How to shock a hot tub
Shock a hot tub when combined chlorine (CC) is 0.5 ppm or more or the water is cloudy. Use the amount on your product’s label.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (8 sections)
Combined chlorine is used-up chlorine: chlorine that has already reacted with sweat, urine, and lotions. A shock label gives an amount for a set volume, so scale it to your tub. Example: if the label says 2 tbsp per 500 gallons, a 400-gallon tub needs 1.6 tbsp (400 ÷ 500 × 2). Non-chlorine shock (MPS) is scaled the same way. Either way, stay out until chlorine is back at or below the limit on your label (4 ppm on many labels).
How the shock dose is worked out
The calculator uses free chlorine (FC) and total chlorine (TC). The difference is combined chlorine (CC): chlorine that has already reacted with sweat, urine, and lotions and now causes the smell and red eyes. If CC is 0.5 ppm or more, the calculator tells you to shock and scales the dose on your product’s label to your tub: enter the label amount and the gallons it treats. If CC is below 0.5 ppm, it only tops FC up to 4 ppm, the soaking limit on many labels. Always follow the product label.
| Tub size | Label amount is per 500 gallons | Label amount is per 1,000 gallons |
|---|---|---|
| 300 gal | 0.6 × the label amount | 0.3 × the label amount |
| 400 gal | 0.8 × the label amount | 0.4 × the label amount |
| 500 gal | 1 × the label amount | 0.5 × the label amount |
Every 10 ppm of chlorine from dichlor also adds about 9 ppm of stabilizer (CYA), which the CDC advises against in hot tubs. If you shock often, unstabilized bleach or liquid chlorine keeps CYA from building up.
Chlorine shock vs MPS: what each does
| Question | Chlorine shock (dichlor, bleach, liquid chlorine) | Non-chlorine shock (MPS, potassium monopersulfate) |
|---|---|---|
| Kills germs | Yes, it is a sanitizer | No, it is an oxidizer only |
| Breaks down sweat, oils, and CC | Yes, at a high enough dose | Yes |
| Reactivates bromide in bromine tubs | Yes | Yes |
| Clears algae or a slimy tub | Yes | No |
| How much | From the label, scaled to your tub | From the label, scaled to your tub |
| Soak again | When FC is at or below your label's limit (often 4 ppm) | Per the label; often soon after |
In short, use chlorine shock when something is wrong (cloudy, slimy, or green water, a strong smell, or sanitizer at zero) and MPS for routine weekly oxidizing when the water is fine. In a bromine tub, either one works. See bromine start-up and shock.
MPS shows up as combined chlorine: test before, not after
MPS left in the water reacts with the DPD reagent used by most test kits and reads as combined chlorine (PHTA/AQUA technical notes). So a test taken soon after MPS can show high CC even when the water is fine, and you might shock again when you don't need to. Test before you add MPS, or wait until the next day. Some test kit makers sell a separate reagent that neutralizes MPS before the CC test.
How to shock a hot tub, step by step
- Test free chlorine, total chlorine, and pH. Bring pH to 7.4–7.6 first if it is off: chlorine works much better at lower pH.
- Take the cover off and remove any floater. Turn on the jets; leave air controls open.
- Measure the dose from the label, scaled to your tub (the calculator does the math). Weigh granules on a kitchen scale.
- Pour it slowly into the water in front of a jet. Dissolve granules as the label directs; never premix two products.
- Run the jets with the cover off for about 30 minutes so the dose mixes and gas can escape.
- Retest. With chlorine shock, wait until free chlorine is at or below your label's limit (4 ppm on many labels) before anyone gets in.
How long to wait before getting in
- Chlorine shock: until free chlorine is back at or below the limit on your label. Many spa chlorine labels set it at 4 ppm; use the number on yours. After a shock, that often takes several hours or overnight with the cover off. Test; don't guess.
- MPS: many labels allow soaking shortly after it has mixed. Check your product.
- Bromine tub after shock: until bromine is back to 6 ppm or less.
Leave the cover open while chlorine drops. A closed cover traps the gas, which can damage the cover and the pillows.
How often to shock
Weekly is a common routine, plus after heavy use: several people, a long session, or anyone wearing lotion or makeup. Shock when you see the signs: CC at 0.5 ppm or more, a "chlorine" smell, cloudy water, or sanitizer that keeps disappearing. If a weekly shock doesn't keep the water clear, the problem is usually the filter or old water. See cloudy or green hot tub. The weekly routine around a shock is in hot tub chemicals for beginners.
Safety
- Never mix shock with other chemicals, including another shock. Add each directly to the water, minutes apart, with the jets running.
- Never add shock to a floater or a feeder that holds tablets.
- No soaking above the swim limit on your chlorine label (4 ppm on many labels). Keep kids and pets away while the cover is off.
- Store shock sealed, in a cool, dry place, away from acids. If someone swallows it or gets it in their eyes, call Poison Control at 1-800-222-1222.
- Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
FAQ
What does shock do for a hot tub?
It breaks down the waste that sanitizer has already reacted with: sweat, oils, urine, and lotions. That removes the harsh smell and frees your regular chlorine or bromine to keep working. Chlorine shock also kills germs and algae; MPS doesn't.
Is hot tub shock the same as chlorine?
Sometimes. Many "spa shock" products are dichlor, the same chlorine as regular spa granules, used at a higher dose. Others are MPS, a non-chlorine oxidizer. Check the active ingredient on the label.
Can you use hot tub shock in a pool?
Yes, if the active ingredient suits pools (dichlor or MPS), but spa packages are small: a 10,000–15,000 gallon pool needs 25–37 times the dose of a 400-gallon tub. Dichlor also adds stabilizer quickly. For pool amounts, use the pool shock calculator.
Sources
- PHTA / AQUA Tech Notes, Potassium monopersulfate (MPS) non-chlorine shock (MPS reads as combined chlorine): PDF
- Taylor Technologies, K-1005 instruction manual (combined chlorine = total − free): PDF
- CDC, Model Aquatic Health Code 2024, Annex (spa label limit, chlorine vs pH): cdc.gov (PDF)
- CDC, Home pool and hot tub water treatment and testing (no CYA in hot tubs): cdc.gov
- PHTA fact sheet, Dichlor: phta.org