The quick answer: to raise free chlorine (FC) by 1 ppm:
| Tub volume | 56% dichlor granules | 10% liquid chlorine | 6% unscented bleach |
|---|---|---|---|
| 800 L (≈211 gal, small inflatable) | 1.4 g | 8 mL | 13 mL |
| 1,000 L (≈264 gal) | 1.8 g | 10 mL | 16 mL |
| 1,514 L (400 gal, typical acrylic) | 2.7 g | 15 mL | 24 mL |
| 1,900 L (≈500 gal) | 3.4 g | 19 mL | 31 mL |
Multiply by the number of ppm you need. For example, raising a 400 gallon tub from 1 to 4 ppm takes 8.1 g of dichlor. The calculator handles your volume, product strength and target.
Weigh granules at first. Scoops and teaspoons vary by product and how packed they are. Weigh one level scoop of your dichlor on a kitchen scale once, and you'll know what each scoop adds.
What level should you aim for?
- Soak range: 3–5 ppm free chlorine, with pH 7.2–7.8. The app and the calculators aim for 4 ppm.
- CDC guidance: at least 3 ppm free chlorine in hot tubs (CDC), and nobody in the water above 10 ppm.
- Product labels: many EPA-registered chlorine labels say not to enter a hot tub above 5 ppm. Above 5, wait and retest.
- With CYA from dichlor, more of your chlorine is held in reserve, so the minimum also rises with CYA (7.5% of CYA, as in pools). Keeping CYA at about 30 to 40 ppm with the method below keeps that at 3 ppm, so 3–5 ppm stays effective. Above 40 ppm CYA, drain.
- Stabilizer and public-health guidance: CDC recommends not using stabilizer (CYA) in hot tubs, because it slows disinfection (CDC). The dichlor-then-bleach method keeps it low on purpose; bleach only, with no stabilizer at all, is the other good option (below).
- pH 7.4–7.6 keeps chlorine comfortable and effective. Check alkalinity and pH before chasing chlorine.
When can you soak? Soak when free chlorine is 3 to 5 ppm and at least 7.5% of your CYA (bromine 4 to 6) and pH is 7.2 to 7.8, at least 15 minutes after adding anything (20 after pH decreaser); after chlorine, bromine or a shock, test again first. Above 5 ppm chlorine or 6 ppm bromine, wait and retest in an hour or two; never soak above 10 ppm chlorine or 8 ppm bromine, or with pH below 7.0 or above 7.8. Below 3 ppm chlorine (4 bromine), add sanitizer first; at pH 7.0 to 7.2, raise pH first; with CYA above 40 ppm, drain and refill. Hot tub limits based on CDC guidance and EPA product labels.
The dichlor-then-bleach method
Many experienced owners use this method because it gives the stability of a little CYA without CYA climbing all fill long.
- After a fresh fill, balance alkalinity and pH, then use dichlor for daily and after-soak doses.
- Track how much dichlor you've added. Each 1 ppm of FC from dichlor adds about 0.9 ppm CYA. Once you've added about 33 ppm of FC worth of dichlor in total, CYA is around 30 ppm. (The app keeps the running total for you.)
- Switch to liquid chlorine or plain bleach for the rest of the fill. It adds no CYA.
- Test CYA if you can, and drain on schedule. See how often to drain a hot tub.
Why not dichlor all the time? CYA keeps rising, so the chlorine you need keeps rising, until the water stops responding and needs draining early.
Bleach from day one (no stabilizer) is a perfectly good choice, and it's what CDC recommends for hot tubs. Chlorine works fastest with no CYA, but it also fades faster in a hot tub, especially uncovered or in sun, so test before every soak and top up more often. In Spa Calc, pick "Chlorine: bleach only, no stabilizer". Whichever method you use, free chlorine has to be at least 3 ppm before anyone soaks.
After-soak dosing
People bring the contaminants, so chlorine demand follows use, not the calendar.
- After a soak, add chlorine to cover what the bathers used. More people and longer soaks need more.
- Test 15–30 minutes after dosing, or before the next soak, and adjust the after-soak amount for next time.
- Over a couple of weeks, your log will show your tub's own pattern, for example "2 people, 20 minutes → about 2 ppm used".
Spa Calc's after-soak suggestion starts from a conservative estimate of what a soak uses; your log shows your tub's real pattern, and the test before the next soak catches any shortfall.
Start-up chlorine (new fill)
- Fill (through the filter housing for acrylic tubs) and start heating.
- Test the fill water. Adjust alkalinity first (often to about 50–80 ppm), then pH to about 7.4–7.6.
- Add dichlor (or bleach, if you run without stabilizer) to reach 3–5 ppm, using the calculator.
- Run the jets with the cover open for a few minutes to mix.
- Test before the first soak.
If the fill water has metals (well water), consider a metal sequestrant before chlorinating. Chlorine can make dissolved iron or copper visible.
Shocking with chlorine
- Chlorine shock is simply a larger dose that oxidizes built-up contaminants. Many owners shock weekly and after heavy use. A common approach is to raise FC well above normal for a short time, using the calculator's shock mode.
- Non-chlorine shock (MPS) oxidizes without raising chlorine. It's quick to soak after, but it doesn't sanitize and can show up as "combined chlorine" on DPD tests.
- Wait before soaking until chlorine is back in your normal range. See hot tub shock: how long to wait.
Chlorine too high?
Overshooting is easy in a small tub. If free chlorine is well above your normal range:
- Don't soak until it's back to 5 ppm or less, and never above 10 ppm.
- Open the cover and run the jets. Heat, aeration and light help chlorine dissipate, often within hours.
- Don't add more at the next soak. Let it fall into range, then resume normal after-soak doses.
- If it's extremely high, partially drain and refill with fresh water, then rebalance.
- Next time, weigh the dose. The table above per 1 ppm makes it easy.
Common mistakes
- Using pool trichlor tablets. They're acidic and add a lot of CYA, which is wrong for spas.
- Dosing by the cap. Products and strengths vary. Weigh or measure in millilitres.
- Ignoring pH and alkalinity. High pH makes chlorine less effective and causes cloudiness and scale.
- Assuming the inflatable volume. "4-person" tubs vary widely in litres. Use your model's figure.
- Mixing chemicals. Add one at a time, with the jets running, and never combine products before they're in the water.
Get the dose without the maths
Spa Calc gives you the chlorine dose for your tub and product in grams, scoops or millilitres. It tracks your dichlor running total and tells you when to switch to bleach. It suggests an after-soak top-up, and its ready-to-soak check compares your readings with the soak range: soak now, wait, or add this first.
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Safety
- Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
- Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
- Add chemicals to water, never water to chemicals.
- Store chemicals sealed, cool and dry, apart from each other.
- Read and follow the product label. It always comes first.