Determine when to drain and refill your spa or hot tub. Based on bather load and water quality parameters to ensure safe soaking conditions. Regular water replacement is necessary to remove the build-up of unfilterable organic waste and dissolved solids.
This tool tells you how often to fully drain and refill a spa based on its small volume and concentrated bather load. Start with the spa's water volume in gallons (or liters); most portable hot tubs hold 250-500 gal (950-1,900 L). Then estimate the average number of bathers using it per day, counting a long soak or a child as a full bather, since each person sheds sweat, skin oils, lotions, and cosmetics into very little water. Optionally note total dissolved solids (TDS) if you track it, because dissolved salts and chemical byproducts accumulate as you keep adding sanitizer and balancers. Unlike a large pool, a spa concentrates contaminants fast: a hot, aerated, heavily used 350-gallon tub can foul in weeks. Record your typical sanitizer type and how cloudy or foamy the water becomes over time, as rising foam and a stubborn chemical smell are practical signals that the dilution limit is approaching and a fresh fill is due.
Method: the widely used drain interval formula is Days between changes = (Volume in gallons / 3) / Average daily bathers. The 'divide by 3' reflects how quickly spa water saturates relative to pool water. Worked example, a 350-gal (1,325 L) spa used by 3 bathers per day: 350 / 3 = 116.7, then 116.7 / 3 = about 39 days, so drain and refill roughly every 5-6 weeks. A lightly used 300-gal (1,135 L) spa with 1 bather: 300 / 3 = 100 days. A busy 400-gal (1,515 L) spa with 5 bathers: 400 / 3 = 133, divided by 5 = about 27 days. As a cross-check, TDS climbing more than about 1,500 ppm above your fresh tap-water baseline also signals a change is overdue, regardless of the calendar count.
Fresh water matters for safety, not just clarity. Once water is saturated with organic waste and dissolved solids, chlorine or bromine struggles to maintain a sanitizer residual, and the warm, aerated environment is exactly where the pathogens CDC Healthy Swimming guidance flags can persist, so over-saturated water is a genuine health risk, not merely an aesthetic one. The common mistake is chasing cloudy, foamy water with ever more chemicals instead of draining, which only raises TDS further and wastes product. Drain to a safe location away from storm drains and plants, since chlorinated water harms vegetation; let levels neutralize or dechlorinate first. Wipe the shell and clean the filter while empty, rinse thoroughly to remove cleaner residue, then refill and rebalance from scratch, starting with alkalinity, then pH, then sanitizer, retesting after the water reaches temperature. Recalculate your interval whenever bather load changes.
The standard formula is (Gallons / 3) / Daily Bathers. For a 300-gallon spa used by 2 people daily, you should drain and refill every 50 days.
Eventually, the water becomes 'saturated' with dissolved solids (TDS) like skin cells, lotion, and chemical byproducts. At this point, the water becomes cloudy and chemicals lose their effectiveness.
Use a specialized spa surface cleaner or a weak vinegar solution to wipe down the shell. Ensure you rinse thoroughly and wipe away any residue before refilling.