How often to change hot tub water

Change your hot tub water every 3 to 4 months. A common rule of thumb gives a number for your tub: gallons, divided by 3, divided by the average number of bathers per day, equals the number of days between changes. Change it sooner if the water foams, stays cloudy, smells, or stops holding its chemistry.

The rule of thumb

Days between changes = tub gallons ÷ 3 ÷ daily bathers

Tub size	1 bather a day	2 bathers a day	4 bathers a day
250 gallons	83 days	42 days	21 days
350 gallons	117 days	58 days	29 days
400 gallons	133 days	67 days	33 days
500 gallons	167 days	83 days	42 days

Count average use, not peak use. Two people soaking four nights a week is a little over one bather a day. The formula is a guide. If it gives a number over 120 days, change at 4 months anyway.

Why the water wears out

Everything you add to a hot tub stays there. Sanitizer, shock, pH adjusters, sweat, skin oils, and the minerals left behind by evaporation all accumulate as total dissolved solids (TDS). The filter cannot remove dissolved material. Only draining does.

• High TDS makes water look flat and feel heavy. A typical guideline is to change when TDS is about 1,500 ppm above where the fresh fill started.
• Cyanuric acid builds up from dichlor chlorine. Past roughly 50–100 ppm, chlorine gets sluggish.
• Combined waste uses up sanitizer faster and faster.
• Calcium creeps up with each top-up in hard-water areas.

Signs it is time

• Foam that returns right after skimming. See foamy hot tub water.
• Cloudiness that survives balancing, shocking, and a clean filter. See cloudy hot tub water.
• A musty or chemical smell a day after shocking.
• pH or alkalinity that will not stay in range.
• You are using noticeably more sanitizer than you did a month ago.
• Skin or eye irritation with readings in range.
• It has been 4 months.

Change it right away if

• Sanitizer sat at zero for several days in a warm tub.
• Someone used soap, bath oil, or bubble bath in it.
• There was a contamination accident.
• Anyone developed a rash after soaking. That can indicate bacteria. Do a line flush and a full change.
• The water is green and slimy.

How to make water last the full interval

1. Shower first. A quick rinse removes most of the lotion, deodorant, and detergent that load the water.
2. Keep sanitizer in range every day, not just on soak days. Chlorine 1–3 ppm or bromine 3–5 ppm.
3. Shock weekly.
4. Rinse the filter every 1 to 2 weeks and soak it monthly. See how to clean a hot tub filter.
5. Use only what the test strip calls for. Every extra dose adds dissolved solids.
6. Keep the cover on. It keeps debris out and slows evaporation.
7. Rinse swimwear in plain water.

What a water change involves

1. Add a line flush product and circulate it per the label.
2. Turn off the breaker and drain. Steps in how to drain a hot tub.
3. Wipe down the shell.
4. Deep clean or replace the filter.
5. Refill through the filter compartment.
6. Restore power, bleed any air, balance the water, and heat.

Plan on 3 to 4 hours of elapsed time, most of it waiting for water to drain and fill, plus heating time afterward. A 240V tub typically gains 3 to 6°F per hour. A 120V plug-in tub gains about 1 to 2°F per hour.

Seasonal timing

Four changes a year fit neatly on the calendar. Many owners in cold climates time one for late fall so the water is fresh through the months when draining is awkward, and the next for early spring. Avoid draining when the temperature is below freezing, and never leave the tub empty in winter unless it is fully winterized.

What it costs

Water for a 400-gallon fill costs a few dollars in most of the US. Reheating it costs a few dollars more. Fresh start-up chemicals add a little. In total, a water change usually costs less than the extra chemicals you would pour into tired water to keep it going another month.

Set a reminder

The easiest way to stay on schedule is to stop relying on memory. Note the fill date, work out your interval from the table above, and set an AskTub reminder for the next change along with filter cleanings.