Hot Tub Shock: What It Is, When to Use It and How to Do It

Updated September 2026. I’ve rewritten this post from scratch. The original muddled up the types of shock and when to use them, so this version explains chlorine and non-chlorine shock properly, how to do it step by step, how long to wait, and the common mistakes.

“Shock your hot tub” sounds dramatic, but it’s one of the simplest and most useful jobs in hot tub care. A shock is a big dose of oxidiser that clears out the gunk your everyday sanitiser can’t keep up with. It gets rid of the sweat, body oils, lotions and used-up chlorine that make water smell, go flat and foam.

Here’s what shock actually does, which type to use, and exactly how to do it.

What is hot tub shock?

Your sanitiser (chlorine or bromine) sits in the water at a low, steady level and kills bacteria. But every soak adds organic waste: sweat, skin cells, deodorant, make-up and so on. As chlorine reacts with that waste, some of it turns into combined chlorine (chloramines). Chloramines are poor sanitisers, and they’re what cause that swimming-pool smell and stinging eyes.

Shock is a concentrated dose of oxidiser that:

• Burns off organic waste the sanitiser hasn’t dealt with
• Breaks down chloramines (or bromamines), freeing up your sanitiser to do its job
• Reactivates bromine. In a bromine tub, used bromine turns back into bromide, and shock converts it back into active bromine.
• Chlorine shock only: gives a burst of sanitiser to knock back any bacteria that have got a foothold

Think of it as a weekly reset. Your sanitiser does the day-to-day work, and shock clears the backlog.

When should you shock a hot tub?

• Once a week under normal use. This is Sundance’s recommendation, and most other manufacturers say the same.
• After heavy use, such as a party, lots of guests or a long session.
• When the water smells of “chlorine” or your eyes sting. That’s chloramines.
• When the water looks dull, hazy or starts foaming.
• When combined chlorine is over about 0.5–1 ppm (total chlorine minus free chlorine). The UK HSE’s spa guidance (HSG282) says it should ideally be zero and not above 1 mg/l.
• After a fresh fill, and after the tub has been sitting unused or unheated for a while.

Types of hot tub shock

When I first wrote this, I described the two types as “chlorine” and “bromine”. That was wrong. The two families are chlorine shock and non-chlorine shock, and you can use either in a chlorine or bromine tub.

	Chlorine shock	Non-chlorine shock (MPS)
What it is	Usually dichlor granules. Liquid chlorine and cal-hypo also work.	Potassium monopersulfate, sold as “oxidiser” or “non-chlorine shock”
Oxidises waste	Yes	Yes
Sanitises (kills bacteria)	Yes	No, it relies on your normal sanitiser
Back in the tub	When free chlorine is back to 3–5 ppm (often a few hours, or overnight)	Usually 15–30 minutes
Downsides	Dichlor adds cyanuric acid, and the smell while it works	Doesn’t sanitise, and it upsets total chlorine tests
Best for	After heavy use, cloudy or smelly water, a fresh fill, or when it’s been sitting unused	Routine weekly shock and quick top-ups after a soak, and reactivating bromine

A sensible routine for most people is MPS weekly, plus a chlorine shock after a big session or if the water’s looking tired. If you’re on dichlor, keep an eye on cyanuric acid, because it builds up every time you use it. I’ve explained why that matters in my guide to hot tub chemical levels and in cyanuric acid in a hot tub.

Using biguanide (Baqua, SoftSwim and similar)? These systems use their own hydrogen peroxide-based oxidiser. Never add chlorine, bromine or MPS shock to a biguanide tub. Use the system’s own products only.

How to shock a hot tub, step by step

1. Test first. Check free and total chlorine (or bromine), pH and alkalinity.
2. Balance the pH to 7.2–7.6. Shock, and chlorine shock in particular, works much better in that range. If alkalinity is way out, fix that first. My chemistry guide covers the order.
3. Clean or rinse the filter if it’s due. There’s no point oxidising the water and then pushing it back through a filthy cartridge. See how to clean a hot tub filter cartridge.
4. Work out the dose from the product label and your tub’s volume. Products vary a lot in strength, so I’ve dropped the one-size-fits-all dose some old guides (including this one) used to give.
5. Take the lid off and turn the jets on, with the air controls open. Doing it in the evening helps, because UV burns chlorine off quickly.
6. Add the shock slowly across the surface, not in one heap. Sundance says to disperse it over the whole water surface so concentrated chemical doesn’t sit on the shell. If the label says to pre-dissolve it, do that in a bucket of tub water, and always add chemical to water, not water to chemical.
7. Leave the lid off and jets running for at least 20 minutes so the gases can escape. If you shut the lid straight away, the fumes are trapped against the underside of your cover and the headrests.
8. Retest before anyone gets in.

How long after shocking can you use a hot tub?

• Non-chlorine (MPS): usually 15–30 minutes, as long as your normal sanitiser is in range. Check the label.
• Chlorine shock: wait until free chlorine has dropped back into the normal range (3–5 ppm) before anyone gets in. That can take a few hours or overnight, depending on the dose and how dirty the water was. The old version of this post said “under 5 ppm, and at least 20 minutes”. The 5 ppm part is right, but most of the time it takes a lot longer than 20 minutes.

A testing gotcha: MPS interferes with the DPD total chlorine test. It shows up as combined chlorine that isn’t really there. AQUA Magazine suggests testing before you add MPS, or waiting around 12 hours before testing total chlorine again. Very high chlorine can also bleach the pink out of a DPD test, so a reading of zero straight after a big chlorine shock might actually mean “off the scale”.

Common shock mistakes

• Thinking a strong chlorine smell means too much chlorine. It’s usually the opposite. It’s chloramines, and the fix is to shock.
• Shocking with the lid on. The gases attack the cover’s underside, headrests and pillows.
• Shocking at high pH. Chlorine shock is far less effective above about 7.8.
• Mixing products. Never mix different chlorine products (dichlor and cal-hypo can react violently), and never mix chlorine with acid. Store them apart.
• Using pool shock at pool doses. A hot tub holds a tiny fraction of a pool’s water, so a pool-sized bag can overshoot badly. Use spa products and spa doses.
• Using shock to cover up tired water. If you’re shocking every day and the water still won’t behave, it’s time to drain and refill.
• Adding it on a windy day, or with people in the tub.

Safety

Wear gloves and eye protection, measure with a dry scoop, and keep shock sealed, cool and dry, away from other chemicals, kids and pets. The CDC’s pool chemical safety guidance says to store each chemical separately and keep incompatible ones apart.

Shock won’t fix it? Troubleshooting

Still seeing…	Check
Cloudy water	Filter, pH, alkalinity and calcium. See cloudy water in a hot tub.
Foam	Detergent on swimwear, body products, old water. See foam in a hot tub.
Chlorine disappears overnight	Heavy organic load, a dirty filter, biofilm in the pipes, or the lid left off in sunlight
Chlorine reads fine but water still dull	Cyanuric acid build-up from dichlor, so dilute or change the water

Need parts?

If shock has shown up a tired filter or a pump that’s not moving enough water, you’ll find replacement cartridges and spa parts at parts4tubs.co.uk in the UK and parts4tubs.com in the USA. For the full weekly routine, see my beginner’s guide to hot tub maintenance, and for chlorine versus bromine, should I use bromine or chlorine?

FAQs

How often should I shock my hot tub?

About once a week with normal use, and again after heavy use, a party, or whenever the water smells or looks dull.

Can I use pool shock in a hot tub?

The chemical may be the same, but pool doses are far too big for a hot tub. Cal-hypo pool shock also adds a lot of calcium. Use a spa product dosed for your tub’s volume.

Should I shock with the cover on or off?

Off, with the jets running for at least 20 minutes, so the gases can escape rather than attack your cover.

Can I use non-chlorine shock with bromine?

Yes. MPS is commonly used in bromine tubs, and it reactivates bromide back into bromine.

How long after shocking can I get in?

Usually 15–30 minutes after non-chlorine shock. After chlorine shock, wait until free chlorine is back to 3–5 ppm.

Why does my hot tub smell of chlorine?

It’s usually chloramines (used-up chlorine), not too much chlorine. Shock it, and the smell should go.

Got a question about shocking your tub? Drop it in the comments below. I read them all.

Cheers,
Andi

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Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi