The quick answer: use the spa industry's water-replacement formula:
Days between drains = (tub volume in US gallons ÷ 3) ÷ average bathers per day
A 400 gallon hot tub used by 2 people a day comes out at (400 ÷ 3) ÷ 2 ≈ 67 days, about every two months. In litres, divide the volume by 11.4 instead of 3, then by bathers per day.
Enter your tub below. The calculator also handles weekend-only use and inflatables.
Why hot tub water wears out
A hot tub holds a small amount of very hot water shared by people, so the water "ages" fast. Every soak adds sweat, skin oils, cosmetics, lotions and detergent residue from swimsuits. Every dose of chemicals leaves something behind, such as sodium, chloride, sulfate or, with dichlor, cyanuric acid.
Sanitizer destroys germs, but the dissolved leftovers keep building up as total dissolved solids (TDS). Eventually:
- sanitizer gets used up faster than it should;
- the water foams;
- it turns dull or cloudy even with good readings;
- it smells "off".
At that point, no chemical fixes it. Fresh water does.
The formula estimates when that happens, from how much water you have and how many people use it.
Worked examples
| Tub | Volume | Bathers/day | Drain every |
|---|---|---|---|
| Large acrylic | 450 gal | 2 | ~75 days |
| Typical acrylic | 400 gal | 2 | ~67 days |
| Typical acrylic | 400 gal | 4 | ~33 days |
| Small acrylic | 250 gal | 2 | ~42 days |
| Inflatable (~800 L) | ~211 gal | 2 | ~35 days |
| Inflatable (~800 L) | ~211 gal | 4 | ~18 days |
The app uses the same rule, rounded, and keeps the interval between 7 and 120 days (a lightly used tub still needs fresh water every four months).
"Bathers per day" is an average. Two people every evening counts as 2. Four people on Saturday and Sunday only works out to about 1.1 per day (8 ÷ 7).
For the volume, use your manufacturer's stated capacity. Inflatable volumes vary a lot by model, so check your manual or how many gallons in a hot tub.
Drain sooner if you see these signs
The formula is a starting point. Change the water early if:
- Foam keeps coming back even after shocking. See hot tub foam.
- Cloudiness persists while sanitizer, pH and alkalinity are all in range.
- Sanitizer disappears fast. Chlorine or bromine is gone within hours of dosing, with no obvious reason such as a party.
- CYA has built up (dichlor users). Above 40 ppm the chlorine you need (7.5% of CYA) rises above the 3 ppm minimum and starts to crowd the 5 ppm soak limit, so Spa Calc tells you to drain. A refill is the only way to lower CYA.
- TDS is high. A TDS meter reading roughly 1,500 ppm above your fill water is a common trigger.
- It smells, even right after shocking.
- After illness, or a heavy party weekend, many owners simply refill.
Draining in summer vs winter
- Summer: more soaking, more sweat and more sunscreen, so the interval shortens. Recalculate with your real bathers per day.
- Winter: use often drops, but don't drain in freezing weather unless you can refill and heat immediately. Water left in plumbing can freeze and crack pipes. Inflatables are usually drained, dried and stored rather than run through hard frosts. Follow your manual.
How to drain and refill (short version)
- The day before: flush the lines. Add a pipe or line-flush cleaner, run the jets per the label, and let it lift biofilm from the plumbing.
- Turn off the power at the breaker or the plug. Never run the heater or pump dry.
- Drain:
- Use the drain valve and a hose, or a submersible pump, which is much faster.
- Check local rules on where spa water may go; sanitized water shouldn't go straight into storm drains or onto sensitive plants.
- Clean:
- Wipe the shell with a non-foaming spa surface cleaner, never household detergent.
- Rinse it.
- Soak or replace the filter.
- Refill through the filter housing (acrylic tubs). This helps prevent air locks in the pump. Inflatables refill per the manual.
- Restore the chemistry:
- Test the fill water.
- Adjust alkalinity, then pH.
- Add the sanitizer: dichlor for a chlorine start, or sodium bromide plus an oxidizer to build a bromine bank.
- See how much chlorine for a hot tub or how much bromine.
- Test before the first soak.
Ways to stretch the time between drains
- Shower before soaking. That means rinsing off lotion, deodorant and hair products, not a full shower.
- Rinse swimsuits in plain water. Detergent residue is a top foam cause.
- Clean the filter regularly. Rinse it every week or two, and give it a chemical clean about monthly with heavy use.
- Dose sanitizer after soaks, not just on a schedule. The organic load arrives with people.
- Chlorine users: switch from dichlor to bleach once CYA reaches about 30 ppm. This is the "dichlor-then-bleach" method, and it keeps CYA from building up and forcing an early drain.
- Keep the cover on. Less debris, less evaporation and less heat loss.
How long does draining take?
It depends on the method. A garden hose on the drain valve is slow; at a few gallons per minute, a 400 gallon tub can take a couple of hours or more. A submersible pump can empty the same tub in well under an hour. Refilling through a hose takes a similar time, so plan on a morning for the whole job, including cleaning and rebalancing.
The app counts the days for you
Spa Calc calculates your drain interval from your tub's volume and how often it's used. It counts down to drain day on your Home Screen, and nudges you early if your log shows the warning signs: rising sanitizer demand, high CYA, or repeated foam notes. Inflatable presets are built in.
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Safety
- Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
- Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
- Add chemicals to water, never water to chemicals.
- Store chemicals sealed, cool and dry, apart from each other.
- Read and follow the product label. It always comes first.