses no more than 1 ppm overnight.
  • Mixing shock with other chemicals. Never put cal-hypo in a skimmer or feeder that has trichlor tablets: it can catch fire or explode.
  • Shocking in the sun. Sunlight burns off unstabilized chlorine within hours. Shock at dusk with the pump running.
  • Swimming too soon. Wait until free chlorine is back in your normal range and within the product label's limit; some labels say 4 ppm or less.

Questions people ask

How much shock for a 10,000 gallon pool?

A 10 ppm shock takes 1.3 lb of 65% cal-hypo (2 one-pound bags) or 102.4 fl oz of 12.5% liquid chlorine. With stabilizer, the shock level is 40% of your CYA, so at 50 ppm CYA you need twice that.

How long after shocking can I swim?

Once free chlorine is back in your normal range and within the product label's limit: some labels say 4 ppm or less, and the CDC maximum is 10 ppm. That usually takes overnight to a day or two, depending on the dose, sun and stabilizer. Test before you swim.

Can I add salt and shock at the same time?

Salt and chlorine don't react, but add one, let it circulate for a few hours, then add the other, so you can test each change. Never add shock in the same spot or bucket as another chemical.

Should I shock a saltwater pool?

Yes, when CC is above 0.5 ppm or after algae, heavy use or a storm. Use liquid chlorine or cal-hypo, or set the generator to boost mode for smaller jumps.

What is the difference between shock and chlorine?

Most shock is chlorine: a bigger dose of it. Cal-hypo is sold as "shock" and liquid chlorine works the same way. Non-chlorine shock (potassium monopersulfate) breaks down chloramines but does not kill algae.