Change your hot tub water every 3 to 4 months as a general rule – but that number is really just a starting point. How many people use your tub, how often, and what your water chemistry looks like all push that timeline shorter or longer. The real trigger for a drain and refill is not the calendar; it is a combination of high total dissolved solids (TDS), chemistry that won’t hold, and water that’s just plain tired. Here’s how to actually know when your tub is ready for a fresh start.
Why Hot Tub Water Wears Out
Unlike a pool, a hot tub holds a relatively small volume of water – usually 300 to 500 gallons – that gets used repeatedly at high temperatures. Every soak deposits body oils, lotions, sweat, and other organics directly into that small reservoir. Chlorine or bromine breaks down those contaminants, but the byproducts stay in the water. Over time, the water becomes saturated with dissolved solids that your sanitizer, shock, and balancing chemicals can no longer work around. This is what’s meant by “water exhaustion,” and it’s the real reason you have to drain a hot tub – not just because it’s been a few months.
Total dissolved solids (TDS) is the measurement that captures all of this buildup. Most manufacturers recommend draining when TDS climbs to 1,500 ppm above your fill water’s TDS baseline. You can test TDS with an inexpensive digital meter, and it’s worth adding to your routine every month or so if you use your tub heavily.
The Simple Formula for Timing Your Water Change
There’s a calculation that hot tub technicians use to estimate a reasonable drain schedule. Take your hot tub’s water volume in gallons, divide it by 3, then divide that number by the average number of bathers per day. The result is the approximate number of days between water changes.
For example: a 400-gallon tub used by 2 people daily works out to 400 / 3 / 2 = roughly 67 days, or about every 9 to 10 weeks. A 400-gallon tub used by just one person every few days stretches closer to the standard 3-to-4-month mark. The formula is not perfect, but it gives you a rational starting point rather than just guessing.
Signs Your Water Is Overdue for a Change
Chemistry numbers will start giving you hints before the water looks visibly bad. Watch for these patterns:
- pH and alkalinity won’t stay in range – you add chemicals, they look fine, then 24 hours later they’re off again. Exhausted water resists balancing.
- Persistent foam that comes back shortly after adding a defoamer. Foam is largely caused by organic buildup, and if it keeps returning, the water is saturated.
- A strong chlorine or chemical smell even when free chlorine is at the right level (3 to 5 ppm). That smell is usually chloramines – spent chlorine bonded to contaminants – which is a classic sign of overloaded water.
- Cloudy water that clears briefly then goes hazy again within a day or two, even with correct sanitizer and balanced chemistry.
- TDS over 1,500 ppm above your source water, confirmed with a digital TDS meter.
If you’re seeing two or more of these at the same time, no chemical fix will work long-term. It’s time to drain.
How to Make Each Fill Last as Long as Possible
The biggest enemy of hot tub water is what people bring in with them. Showering before getting in – without soap, just a rinse – removes a significant portion of the oils, lotions, and cosmetics that would otherwise end up in your water. It sounds fussy, but it genuinely extends how long your water stays manageable. Hair products are especially rough on water chemistry, so rinsing your hair before soaking (or keeping hair out of the water) makes a noticeable difference.
Keeping your sanitizer level consistent matters more than any single shock treatment. Chlorine that’s allowed to drop to zero even briefly lets organic contamination get ahead of you, and you’re playing catch-up for days afterward. Aim to keep free chlorine between 3 and 5 ppm and test at least three times a week if you use the tub regularly. AquaDoc makes a granular dichlor specifically sized for hot tub dosing, which is easier to manage in small volumes than the big-pool formulations sold at box stores.
Running your circulation pump on a schedule – at least two full cycles per day – also helps your filter and sanitizer do their job. A filter that’s past due for a chemical soak will pass oils back into the water faster than fresh sanitizer can handle them.
What to Do When It’s Finally Time to Drain
Before you drain, it’s worth doing a line flush. Add a pipe purge product to the water while the jets are running, let it circulate for 30 to 60 minutes, then drain. This clears out the biofilm that builds up inside your plumbing lines – the stuff you never see until it flakes off into your fresh fill. Skipping the line flush and just refilling is one of the most common mistakes owners make, because that biofilm will contaminate your new water within the first week.
- Add a line flush product and run jets for 30 to 60 minutes.
- Drain completely using the drain spigot or a submersible pump.
- Wipe down the shell with a diluted white vinegar solution or a hot tub surface cleaner.
- Rinse the shell and remove any cleaning residue before refilling.
- Chemical-soak your filter cartridge while the tub is empty – or replace it if it’s been more than a year.
- Refill slowly through the filter housing (not the footwell) to prevent air locks in the lines.
- Balance your water chemistry before turning on the heater: total alkalinity first (80 to 120 ppm), then pH (7.4 to 7.6), then calcium hardness (150 to 250 ppm), then sanitizer.
Most hot tubs take 1 to 2 hours to refill from a standard garden hose. Have your startup chemicals ready so you can balance and sanitize the same day.
Frequently Asked Questions
How often should you change hot tub water?
Every 3 to 4 months is the standard recommendation for a hot tub used by 2 to 4 people regularly. If you use it more heavily or host guests often, plan for every 6 to 8 weeks instead.
What happens if you don’t change hot tub water often enough?
Water that’s overdue for a change becomes saturated with dissolved solids, bodily fluids, and chemical byproducts. At that point, no amount of shocking or balancing will make it truly clean – you have to drain it.
Is there a formula to calculate when to change hot tub water?
Yes. Divide the hot tub’s gallon capacity by 3, then divide again by the average number of daily bathers. The result is roughly how many days you can go between full water changes. It’s a useful estimate, not a hard rule.
Can you tell when hot tub water needs changing without testing?
Sometimes. Persistent foam, a strong chemical smell even with proper chlorine levels, or water that looks dull despite treatment are all signs the water is exhausted. A TDS reading above 1,500 ppm over your fill water baseline confirms it for certain.
How long does it take to refill a hot tub after draining?
Most residential hot tubs take 1 to 2 hours to refill with a standard garden hose. Have your startup chemicals ready so you can balance the water the same day and get back to soaking within 24 hours.
The calendar reminder is useful, but the real skill is reading your water. Once you know what exhausted water looks and smells like, you’ll catch it well before it turns into a problem – and your tub will be a lot more enjoyable in between drains. For more on hot tub hygiene and waterborne illness prevention, the CDC has a readable overview worth bookmarking. The Pool and Hot Tub Alliance also publishes consumer guidance on water care that aligns with industry standards.