Hot tub water can't be filtered or sanitized forever. Body oils, lotions, sweat, urea (a common byproduct of perspiration), and the chemical byproducts of sanitization (chloramines, bromamines) accumulate until the water reaches a level where chemistry can't keep up. Total Dissolved Solids (TDS) is the technical measure. Most residential hot tubs hit the "needs to be changed" point at 3 to 4 months for typical use. The formula below gives a more specific answer based on actual use.
Quick schedule
- Default: every 3 to 4 months for typical residential use.
- Bather-load formula: tub gallons ÷ 3 ÷ daily-average bathers = days between changes.
- Heavy use (daily, multiple bathers, parties): every 6 to 10 weeks.
- Light use (weekly, 1-2 bathers): can stretch to 5 to 6 months.
- Sooner whenever: foam, cloudy water, persistent odor, chemicals "won't balance," or TDS test shows greater than 1,500 ppm above source water.
The bather-load formula
Common industry formula: tub volume in gallons ÷ 3 ÷ average bathers per day = days between water changes.
- 400-gallon tub, 2 bathers/day average: 400 ÷ 3 ÷ 2 = 67 days (~ 2 months).
- 400-gallon tub, 1 bather/day: 400 ÷ 3 ÷ 1 = 133 days (~ 4.5 months).
- 250-gallon tub, 2 bathers/day: 250 ÷ 3 ÷ 2 = 42 days (~ 6 weeks).
- 500-gallon tub, 4 bathers/day: 500 ÷ 3 ÷ 4 = 42 days (~ 6 weeks).
The formula isn't precise but it's a useful starting point. Adjust based on actual water quality between changes.
Signs the water needs changing now
- Foam that doesn't clear with defoamer or fresh water addition.
- Cloudy water despite balanced chemistry and clean filter.
- Persistent odor that returns within hours of shocking.
- Difficulty maintaining sanitizer levels — you keep dosing and the test reads low within hours.
- Skin irritation after soaking that wasn't there with fresh water.
- TDS measurement above 1,500 ppm above source water TDS (per industry guidance from Pool & Hot Tub Alliance).
- Sanitizer demand keeps rising beyond normal use.
What causes TDS to build up
- Body oils, sweat, lotions: the main residential source. Each bather adds significantly more than people realize.
- Sanitizer byproducts: chloramines (from chlorine + nitrogen from sweat/oils) and bromamines.
- pH adjusters and other chemicals: each dose adds dissolved solids.
- Evaporation: water evaporates; dissolved solids don't. Topping off concentrates them.
- Hard tap water: the starting TDS is already higher.
Rinsing off in the shower before using the hot tub significantly reduces oil/lotion load. So does not wearing perfume, deodorant, or makeup into the tub.
How to change the water
- Turn off power at the breaker or spa-side disconnect.
- Remove and clean the filter cartridges. See hot tub filter cleaning schedule.
- Drain via the built-in drain or with a submersible pump. Direct the water somewhere acceptable (not on lawn if you've just shocked; not to storm drain if local rules prohibit).
- Once empty, clean the interior shell with a non-foaming spa cleaner. Rinse thoroughly to remove all residue.
- Wipe down the cover underside and the seal area.
- Replace the filter (clean or new) and any filter wells.
- Refill with fresh water through the filter intake (helps prime the pump and prevent airlock).
- Restore power. The heater will start as water reaches the level sensor.
- Add start-up chemicals: balance pH, alkalinity, calcium hardness, then sanitizer.
- Wait 24 hours and retest before getting in.
The "dichlor then bleach" method
An alternative sanitization approach that some hot tub owners use to extend water life. Use dichlor (stabilized chlorine) until cyanuric acid reaches 30 ppm, then switch to unscented bleach (sodium hypochlorite) for ongoing sanitization. This method can roughly double the time between water changes because it doesn't keep adding cyanuric acid (which raises TDS without doing useful work after the first 30 ppm).
This is more advanced than basic hot tub care; read up before switching methods, and confirm your tub manufacturer doesn't have a specific sanitization requirement.
What changes the schedule
- More bathers, more often: shortens it.
- Pets in the tub: add bather-load equivalent and possibly more.
- Lotions, perfumes, makeup not rinsed off before getting in: shortens it significantly.
- Salt-water systems: follow the manufacturer's specific schedule, usually similar to traditional systems.
- UV or ozone systems: reduce chemical load on the water but don't change the bather-load math much.
- Hard tap water: starts higher, hits TDS limit sooner.
- Filtered fill water (RO, water softener): reduces starting TDS, extends water life.
Common mistakes
- Topping off without changing for 6+ months. Evaporation concentrates TDS faster than people realize.
- Skipping the rinse cycle when changing water. Soap and cleaner residues foam the new water for weeks.
- Refilling without cleaning the filter. The dirty filter contaminates the fresh water.
- Adding chemicals before the heater catches up. Cold water absorbs differently; some chemicals need warm water to dissolve.
- Not rinsing off before getting in. The single biggest controllable variable in water life.
- Letting the tub sit empty for days during the change. Pumps and seals can dry out.
Storing the tub between uses
If you'll be away for more than 2 weeks, options:
- Leave it running: the tub manages itself with normal chemistry maintenance, but no one is testing.
- Drain it: for trips longer than a month, draining and leaving the cover off (under shelter) is cleaner.
- Reduce temperature and add long-acting sanitizer: for shorter trips. Cooler water (90°F) reduces chemical demand.
Good maintenance rhythm
- Every 3 to 4 months (or per bather-load formula): full drain and refill.
- Weekly: test water chemistry (pH, sanitizer, alkalinity).
- Every 1 to 2 weeks: rinse filter.
- Monthly: chemical soak the filter.
- Quarterly (with water change): deep clean filter, clean cover underside.
- Yearly: inspect cover, jets, equipment closet for moisture damage.
- Every 12 to 18 months: replace filter cartridges.
- Before guests or parties: test water and shock the night before.