Hot Tub Chemical Calculator

How it is calculated

The tool takes the spa volume in US gallons and the chlorine reading before and after in ppm:

increase = target − current (ppm)
dose (oz) = gallons × increase × 0.0001335 ÷ available-chlorine fraction

The factor 0.0001335 is the weight in ounces of 1 ppm of chlorine in one US gallon. Because a product supplies only part of its weight as chlorine, the dose is divided by the product's fraction: dichlor 0.56, trichlor 0.90, calcium hypochlorite 0.65. An unknown or missing product is treated as dichlor, the usual hot tub choice, and the note says so. A target above 5 ppm adds a warning that it is high. If the target is not above the current reading, or the volume is zero or negative, the tool shows no result. It only raises chlorine; it does not test or adjust pH, alkalinity or stabilizer.

Common mistakes

• Entering the volume in litres. The field is in US gallons; a 1500-litre spa is roughly 396 gallons.
• Typing the target as the amount to add. The target is the final reading, and the tool subtracts the current reading itself, so enter 4, not 3.
• Entering a current reading of 0 when the test strip just was not read. A true reading changes the dose, since the tool only doses the gap.
• Leaving the product on dichlor while actually adding trichlor or calcium hypochlorite. The dose differs for each (0.29, 0.18 and 0.25 oz in the example), so choose the product you really use.
• Entering a target far above normal. Above 5 ppm the tool still gives a dose but warns that it is high for a hot tub; do not treat a larger number as safer.