Where can I find my hot tub's capacity in litres?

Start with the manufacturer's handbook or specification plate. UK hot tubs generally contain between 800 and 1,500 litres, while swim spas hold far more. If no capacity is listed, multiply water length by width and average depth in metres, then multiply that result by 1,000 to estimate litres.

Does the calculator give an exact chemical dose?

No. These cautious guide quantities follow the rates printed on Zagosa Spa & Pool labels, but the pack label remains definitive. Add the chemical, circulate the water and test again before either bathing or applying another treatment.

Which readings should I target in hot tub water?

Aim for free chlorine 3 to 5 ppm or total bromine 4 to 6 ppm, alongside pH 7.2 to 7.6, total alkalinity 80 to 120 ppm and calcium hardness 150 to 250 ppm. Use strips or a photometer to check the water at least two or three times weekly.

In what order should I balance the chemicals?

Balance total alkalinity before correcting pH, and deal with the sanitiser last. Because alkalinity stabilises pH, adjusting pH while alkalinity sits outside 80 to 120 ppm means the reading is likely to wander again.

Hot Tub Chemical Dosing Calculator

Provide the hot tub volume to calculate label-aligned guide amounts for balancers, shock and sanitiser. Follow the product label, then circulate and retest after dosing.

Chlorine top-up using stabilised granules: 1.4 g per 1,000 litres - top-up that increases free chlorine by approximately 1 ppm
Bromine top-up using granules: 3 g per 1,000 litres - addition that increases total bromine by approximately 1 ppm
Bromine tablet dispenser refill (20 g tablets): 40 g per 1,000 litres - tablet refill for maintaining free bromine at 4-6 ppm
Routine non-chlorine shock (MPS): 17 g per 1,000 litres - weekly oxidation treatment for routine care
Calcium hypochlorite shock for problem water: 7.5 g per 1,000 litres - corrective shock treatment for algae or severe cloudiness
Raise total alkalinity with sodium bicarbonate: 17 g per 1,000 litres - addition that lifts total alkalinity by approximately 10 ppm
Lower pH and alkalinity with sodium bisulphate: 16 g per 1,000 litres - treatment that drops pH by approximately 0.2 (and lowers alkalinity)