How much chlorine to add to a hot tub on first use?
To bring a freshly filled 400 gallon hot tub from 0 to 4 ppm of free chlorine, add about 1.6 fl oz of 12.5% liquid chlorine, with the jets running. Plain 6% bleach works too: about 3.4 fluid ounces.
How to work it out
- In short: work out how much chlorine the rise takes for your tub’s size, then how much of your product holds that much.
- 4 ppm × 1,514 L (400 gal) = 6.06 g of chlorine
- 6.06 g ÷ 125 g per litre (12.5%) = 48 ml = 1.6 fl oz
Fresh fill water has no sanitizer at all, and hot tub water heats up to ideal growing conditions for germs. Getting chlorine into range before the first soak — and keeping it there — matters more than any other step.
Adjust for your project
More options: Users
- To shock this tub
- 4.1 fl oz raises it about 10 ppm; then wait until free chlorine is back to 5 ppm or less before soaking (one EPA-registered chlorine label bars spa use above 5 ppm)
- Drain and refill about every (rule of thumb)
- 67 days
- The drain interval uses a common spa-trade rule of thumb — gallons ÷ 3 ÷ people using it per day = days between drains — not a standard. Your manual’s schedule comes first.
- Hot water and jets burn chlorine off fast. Test before each soak and keep at least 3 ppm (CDC).
400 gallon tub from 0 ppm, 12.5% liquid chlorine, by target
| Target FC | Add |
|---|---|
| 3 ppm | 1.2 fl oz |
| 4 ppm | 1.6 fl oz |
| 5 ppm | 2 fl oz |
| 6 ppm | 2.5 fl oz |
| 8 ppm | 3.3 fl oz |
| 10 ppm | 4.1 fl oz |
The start-up sequence
- Fill the tub, start the circulation, and let it run for a few minutes.
- Test alkalinity and pH. Adjust alkalinity first if it is low (see how much baking soda to raise alkalinity in a hot tub), then pH.
- Add the chlorine: about 1.6 fl oz of 12.5% liquid chlorine (or 3.4 fl oz of 6% bleach) in 400 gallons gives 4 ppm. Pour it in slowly with the jets on and the cover open.
- Heat the water, then test again before anyone gets in. The CDC minimum for hot tubs is 3 ppm.
Hot water and jets use chlorine quickly, and every soak adds more demand. Expect to add a smaller dose after each use, and test before the next soak. With two people using it most days, the spa trade’s rule of thumb (gallons ÷ 3 ÷ daily users) suggests draining and refilling about every 67 days — your manual’s schedule comes first.
- Why not granules? Dichlor granules are sold for spas and are easy to measure, but each dose adds stabilizer (about 3.6 ppm of CYA for this one), and the CDC recommends no cyanuric acid in hot tubs. If you use them, drain and refill more often.
- Bleach instead? Plain 6% bleach works too, and adds no stabilizer — see how much bleach to put in a hot tub.
- Bromine tub? The start-up is different — see how much bromine to start a hot tub.
Different size? Use the full Hot Tub Chemical Calculator (Chlorine, Bleach, Bromine).
What you’ll need
- Liquid pool chlorine (sodium hypochlorite)Budget price
Adds chlorine without stabilizer or calcium. Strength is printed on the label — commonly 10% or 12.5% — and it weakens with age and heat, so buy it fresh.
See options on Amazon (opens in a new tab) - Hot tub test strips (chlorine or bromine)Budget price
Hot tubs move faster than pools; a strip before each soak tells you whether the sanitizer is still there.
See options on Amazon (opens in a new tab) - Measuring pitcher (marked in oz and ml)Budget price
Measure doses instead of glugging from the jug. Keep one for acid and a separate one for chlorine.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much chlorine granules for a 300 gallon hot tub on first fill?
About 0.3 oz of 56% dichlor to reach 4 ppm, adding about 3.6 ppm of stabilizer, which the CDC recommends keeping out of hot tubs.
How much liquid chlorine to shock a 400 gallon hot tub?
About 4.1 fl oz of 12.5% liquid chlorine to reach 10 ppm from zero, then wait until it falls back to 5 ppm or less before soaking.
Other common sizes
- How much bleach to put in a hot tub — 2.6 fl oz
- How much bromine to start a hot tub — 6.4 fl oz of bromide booster
Sources
- CDC Healthy Swimming — Staying healthy in hot tubs (opens in a new tab) — No hotter than 104°F (40°C); chlorine at least 3 ppm, bromine 4–8 ppm, pH 7.0–7.8; no children under 5.
- CDC Healthy Swimming — Home pool and hot tub water treatment and testing (opens in a new tab) — pH 7.0–7.8; free chlorine at least 1 ppm in pools (2 ppm with cyanuric acid) and 3 ppm in hot tubs; no CYA in hot tubs; test twice a day.
- CDC Healthy Swimming — Operating public pools, hot tubs and splash pads (opens in a new tab) — Hot tubs: at least 3 ppm free chlorine or 4 ppm bromine; water no hotter than 104°F (40°C).
- CDC — Model Aquatic Health Code, 5th edition (2024), code (opens in a new tab) — Free chlorine minimum 1.0 ppm (2.0 with CYA, 3.0 in spas); pH 7.0–7.8; CYA at or below 90 ppm; CYA:FC above 45:1 is an imminent hazard; TA 60–180 ppm; turnover tables.
- Leisure Time — Spa 56 chlorinating granules (dichlor) label (opens in a new tab) — 99% sodium dichlor dihydrate, 55% available chlorine; ½ oz per 500 gallons for a 2–3 ppm residual; 1 oz per 500 gallons to shock.
- Leslie’s — Chlor Brite spa shock (dichlor) label (opens in a new tab) — Half an ounce per 500 gallons; 1.5 oz per 500 gallons after use.
- Leisure Time — Reserve (sodium bromide) label (opens in a new tab) — Sodium bromide 32.18%; 4 oz per 250 gallons at start-up; 1 oz per 250 gallons weekly.
- Leslie’s — Bromine Tabs Plus label (opens in a new tab) — 98% active ingredient; 3 tablets per 300 gallons in a floater or feeder.
- SpaGuard — Brominating Tabs (opens in a new tab) — BCDMH 98%; target active bromine 3–6 ppm.
- OxyChem — Soda Bleach Solutions Handbook (re-hosted copy) (opens in a new tab) — Trade percent = grams per litre of available chlorine ÷ 10; weight % NaOCl ≈ 1.05 × weight % available chlorine; strength and specific gravity table.
- US EPA reg. 42177-80 — sodium dichlor label (ProPools copy) (opens in a new tab) — Dihydrate, 56% available chlorine; 2–3 oz per 10,000 gallons daily; 1 lb per 10,000 gallons to superchlorinate.
- CDC — Pool chemical injuries press release (2014) (opens in a new tab) — Never mix pool chemicals, especially chlorine products with acid; add chemical to water, never water to chemical.
- CDC — Pool chemical safety: storage poster (opens in a new tab) — Separate acid and chlorine; don’t mix different types of chlorine; keep chemicals cool, dry and in original containers.