Hot Tub Chemical Calculator
A 350-gallon spa is roughly 1/30th of a swimming pool 鈥?chemistry moves 30脳 faster. This calculator converts your test reading into household measurements (teaspoons and tablespoons), because that's how hot tub chemicals are actually dosed.
Target ranges cheat sheet
- Chlorine spa: FC 2鈥? ppm (with CYA) 路 Bromine spa: 3鈥? ppm
- pH: 7.4鈥?.6 路 Total alkalinity: 80鈥?20 ppm 路 Calcium hardness: 150鈥?50 ppm
- Establishing a bromine spa: build sodium bromide to ~30 ppm, then activate with oxidizer
How the math works
- Pure chemical (oz) = 螖ppm 脳 Gallons 脳 8.34 梅 1,000,000 脳 16
- Product weights per teaspoon: dichlor 鈮?5.5 g, MPS 鈮?4.5 g, baking soda 鈮?4.8 g
- Bleach: 1 gallon of 6% 鈮?0.54 lb available chlorine 鈫?fl oz = lbs 梅 0.54 脳 128
Small tub, big consequences
- Dose in halves. Calculate the dose, add half, run jets 15 minutes, retest. In 300 gallons, overshooting by one scoop is easy and unpleasant.
- Run the jets, cover off. Cover-on dosing traps concentrated chemical near the water surface.
- Wait before soaking. After chlorine or MPS, wait until the reading is back in range 鈥?usually 20鈥?0 minutes with jets on.
Frequently asked questions
How much chlorine to raise 1 ppm in a 350 gallon hot tub?
About 0.07 fl oz of 6% bleach (roughly 陆 teaspoon of dichlor granules at 56%). Tiny amounts 鈥?which is exactly why you dose in halves and retest.
Can I use pool chemicals in my hot tub?
The same chemistry, yes 鈥?bleach is bleach. But pool-sized dosing habits will wreck a spa: always recalculate for your tub's small volume, and never add pool-strength products without converting the dose.
Why does my hot tub use so much sanitizer?
High heat burns chlorine fast, and bather load per gallon is enormous 鈥?two people in 350 gallons is like 40 people in a pool. If usage is heavy, an MPS oxidizer after each soak keeps the chlorine demand manageable.