Hot Tub Chemicals for Beginners: What You Need

For a 400-gallon tub on fresh water, start-up is usually about 11 g (0.4 oz) of 56% dichlor to reach 4 ppm chlorine, plus baking soda and acid only if your test says so.

The 5 things you need (and what you can skip)

Spa aisles sell dozens of bottles. You need five kinds of product, and each has one job:

The short list
What	Typical product	Its job
1. Test kit	Strips for chlorine or bromine, pH, and alkalinity, or a drop kit	Tells you what the water needs. Every dose starts here.
2. Sanitizer	Chlorine: dichlor granules, liquid chlorine, or plain bleach. Or bromine: sodium bromide plus tablets	Kills germs. Chlorine 3–4 ppm (or up to your label's limit) or bromine 4–6 ppm.
3. pH down and pH up	Dry acid (sodium bisulfate) and soda ash (sodium carbonate)	Keeps pH at 7.4–7.6 for comfort and to protect the heater.
4. Alkalinity increaser	Baking soda (sodium bicarbonate)	Holds total alkalinity at 80–120 ppm so pH stays steady.
5. Shock	MPS (potassium monopersulfate, non-chlorine) or chlorine shock	Breaks down sweat and lotions; reactivates bromine.

"Spa" products and pool products often contain the same active ingredient in smaller packages. "Alkalinity increaser" is baking soda; "pH decreaser" is usually sodium bisulfate. Compare the active ingredient and percentage on the label, not the name on the front.

What you can usually skip at the start:

Stabilizer (cyanuric acid): the CDC recommends against it in hot tubs.
Clarifiers and foam removers: they treat symptoms. Cloudy or foamy water usually means low sanitizer, a dirty filter, or old water.
Calcium increaser: only if your tap water tests below about 100 ppm calcium hardness, or your tub's manufacturer calls for it.
Metal remover: only if you fill from a well or your water has iron or copper.
Scented additives: fragrance oils can cause foam and use up sanitizer.

First fill: step by step

Do this with the heater on and the jets running. Wait about 30 minutes between steps so each product mixes before the next.

Clean before you fill. Wipe the shell, rinse the filter, and, for a used tub, run a jet line cleaner through the old water before draining (follow its label).
Fill through the filter well with a garden hose. A hose filter helps if your water has metals.
Know your gallons. Check the spec sheet or time the fill with a bucket. All doses depend on it.
Test the fresh water: chlorine or bromine, pH, total alkalinity (TA), and, if you can, calcium hardness (CH).
Alkalinity first. Bring TA into 80–120 ppm. In a 400-gallon tub, raising TA from 60 to 90 ppm takes about 76 g (2.7 oz) of baking soda.
Then pH. Bring pH into 7.4–7.6 with small doses of dry acid or soda ash. Retest after 30 minutes.
Calcium if needed. If your water is very soft (below about 100 ppm), raise it with calcium chloride. In a 400-gallon tub, 109 g (3.8 oz) of 77% calcium chloride adds about 50 ppm. Dissolve it in a bucket of tub water first. It gets hot.
Sanitizer. Chlorine: raise it to 3–4 ppm. Many labels set 4 ppm as the limit for soaking, so check yours. From zero to 4 ppm in 400 gallons takes about 11 g (0.4 oz) of 56% dichlor, or 98 mL (3.3 fl oz) of 6% bleach. Bromine: build up a reserve of sodium bromide (the bromide bank) and activate it with shock, as described on the bromine start-up page.
Retest the next day before the first long soak. Fresh water often shifts pH and TA for a day or two.

Hot tub chemical calculator

Sanitizer

Hot tub volumeDon't know it? Use the volume calculator.

Chlorine (FC)ppm

Total alkalinity (TA)ppm

Calcium hardness (CH) (optional)ppm

Product you have

Acid you have

More options (total chlorine, temperature, and swim limit)

Total chlorine (TC) (optional)ppmOptional. Needed to see combined chlorine (CC = TC − FC).

Water temperature (optional)°FOptional. Used for water balance (LSI).

Swim limit on your chlorine label (optional)ppmThe "do not swim above" number on your chlorine label. Many say 4 ppm. The label wins over any calculator.

Result for example values. Enter your own numbers.

Don't soak yet

Chlorine is low. After the acid step, wait 30 minutes with the jets running.

Your targets: FC 3–4 ppm (minimum 3; your label's swim limit: 4 ppm), pH 7.4–7.6, TA 80–120. If your product's label lists different levels, the label wins.

Add chlorine

0.3 oz (about 1½ tsp)8.1 g

Dichlor granules 56%

FC 1 → 4 ppm

Chlorine is below the hot tub minimum (3 ppm).

Also adds about 2.7 ppm stabilizer (CYA).

Wait 30 min with the jets running.

Lower pH: add acid

0.3 oz9.2 g

Dry acid 93.2%

pH 7.8 → 7.5

pH is above 7.6.

Retest FC and pH in 30 minutes.

Estimates, not guarantees. Always follow the product label. How we calculate

The calculator above turns your own first test into these steps, in order and sized for your tub. For more detail on the numbers, use the full hot tub calculator page. Always follow the product label.

Weekly routine

Every time you soak: check the sanitizer before you get in, and top it up after. The CDC advises testing chlorine and pH at least twice a day when a pool or hot tub is in use. For chlorine amounts, see how much chlorine to add.
Two or three times a week if the tub sits unused: test sanitizer and pH.
Weekly: test TA; shock (MPS or chlorine); rinse the filter with a hose.
Monthly: soak the filter in a filter cleaner; wipe the waterline.
Every few months: drain, clean, and refill.

If a weekly shock doesn't keep the water clear, see cloudy or green hot tub. The full routine is also on the hot tub maintenance schedule.

Test kit: strips or drops

Strips are fast and good enough for daily checks. Their limits: color steps are wide (for example, TA in steps of 40 ppm), they fade when stored in heat or humidity, and they are hard to read in poor light. Keep the lid closed and check the expiration date. Our guide to reading test strips covers how to match colors.

Drop kits (DPD or FAS-DPD for chlorine, titration for TA and CH) take a few minutes but give finer readings. They are the better tool when something is wrong or when you set up a new fill. Many owners use strips daily and drops weekly.

Whatever you use, test water from elbow depth, away from a jet, before adding anything.

Mistakes beginners make

Using pool-sized amounts. A 400-gallon tub needs about 1/37 of the dose for a 15,000-gallon pool. A "capful" can be far too much.
Adding several products at once, or premixing them in a cup. Add one, wait 30 minutes with the jets on, then add the next.
Closing the cover right after dosing. Leave it open while the product mixes and gas escapes.
Adjusting pH but ignoring alkalinity. If TA is off, pH won't stay where you put it.
Using dichlor forever. Each dose adds stabilizer. After a few months, chlorine works poorly. Drain on schedule or switch to liquid chlorine or plain bleach.
Scented or "splash-less" bleach. Use only plain, unscented bleach with the percentage on the label.
Forgetting the filter. Many "chemical" problems are a clogged filter.
Soaking too hot or too long. Keep the water at 104 °F or lower (CPSC).

Printable start-up checklist

Print this page from your browser. Menus and buttons don't print.

Shell wiped, filter rinsed, tub filled through the filter well.
Gallons known: ______ gal.
First test: sanitizer ____ pH ____ TA ____ CH ____.
TA adjusted to 80–120 ppm.
pH adjusted to 7.4–7.6.
Calcium checked (150–250 ppm target; add only if very low).
Sanitizer up: chlorine 3–4 ppm (or your label's limit) or bromine 4–6 ppm.
Heater set to 104 °F or lower.
Next-day retest done, first soak OK.

Safety

Never mix chemicals with each other, especially chlorine and acid. Add each to the water separately.
Add chemicals to water, never water to chemicals. Wear gloves and eye protection.
Store products sealed, in a cool, dry place, out of reach of children, with acids away from chlorine.
If someone swallows spa chemicals or gets them 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 chemicals do I put in my hot tub when I first fill it?

After testing: baking soda if alkalinity is below 80 ppm, dry acid or soda ash to bring pH to 7.4–7.6, calcium only if the water is very soft, and then your sanitizer up to 3–4 ppm chlorine (or your label's limit) or 4–6 ppm bromine. Add them one at a time, about 30 minutes apart.

Should I shock my hot tub when I first fill it up?

With chlorine, raising it to 3–4 ppm is enough for clean tap water. With bromine, you do shock at start-up, because shock turns the sodium bromide bank into active bromine. Follow your tub manufacturer's manual if it says otherwise.

Do spa chemicals expire?

Some do. Liquid chlorine and bleach lose strength steadily, and much faster in heat. PHTA data show 12.5% chlorine losing half its strength in about 48 days at 95 °F. Granules and tablets keep well if sealed and dry. Test strips and drop reagents have printed expiration dates; out-of-date reagents give wrong readings.

Sources

CDC, Home pool and hot tub water treatment and testing (3 ppm chlorine, testing frequency, no CYA in hot tubs): cdc.gov
CDC, Model Aquatic Health Code 2024, Annex (spa chlorine and bromine ranges): cdc.gov (PDF)
U.S. Consumer Product Safety Commission, hot tub temperatures: cpsc.gov
PHTA fact sheet, Sodium hypochlorite (strength loss in storage): phta.org
Taylor Technologies, K-1005 instruction manual (test methods, adjust alkalinity before pH): PDF