Estimate how much liquid chlorine to add.
Spas and hot tubs hold far less water than swimming pools, so even a small amount of liquid chlorine can create a noticeable Free Chlorine increase. Accurate dosing matters because adding too much can make the water uncomfortable, while adding too little may leave the spa under-sanitized.
A 300- to 500-gallon spa responds much faster to chemical additions than a full-size pool. Before dosing, confirm the manufacturer’s stated water capacity or calculate the actual volume as closely as possible.
The target rise depends on the current test result, recent bather load, water temperature, and the sanitation method being used. Test the water first, calculate only the increase needed, then circulate and retest before adding more.
With the circulation system running, slowly add the measured dose to the water according to the product label. Leave the cover open long enough for strong fumes to dissipate, allow the water to circulate, and test again before using the spa.
Spa doses may be only a fraction of a fluid ounce. A small marked measuring cup, syringe-style measuring tool, or milliliter container can provide better accuracy than estimating by eye.