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Drain and refill a hot tub every three to four months under normal household use. The industry sizing formula is more precise: divide the tub volume in gallons by three times the average number of bathers per day, and the result is the number of days between water changes. A 400-gallon tub used by two people daily gives 400 divided by 6, or about 67 days. The same tub used by two people twice a week gives roughly 230 days. Override the calendar and drain early if total dissolved solids exceed the fill water by 1,500 ppm, or if foam and cloudiness persist despite correct chemistry.
How does the bather load formula work?
Water quality in a spa is governed by concentration, not by time. A hot tub holds roughly one two-hundredth of the water of a backyard pool while carrying a similar number of people, so every soak adds a proportionally enormous load of sweat, skin cells, oils and cosmetics. The standard spa-industry formula captures that directly.
Days between water changes = gallons ÷ (3 × average bathers per day)
| Tub volume | 2 people, twice a week | 2 people daily | 4 people daily |
|---|---|---|---|
| 250 gallons | ~146 days | ~42 days | ~21 days |
| 350 gallons | ~204 days | ~58 days | ~29 days |
| 400 gallons | ~233 days | ~67 days | ~33 days |
| 500 gallons | ~292 days | ~83 days | ~42 days |
| Inflatable spa, ~200 gal | ~117 days | ~33 days | ~17 days |
Two caveats keep this honest. Cap the answer at about six months no matter how light the use, because biofilm and scale accumulate on a timeline of their own. And treat any tub used with lotions, fake tan or after workouts as carrying a heavier load than the headcount suggests.
What test result tells me it is time?
Total dissolved solids is the direct measurement. Test your fill water first and write the number down, since municipal supply commonly runs 100 to 400 ppm and well water can start far higher. Drain when the tub reads roughly 1,500 ppm above that baseline. A cheap TDS meter costs under $20 and removes the guesswork entirely. Cyanuric acid is the second trigger and the one most owners never check: dichlor, the granular chlorine most spa owners use, adds about 9 ppm of cyanuric acid for every 10 ppm of free chlorine it delivers. Once it passes roughly 100 ppm your chlorine is still present on a test strip but chemically hobbled.
| Reading | Drain trigger | Why |
|---|---|---|
| Total dissolved solids | 1,500 ppm over fill water | Sanitizer efficiency falls, water feels dull and salty |
| Cyanuric acid | Above 90 - 100 ppm | Locks up chlorine; only dilution removes it |
| Calcium hardness | Above 400 ppm | Scale on heater element and jet faces |
| Combined chloramines | Above 0.5 ppm and will not shock down | Organic load exceeds what shocking can clear |
| Persistent foam | Any, with correct pH and sanitizer | Surfactant saturation from oils and detergent |
What are the signs the water has gone off?
Foam that returns within minutes of the jets starting is the classic tell, and it means the water is saturated with surfactants from body oils, shampoo residue and detergent left in swimwear. Water that feels slick or slimy on the shell, a musty smell distinct from chlorine, cloudiness that does not clear after shocking, and skin irritation after a soak all point the same way. So does chemical demand that keeps climbing: if you are adding more sanitizer each week to hold the same reading, the water is telling you it is spent.
One misconception is worth correcting. A strong chlorine smell does not mean too much chlorine, it means too little free chlorine and too many chloramines. The fix is shocking, and if shocking will not clear it, the fix is fresh water.
How do I make the water last longer?
- Shower before soaking. A 30-second rinse removes most of the lotion, deodorant and product that would otherwise end up in the water and the filter.
- Rinse swimwear in plain water and skip the detergent. Trapped detergent is the leading cause of foam.
- Keep the cover on. It blocks debris, and a cover in good condition is also the largest single factor in running cost.
- Clean the filter on schedule. Filter condition and water life are directly linked, covered in how long hot tub filters last.
- Use bromine or a mineral cartridge with chlorine instead of dichlor alone, so cyanuric acid stops accumulating.
- Shock weekly to burn off combined chloramines before they build.
Nothing on that list buys unlimited time. It moves a three-month interval to four or five months, which over a decade is a meaningful amount of water, heat and chemistry saved. Set-temperature choices affect chemical demand too, since sanitizer is consumed faster at higher temperatures — see what temperature a hot tub should be set to.
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