How do I work out how many litres my hot tub holds?

Start with the owner's manual or the specification sticker on the cabinet. As a rule of thumb, UK hot tubs sit somewhere between 800 and 1,500 litres, with swim spas well above that. No figure to hand? Measure the water's length, width and average depth in metres, multiply the three together, then multiply by 1,000 to get a rough litre count.

Are the doses this calculator gives exact?

No. They are deliberately cautious starting doses that follow the rates printed on Zagosa product labels, and the label on the pack always takes priority. Add the dose, give the water time to circulate, then retest before anyone bathes and before adding anything else.

What readings should I be aiming for?

Free chlorine 3 to 5 ppm or total bromine 4 to 6 ppm, pH 7.2 to 7.6, total alkalinity 80 to 120 ppm and calcium hardness 150 to 250 ppm. Check the water with test strips or a photometer at least two or three times each week.

Which reading should I fix first?

Total alkalinity first, pH second, sanitiser last. Alkalinity is the buffer that holds pH steady, so if you adjust pH while alkalinity is outside 80 to 120 ppm the reading will simply slide back out again.

Hot Tub Chemical Dosing Calculator

Type in how many litres your hot tub holds and get a guide dose in grams for chlorine or bromine sanitiser, shock treatments and the balancers that move pH and alkalinity. Every rate matches the Zagosa product label, and the label always has the final say.

Chlorine top-up (stabilised chlorine granules): 1.4 g per 1,000 litres, top-up dose to raise free chlorine by about 1 ppm
Bromine top-up (bromine granules): 5 g per 1,000 litres, top-up dose to bring total bromine back into range
Bromine tablet dispenser top-up (20 g tablets): 40 g per 1,000 litres, dispenser top-up to hold total bromine at 4 to 6 ppm
Weekly non-chlorine shock (MPS): 17 g per 1,000 litres, regular weekly oxidising dose
Chlorine shock for problem water (calcium hypochlorite): 7.5 g per 1,000 litres, shock dose to deal with algae and heavy cloudiness
Raise total alkalinity (sodium bicarbonate): 17 g per 1,000 litres, dose to raise total alkalinity by about 10 ppm
Lower pH and alkalinity (sodium bisulphate): 16 g per 1,000 litres, dose to lower pH by about 0.2 (also reduces alkalinity)