How often should you drain your hot tub?

No chemical routine, however disciplined, keeps spa water good forever. Every dose of sanitizer, every soak, every splash of lotion leaves something dissolved behind — and dissolved solids never leave. They accumulate until the water is chemically “full,” at which point it clouds easily, foams easily, resists your chemicals, and generally behaves like a teenager. The only fix is the honest one: drain it and start fresh.

The short version: drain every 3–4 months with typical use. For your exact spa, use the classic formula: gallons ÷ daily bathers ÷ 3 = days between drains. And when the water starts fighting you before the calendar says so, believe the water, not the calendar.

Why water wears out

Two things build up in spa water that nothing removes:

• Total dissolved solids (TDS). Every chemical you add stays behind as a dissolved salt after it does its job. Add sweat, body oils, and lotion residue, and month by month the water becomes saturated — cloudy-prone, foam-prone, and slow to respond to treatment. It’s why a spa that foams at the slightest provocation late in its fill was fine in month one.
• Cyanuric acid (CYA), for chlorine spas using dichlor. Every dichlor dose adds stabilizer that only leaves by draining. Past about 30 ppm, chlorine works measurably slower in hot water — the water can test “fine” and still misbehave.

This is why late-fill water problems often can’t be solved by adding anything. If you’ve balanced everything, shocked properly, cleaned the filter, and the water still turns, it isn’t a chemistry puzzle anymore — it’s a drain.

The formula for your spa

The widely-used industry rule of thumb:

Gallons ÷ daily bathers ÷ 3 = days between drains

• A 300-gallon spa, 2 people daily: 300 ÷ 2 ÷ 3 = 50 days.
• The same spa used by 2 people twice a week (≈0.6 bathers/day): about 5–6 months.
• A 250-gallon inflatable with a family of 4 in it daily: 250 ÷ 4 ÷ 3 = 3 weeks — small tubs with heavy use really do turn over that fast.

Treat the result as a ceiling, not a promise. Heavy lotion use, parties, and hot climates all shorten it.

Signs the water is done (whatever the calendar says)

• It clouds or foams easily even though every reading is in range.
• A smell that shocking clears for a day, then comes back.
• Chemical doses stop “sticking” — you add more and more to hold the same numbers.
• CYA reads above 30 ppm (chlorine spas).
• The water just feels flat or slick instead of crisp. Owners know.

How to drain and refill, cleanly

The short version below covers the essentials; for the full walkthrough with equipment recommendations, see how to drain and refill a hot tub, step by step.

1. Flush the lines first (every drain or two). Add a line-flush product to the old water and run the jets as directed. Biofilm hides in the plumbing where sanitizer can’t fully reach — skip this and it seeds the fresh fill within days. Expect alarming gunk; that’s the point.
2. Kill power, then drain via the drain valve or a submersible pump (a pump turns 2+ hours into 20 minutes). Never let pumps or the heater run dry.
3. Wipe down the shell with a spa-safe cleaner and rinse well — household cleaners leave foaming residue. If you're dealing with actual mold or mildew spots rather than routine grime, that needs a different approach — see hot tub mold and mildew: prevention and removal.
4. Deep-clean or replace the filter while the tub is empty — a fresh fill through a dirty filter is a head start on cloudy water. Our filter cleaning guide covers when a rinse is enough and when it isn’t.
5. Refill through the filter compartment with the hose — filling there pushes air out of the plumbing and prevents airlocks that leave pumps sputtering.
6. Heat, then balance in order: total alkalinity first (80–100 ppm), then pH (7.4–7.6), then calcium hardness (150–250 ppm), then sanitizer — and finish with a shock dose to set a clean baseline. Alkalinity leads because it controls how pH behaves; dose out of order and you’ll redo the work.

Frequently asked questions

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