Hot Tub Shock: When, Which Type, and How Long to Wait

The quick answer:

• Non-chlorine shock (MPS) is the "soak again soon" option. Many labels allow soaking after about 15 minutes; follow yours. In a bromine tub, MPS turns the bromide reserve back into bromine, and a full label dose can read about 27 ppm, far above the 8 ppm maximum, so always test bromine first.
• Chlorine shock sanitizes as well as oxidizes. Wait until free chlorine is back to 5 ppm or less before soaking (many chlorine labels say not to get in above 5 ppm), and never get in above 10 ppm.
• Either way, test before you get in.

The ready-to-soak checker below compares your readings with those limits: soak, wait, or fix first.

What "shock" actually does

Every soak adds sweat, skin oils and cosmetics. Your sanitizer works on germs, but some of the organic load and the combined chlorine (chloramines) left behind need a stronger oxidizing dose to burn off. Shock provides that dose. It:

• clears the "chlorine smell" (combined chlorine);
• reduces dull, tired-looking water;
• in bromine tubs, regenerates bromine from the bromide bank.

Chlorine shock vs non-chlorine shock (MPS)

Use chlorine shock when the water has a problem: cloudiness, slime, a strong smell, after an illness or a heavy party, or when sanitizer has been at zero.

Use MPS for routine weekly oxidation when the water is basically fine, especially if you want to soak again soon, or to recharge a bromine tub.

How much shock?

Chlorine shock: raise free chlorine to the shock level for a short time: 40% of your CYA, never below 10 ppm. At CYA 30 that is 12 ppm; with no CYA it is 10 ppm. Or follow your product's label for a "shock" dose. Per 1 ppm:

To go from 3 to 12 ppm in a 400 gallon tub at CYA 30: 136 mL of 10% liquid chlorine (or 24 g of dichlor, which also adds about 0.9 ppm of CYA for every 1 ppm of chlorine). Nobody soaks until it's back to 5 ppm or less.

If you use the dichlor-then-bleach method, shock with bleach or liquid chlorine once CYA is around 30 ppm, so CYA doesn't climb.

MPS: follow the label dose for your volume. Strengths vary by product. The calculator converts it for your tub.

How long to wait: the details

After MPS:

• Follow the label. Many say about 15 minutes with the jets running.
• Then test your sanitizer. MPS doesn't add chlorine, so you still need 3–5 ppm FC. In a bromine tub MPS does raise bromine: soak only at 4–6 ppm. For routine use, Spa Calc sizes a smaller dose from your reading that brings bromine back to about 5 ppm.

After chlorine shock:

• Leave the cover open and run the jets. Heat, aeration and light help chlorine dissipate.
• Test before getting in. Soak when free chlorine is 3 to 5 ppm and at least 7.5% of your CYA (bromine 4 to 6) and pH is 7.2 to 7.8, at least 15 minutes after adding anything (20 after pH decreaser); after chlorine, bromine or a shock, test again first. Above 5 ppm chlorine or 6 ppm bromine, wait and retest in an hour or two; never soak above 10 ppm chlorine or 8 ppm bromine, or with pH below 7.0 or above 7.8. Below 3 ppm chlorine (4 bromine), add sanitizer first; at pH 7.0 to 7.2, raise pH first; with CYA above 40 ppm, drain and refill. Hot tub limits based on CDC guidance and EPA product labels.
• How long that takes depends on the dose, temperature, sunlight and CYA. Often it's a few hours; sometimes it's overnight. Testing beats guessing.

After pH or alkalinity adjusters: wait about 15–30 minutes with the jets running, then retest.

After a fresh fill: soak only once sanitizer, pH and alkalinity are all in range.

How often to shock

• Weekly with regular use.
• After heavy use: a party, several people, long soaks.
• When you notice a chlorine smell, dull water, cloudiness or foam.
• Bromine tubs: weekly shock keeps the bromide bank recharged.

If you're shocking more than twice a week and the water still looks tired, it's probably time to drain. See how often to drain a hot tub.

How to shock (step by step)

1. Test sanitizer and pH. Adjust pH to about 7.4–7.6 first; shock works better in range.
2. Remove the cover and turn on the jets. Open the air controls if your tub has them.
3. Add the shock per the calculator or label. Pre-dissolve granules if the label says to, and never add them to a closed skimmer basket.
4. Leave the cover open for at least 20 minutes, longer for chlorine shock. Trapped gas can damage the cover's underside and nearby fittings.
5. Test before soaking.

Test reading weirdness after MPS

MPS reacts with the total chlorine part of a standard DPD test, so after MPS, your kit may show high combined chlorine that isn't really there. Some kits include an MPS neutralizer reagent. Otherwise, test sanitizer before adding MPS, or wait a day for an accurate combined chlorine reading.

Storing shock safely

Keep shock products sealed, dry, cool and out of reach of children and pets. Store chlorine-based and MPS shocks apart from each other and from pH products. Moisture or contamination can cause dangerous reactions. Use a clean, dry scoop for each product.

Common mistakes

• Treating MPS as a sanitizer. It isn't one.
• Getting in right after chlorine shock without testing.
• Shocking with the cover closed.
• Using dichlor for every shock all fill long, which makes CYA climb.
• Mixing shock with other chemicals in a bucket or scoop. Add one at a time.

Shock with confidence in Spa Calc

Log a shock in Spa Calc and it:

• starts a wait-before-soaking timer based on the product;
• tells you what to test before getting in;
• sizes every chlorine or MPS dose for your tub's volume;
• reminds you to shock weekly.

One hot tub? Every feature, free. No account, no ads, and iCloud backup is included.

Safety

• Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
• Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
• Add chemicals to water, never water to chemicals.
• Store chemicals sealed, cool and dry, apart from each other.
• Read and follow the product label. It always comes first.