Spa and Hot Tub Chemical Dose Calculator

Every dose is the same mass balance, scaled from a per-10,000-gallon industry factor down to your spa. The only reason the answers come out in ounces and spoonfuls is the volume ratio: a 400-gallon spa is one twenty-fifth of a 10,000-gallon pool.

amount  = delta_ppm x (spa_gallons / 10,000) / ppm_per_unit_per_10k

ppm_per_unit_per_10k is the product's own factor, corrected for the
strength you typed:   effective = factor x (your_strength / label_strength)

Bromine:
  chlorine_equivalent_ppm = delta_bromine_ppm / 2.2538
  sodium_bromide_oz       = reserve_ppm x 1.2877 x gallons x 0.000133526

Sanitiser factors (per 10,000 gallons, packaged product): dichlor 56% raises free chlorine 5.6 ppm per pound, so 0.35 ppm per weight ounce; cal-hypo 65% raises it 6.5 ppm per pound, so 0.40625 ppm per ounce; liquid chlorine raises it 10 ppm (10%) or 12.5 ppm (12.5%) per gallon, so 0.078 or 0.098 ppm per fluid ounce. These are the same packaged-product factors the pool chemical dosing calculator carries, restated per ounce because a spa dose is never a pound.

Balance factors (per 10,000 gallons): sodium bicarbonate 1.5 lb raises total alkalinity 10 ppm, so 0.4167 ppm per ounce; calcium chloride 77% at 1.25 lb raises calcium hardness 10 ppm, so 0.5 ppm per ounce; soda ash raises pH about 0.2 per pound. pH and hardness moves are buffer-dependent and are flagged approximate on the result.

Acid: the cited anchor is muriatic acid at 31.45%, about 0.2 pH per quart per 10,000 gallons, which is 0.00625 pH per fluid ounce. Dry acid is derived from that anchor through Orenda's published equivalence, 10.5 lb of 93.2% sodium bisulfate to one gallon of muriatic: 0.8 pH per 10.5 lb, so 0.004762 pH per weight ounce. Molar acid equivalence agrees independently: a quart of 31.45% muriatic carries roughly 9.5 moles of hydrogen ion, which is about 2.5 lb of sodium bisulfate. Sulfate carry-in is Orenda's figure, 9.6 ppm per pound per 10,000 gallons, scaled to your spa volume.

Bromine: one mole of oxidiser converts one mole of bromide to hypobromous acid, and both residuals are reported as the diatomic equivalent, so 1 ppm of chlorine equivalent reads as 159.808 / 70.906 = 2.2538 ppm of bromine. The bromide bank is pure mass balance: sodium bromide is 102.894 / 79.904 = 1.2877 times the weight of the bromide it delivers, and 1 ppm in one US gallon is 3.785412 mg, which is 0.000133526 oz. At a 30 ppm reserve that lands on 0.52 oz per 100 gallons, which is the field convention it is usually quoted as. Molar masses are PubChem values.

Spa target bands, and why they are not the pool bands: free chlorine 3 to 5 ppm (CDC asks for at least 3 ppm in a hot tub against 1 ppm in a pool, and says to raise it with the operator above 10 ppm); bromine 4 to 6 ppm inside CDC's 4 to 8; pH 7.4 to 7.6 inside CDC's acceptable 7.0 to 7.8; total alkalinity 80 to 120 ppm, the recommended spa range (a wider permitted range exists in the ANSI/APSP/ICC standard, but we could not retrieve that standard first-hand, so we quote no figure for it rather than repeating one second-hand); calcium hardness 150 to 250 ppm. Hardness is the one the sources split on: Taylor Technologies gives 150 to 250 for spas against 200 to 400 for pools, while some spa service writing carries the pool band across unchanged. We default to the spa-specific figure and say so rather than hiding the disagreement.

Cyanuric acid is deliberately not a dosable parameter here. CDC recommends against cyanuric acid and CYA-bearing chlorine in hot tubs, so the only CYA number this page produces is a warning: how much a dichlor dose adds to your volume, at 0.9 ppm CYA per 1 ppm of free chlorine, which is a fixed molar property of the molecule and not a formulation variable.

Sources: CDC Healthy Swimming (hot tub free chlorine at least 3 ppm, bromine 4 to 8 ppm, pH 7.0 to 7.8, no cyanuric acid in hot tubs); Taylor Technologies (spa calcium hardness 150 to 250 ppm); Pool & Spa News (spa service ranges, aeration and temperature effects); Orenda Technologies (sodium bisulfate acid equivalence and sulfate carry-in); PubChem (molar masses).