How often to drain your hot tub

Days between drains = (tub volume in US gallons ÷ 3) ÷ average soakers per day. A 400 gallon tub used by two people a day works out to about 67 days.

Drain interval calculator

67days

between drains

about 2.2 months

400 gal ÷ 3 ÷ 2 soakers a day = 67 days.

Draining at this rate takes about 1.3 hours. Refilling with a hose takes about as long again.
Drain sooner if the water stays cloudy or foamy after shocking, smells even when sanitizer is in range, or if CYA climbs above 40 ppm.
Flush the plumbing with a line cleaner before you drain, then clean the filter.

Save this test and get a reminder before your next soak.

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Why tubs need draining

Every soak adds sweat, oils and lotion residue, and every dose leaves salts behind. Total dissolved solids build up until the water stops responding to chemicals: it foams, clouds, or eats chlorine. The formula estimates how long a tub of your size takes to get there.

Drain sooner when

the water stays cloudy or foamy after shocking,
it smells even when sanitizer is in range,
stabilizer climbs above 40 ppm (dichlor users),
a TDS meter reads about 1,500 ppm above your fill water.

Questions

How often should I drain an inflatable hot tub?

Use the same formula with the tub’s volume. A 177 gallon inflatable used by two people a day comes out at about 29 days.

How long does it take to drain a hot tub?

With a garden hose at a few gallons per minute, a 400 gallon tub can take a couple of hours. A submersible pump is much faster.