Hot Tub Chemical Calculator

In a 400-gallon spa, dichlor 56% needs 0.095 oz per 1 ppm — about 0.47 teaspoon (2.7 g). The formula is: oz = ppm rise × gallons ÷ (74.94 × % available chlorine). So raising a 400-gallon spa from 1 to 5 ppm takes 0.38 oz (about 1.9 tsp / 10.8 g) and adds roughly 3.6 ppm of cyanuric acid. Always pre-dissolve granules in a bucket of spa water first.

Both sanitize well. Chlorine (dichlor granules) dissolves fast and costs less, but burns off quickly in hot water and adds CYA with every dose. Bromine stays stable in hot water, has a milder odor, and works through slow-dissolve tablets in a floater — ideal if you soak infrequently. Bromine costs more and some people dislike the smell. Targets: 3–5 ppm free chlorine, or 4–6 ppm bromine (as Br₂).

Three reasons: heat accelerates sanitizer loss, the small water volume means every bather is a large contaminant load, and jets aerate the water. A rule of thumb from Trouble Free Pool: at 104°F with no ozonator, each person-hour of soaking needs about 3.5 tsp of dichlor to oxidize bather waste — so two people soaking 30 minutes need ~3.5 tsp after the soak. An ozonator roughly halves that.

Not recommended. Trichlor is strongly acidic and dissolves slowly; in a small spa volume it crashes pH and total alkalinity, which can corrode heaters and pumps. Spas should use dichlor granules or bromine tablets/tabs designed for hot water. Pool and spa chemistry share the math, but not the products.

Shock weekly, after heavy use, or whenever water looks cloudy or smells. Chlorine shock means raising free chlorine to ~10 ppm — in a 400-gallon spa that is 0.95 oz of dichlor 56% (about 4.75 tsp). Non-chlorine MPS shock is dosed at 1 oz per 250 gallons (1.6 oz for 400 gal). Run the jets 20–30 minutes with the cover open, and don't soak until free chlorine is back to 3–5 ppm.

Dichlor adds ~0.9 ppm of cyanuric acid per ppm of chlorine, and CYA only leaves by draining. A 400-gallon spa used three times a week by two people can add ~32.9 ppm CYA per week — passing 50 ppm in about 1.5 weeks. Keep CYA at 20–50 ppm. Above ~50 ppm chlorine gets 'locked' and sanitizes poorly. The fix: once CYA hits 30–50 ppm, switch daily sanitation to plain unscented bleach (zero CYA), or drain and refill — most spas need it every 2–4 months.

Total alkalinity, always. TA of 80–120 ppm buffers pH; low TA lets pH swing wildly, high TA pushes pH constantly upward. Raise TA with baking soda first (a 400-gallon spa going 60→100 ppm needs ~3.59 oz, about 6 tbsp), retest after 30 minutes, then fine-tune pH. Chasing pH with low TA is a losing battle.

Usual suspects in order: low sanitizer, heavy bather load, high pH/TA, a dirty filter, or simply old water (high TDS). Work the checklist: test and balance (FC 3–5 ppm, pH 7.2–7.8), shock to ~10 ppm, rinse or deep-clean the filter cartridge, and if the water is more than 3–4 months old, drain and refill — no chemical fixes exhausted water.

Never. Mixing dry spa chemicals — especially chlorine with acids or with MPS — can release toxic gas or react violently. Add chemicals to water (never water to chemicals), one product at a time, with the jets running and several minutes between additions. Store everything cool, dry, sealed, and away from children and pets.

Wait at least 20–30 minutes with the jets running after a normal dose. After shocking, wait until free chlorine has fallen back to the 3–5 ppm range — test before you soak, don't guess. Leave the cover open while shocking so gases can escape.

Balance first, sanitize last: 1) total alkalinity (baking soda), 2) pH increaser/decreaser, 3) calcium hardness, 4) sanitizer (dichlor or bromine). Retest between steps — each adjustment changes the water the next chemical sees.

Yes — plain, unscented household bleach (no splashless, no scent, no additives) is an effective spa sanitizer and adds zero CYA, which makes it the ideal switch once dichlor has built CYA to 30–50 ppm. Dose it with the same mass formula using its % sodium hypochlorite from the label.

Usually calcium scale: calcium hardness above ~250 ppm combined with high pH precipitates calcium carbonate. Bring pH down to 7.2–7.4 and keep CH at 150–250 ppm. White flakes can also be biofilm sloughing off old plumbing — purge with a spa flush product before your next drain.

Different jobs. MPS oxidizes organics (sweat, oils, lotions) without adding chlorine or CYA, and you can usually soak sooner afterward. But MPS does not sanitize — it won't kill pathogens. Use MPS for routine weekly oxidation and chlorine shock when the water is cloudy, smells, or combined chlorine reads above 0.5 ppm.