You’ve had a solid season with the inflatable tub, the temperatures are dropping, and now you’re staring at a slightly-used pool of warm water wondering exactly what you’re supposed to do with it before you pack the whole thing away. The drain is easy – but what goes in the water first, how much, and does it even matter?
It matters more than most people think, and the answer is specific: add a closing shock dose that pushes your sanitizer level to 10 to 20 ppm – whether you’re using bromine or chlorine – then run the jets for 30 minutes before draining. That single step kills off the bacteria and biofilm that would otherwise sit in the liner and hoses all winter. This article walks through the exact amounts by tub size, which sanitizer wins for a closing dose, and the full shutdown sequence so nothing gets missed.
Why a Final Sanitizer Dose Actually Matters
An inflatable hot tub that gets drained without a closing shock is not a clean hot tub. Even clear, well-maintained water carries a small biofilm load on the shell interior, around the jets, and in the short hoses connecting the pump. When you store the deflated tub for four to six months, that residue has nowhere to go – it just sits there, potentially staining the liner and creating an odor problem you will smell the moment you refill in spring. A properly dosed closing treatment gives the sanitizer time to sweep through the entire water volume and contact every surface before it all goes down the drain.
This is also why the closing dose is higher than your normal weekly maintenance target. You are not trying to maintain comfortable water for soaking – you are doing a one-time sanitizing flush. Think of it less like topping up your sanitizer and more like running the water through a car wash before you park it for the winter.
Bromine vs Chlorine for a Winterizing Close
Both sanitizers will do the job. The practical difference comes down to what you already have on hand and how the chemistry behaves during a closing dose.
- Granular chlorine shock: Non-stabilized chlorine – either calcium hypochlorite (cal-hypo) or sodium dichlor – acts fast, typically reaching full potency within 15 to 30 minutes. It also dissipates relatively quickly, so by the time you drain, most of the active chlorine has already done its work and broken down. That is a genuine advantage at closing: you are not sending a tank of heavily chlorinated water through your lawn in ways that could affect grass or soil.
- Bromine granules: If bromine is already your maintenance sanitizer, sticking with it for the closing dose is perfectly sensible. Bromine is more stable at higher water temperatures and slightly slower to off-gas, which means it holds that elevated closing concentration a bit longer. The trade-off is that it does not dissipate as fast as chlorine, so let it run the full 30 minutes before draining.
- Bromine tablets: Skip these for closing. Slow-dissolve tablets are designed for ongoing maintenance, not a fast closing flush. You need granules or shock to hit the target concentration quickly.
For most people closing an inflatable for the first time, granular chlorine shock is the simpler choice – it’s widely available, dissolves fast, and leaves no residue in the liner. If you want more detail on how these two sanitizers compare day-to-day, understanding why sanitizer readings drop unexpectedly is a useful companion read.
Exact Closing Doses by Tub Size
The target is a free chlorine level of 10 ppm, or a bromine level of 10 to 20 ppm, measured after the chemical has circulated for at least 15 minutes. Here is how that translates into actual product amounts across the common inflatable hot tub size range.
| Tub Size | Typical Volume | Chlorine Shock (68% dichlor) | Bromine Granules | Target Level |
|---|---|---|---|---|
| 2-person | 180-250 gal | 0.75 – 1 oz (21-28 g) | 1 tsp (approx 5 g) | 10 ppm Cl / 10-20 ppm Br |
| 4-person | 300-350 gal | 1.25 – 1.5 oz (35-42 g) | 1.5 tsp (approx 7-8 g) | 10 ppm Cl / 10-20 ppm Br |
| 6-person | 400-500 gal | 1.75 – 2 oz (50-57 g) | 2 tsp (approx 10 g) | 10 ppm Cl / 10-20 ppm Br |
Why These Differences Matter
Inflatable hot tubs hold a lot less water than hard-shell spas, which means small changes in dose have a bigger effect on concentration. Under-dose a 200-gallon two-person tub by just half an ounce of shock and you might land at 5 ppm instead of 10 – not enough for a meaningful closing flush. Over-dose and you’re draining water with an active chlorine level above 10 ppm into your garden, which can burn grass or harm plants if it lands in the same spot repeatedly. The ranges in the table are sized to hit the target without excess.
Worked example: say your inflatable holds 250 gallons and you’re using granular sodium dichlor at 68% strength. You need to raise chlorine by about 8 to 10 ppm from a typical maintenance level of 2 ppm. Using the standard dose of roughly 1 oz per 250 gallons per 10 ppm rise, you’d add 1 oz dissolved in a bucket of warm water. Test after 15 minutes of circulation – you should read 10 to 12 ppm. If you’re a little short, add another quarter ounce and retest. This is also the method described in a detailed hot tub shock dosage guide that walks through the same math for different tub sizes.
Step-by-Step: The Full Closing Sequence
Getting the chemical dose right is only part of the job. The order matters too, because adding sanitizer at the wrong stage means it either dilutes before doing anything useful or sits in a tub with pH swings that undercut its effectiveness. For a full walkthrough of the mechanical side – pump drainage, valve positions, and deflation – this complete winterization guide covers the physical steps in detail alongside the chemistry.
- Test and balance the water first. Check pH (target 7.4 to 7.6) and total alkalinity (target 80 to 120 ppm). Sanitizer works best at the right pH – at pH 8.0 or above, chlorine loses more than half its effectiveness. Balance before you shock.
- Run the jets and heater to at least 80°F. Warmer water dissolves granules faster and lets sanitizer circulate into the pump housing and hose connections – the spots that don’t get a good rinse just from draining.
- Pre-dissolve the chemical in a bucket. Add the measured dose to a bucket of warm water, stir until fully dissolved, then pour slowly around the perimeter with the jets running. Never drop undissolved granules directly onto the liner – it can bleach or weaken the material.
- Circulate for at least 30 minutes. Keep the jets on for the full 30 minutes to push treated water through the pump and every jet outlet.
- Test the water again. Confirm you’re at 10 to 20 ppm before draining. AquaDoc makes a 5-in-1 test strip designed for inflatable spa volumes that reads bromine and chlorine side by side – a detail that matters when you’re switching sanitizer types at closing.
- Drain completely. Follow your manufacturer’s drain process and tilt the tub to get the last inch of standing water out of the floor. That residual puddle is where most of the biological growth starts.
- Wipe down the interior. A clean cloth or microfiber towel along the shell interior removes any film the sanitizer loosened. Let it dry fully before deflating and folding – trapping moisture inside a stored inflatable is how mold gets started.
If you’re on the fence about closing the tub at all versus keeping it running through winter, keeping an inflatable hot tub running in cold weather is a real option for some climates – though it comes with its own chemical challenges, including faster sanitizer consumption and pH swings driven by cold temperatures.
Common Mistakes That Undermine the Closing Dose
- Skipping the pH check: At pH 8.0, chlorine is only about 20% active. At pH 7.4, it’s closer to 60%. Adding a full closing dose without balancing pH first wastes most of your chemical.
- Using stabilized chlorine (trichlor): Trichlor tablets or granules add cyanuric acid (CYA) to the water. For a closing dose you are about to drain, you are just adding unnecessary stabilizer that goes down the drain. Use non-stabilized shock for the closing flush.
- Adding chemical to cold water: Below 70°F, granules dissolve slowly and can settle on the liner floor. Always pre-dissolve in warm water from a separate bucket.
- Draining immediately after dosing: Ten minutes is not enough. You need the full 30-minute circulation to push treated water through the pump internals and every jet port. Cutting this short defeats the purpose of the closing dose entirely.
- Forgetting the filter: Pull the filter cartridge out before the final shock dose, give it a proper soak in a filter cleaner solution, rinse well, and let it dry separately before storage. Storing a damp, uncleaned filter is a reliable way to start next season with a clogged, odor-producing mess. The inflatable hot tub chemicals guide from Inflatable Hot Tubs Reviews also flags the filter as one of the most commonly skipped closing steps.
Frequently Asked Questions
How much chlorine should I add when winterizing an inflatable hot tub?
Add enough granular non-stabilized chlorine shock to reach 10 ppm in the water before draining. For a typical 200 to 250 gallon two-person inflatable, that is roughly 1 oz (28 g) of 68% dichlor or sodium hypochlorite shock, pre-dissolved in warm water. Circulate for 30 minutes, retest, and adjust by a quarter ounce if you are below 10 ppm.
What bromine level do you need before closing a hot tub for winter?
Target 10 to 20 ppm for the closing dose – about double the normal maintenance range of 3 to 5 ppm. From a maintenance level of 3 to 4 ppm, you need to raise bromine by 6 to 16 ppm. Use approximately 1 teaspoon of bromine granules per 100 gallons to achieve that increase, then verify with a test strip or drop test after 15 minutes of circulation.
Can I just drain my inflatable hot tub without adding chemicals first?
Technically yes, but the result is a liner, shell interior, and pump housing that go into storage carrying whatever bacteria and biofilm are already in the water. A closing shock takes 30 minutes and prevents the staining, odors, and microbial load that make spring startup harder. It is the single highest-return step in the entire winterizing process.
Should I use bromine or chlorine to close an inflatable hot tub for winter?
Granular chlorine shock is the more practical choice for most people because it works fast and dissipates before you drain, leaving minimal residue. Bromine is equally effective if it is already your maintenance sanitizer – just dose up to 10 to 20 ppm and circulate for the full 30 minutes. Avoid slow-dissolve tablets of either type for the closing dose; you need granules or shock to hit the target concentration quickly. For more on how inflatable tub chemistry works in general, the White Teamed SPA guide to inflatable hot tub chemicals covers the full range of options.
How much sanitizer does a 400-gallon inflatable hot tub need for winterizing?
For a 400-gallon tub, use roughly 1.75 to 2 oz (50 to 57 g) of granular chlorine shock to reach 10 ppm, or about 2 teaspoons of bromine granules to reach 10 to 20 ppm. Pre-dissolve in a bucket of warm water, add with jets running, and test after 15 minutes. Adjust by small increments – a quarter ounce of shock at a time – if you fall short of the target.
The short answer to the whole question: raise your sanitizer to 10 to 20 ppm using granular shock or bromine granules sized to your tub’s actual volume, run the jets for 30 minutes, test it, then drain. That one step is what separates a hot tub that is ready to refill in spring from one that needs a full deep clean before you can use it again.