Hot Tub Shock Calculator
Shocking a spa isn't about hitting a giant ppm number like a pool — it's a small, precise oxidizing dose after heavy use or weekly. Two products, two jobs:
- MPS (non-chlorine shock): the routine choice — oxidizes organic waste, you can soak ~15 minutes after
- Dichlor (chlorine shock): after heavy bather loads, algae events, or when chlorine has crashed
When to shock a hot tub
- Weekly as part of maintenance (MPS)
- After every heavy soak — 4+ people, or a long party session
- When water looks tired: dull, slightly cloudy, or smelling "chlorine-y" (that smell is used-up chlorine, not too much)
- Never mix products — add one, circulate, retest before adding another
How the math works
- MPS (oz) = Δppm × Gallons × 8.34 ÷ 1,000,000 × 16 ÷ 0.43
- Example: +2 ppm in a 350-gallon spa ≥ 0.27 oz of MPS — about 1.5 teaspoons
- Dichlor works the same at 56% strength
Frequently asked questions
How long after shocking can I get in?
MPS: about 15 minutes with jets running, once the reading drops back into range. Dichlor: wait until FC returns to 2–4 ppm — usually 30–0 minutes for a routine dose.
Should I use chlorine or non-chlorine shock?
MPS for the weekly routine (no chlorine spike, soak sooner). Dichlor after heavy use or a sanitation problem. Bromine spas need MPS regularly — bromide must be re-activated by an oxidizer.
My shock says "1 tablespoon per 250 gallons" — why is my dose different?
Label doses assume a specific product strength and are deliberately conservative. Trust your test kit: calculate for the actual ppm change you measured.