Hot tub chemical calculator: chlorine, bleach or bromine
Hot tubs are small and hot, so doses are small and sanitizer disappears fast. Enter the tub’s volume and what you use; the calculator scales the dose to your tub.
Enter your measurements
More options: Users
- To shock this tub
- 8.5 fl oz (1.1 cups) 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
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 ÷ 60 g per litre (6%) = 101 ml = 3.4 fl oz
- 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).
Quick answers for common sizes
| Example | Answer |
|---|---|
| How much chlorine to add to a hot tub on first use | 1.6 fl oz |
| 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 |
Chlorine dose for this tub, by target (from the level above)
| Target FC | Add |
|---|---|
| 3 ppm | 2.6 fl oz |
| 4 ppm | 3.4 fl oz |
| 5 ppm | 4.3 fl oz |
| 6 ppm | 5.1 fl oz |
| 8 ppm | 6.8 fl oz |
| 10 ppm | 8.5 fl oz (1.1 cups) |
A hot tub is a few hundred gallons of water at about 100°F with several people in it — conditions that use up sanitizer quickly. The CDC recommends at least 3 ppm of free chlorine or 4–8 ppm of bromine, pH 7.0–7.8, water no hotter than 104°F (40°C), and testing before each use.
Chlorine: how the dose is worked out
grams of chlorine = ppm rise × litres of tub water ÷ 1,000 dichlor (g) = grams ÷ (available chlorine % ÷ 100) liquid (ml) = grams ÷ (label % × 10) × 1,000 100 gal = 378.5 L
In 400 gallons, 1 ppm is just 1.5 g of chlorine: about 2.7 g of 56% dichlor or 25 ml (0.85 fl oz) of 6% bleach. Spa dichlor labels give scoop measures per 500 gallons — one says ½ oz per 500 gallons for a 2–3 ppm residual and 1 oz per 500 gallons to shock; by the chemistry, ½ oz in 500 gallons is about 4 ppm, the difference being what the water uses up straight away.
| Tub | +1 ppm dichlor 56% | +3 ppm dichlor 56% | +3 ppm bleach 6% | +3 ppm chlorine 12.5% |
|---|---|---|---|---|
| 250 gal | 0.1 oz | 0.2 oz | 1.6 fl oz | 0.8 fl oz |
| 300 gal | 0.1 oz | 0.2 oz | 1.9 fl oz | 0.9 fl oz |
| 400 gal | 0.1 oz | 0.3 oz | 2.6 fl oz | 1.2 fl oz |
| 500 gal | 0.1 oz | 0.4 oz | 3.2 fl oz | 1.5 fl oz |
Worked pages: how much chlorine to add to a hot tub on first use, how much bleach to put in a hot tub and how much bromine to start a hot tub.
Chlorine or bromine?
- Dichlor granules dissolve fast and are easy to measure, but each dose adds stabilizer, which the CDC recommends keeping out of hot tubs. Drain and refill on the schedule your manual suggests.
- Liquid chlorine or plain bleach adds no stabilizer; it is just harder to measure in small amounts and raises pH a little.
- Bromine works in a different way: bromide in the water is turned into active bromine by an oxidizer, and the used bromine turns back into bromide. It holds up well in hot water and is dosed by tablets in a floater. The CDC recommends 4–8 ppm.
- Never mix chlorine and bromine products in the same floater, and don’t switch sanitizers without draining.
Hot tub routine
- Test sanitizer and pH before each use.
- Add sanitizer with the jets or circulation running and the cover open; wait for it to mix before getting in.
- Keep the temperature at or below 104°F (40°C).
- Drain, clean and refill on the schedule in your manual — small volumes build up dissolved solids and stabilizer fast.
Not sure of the volume? Start with the hot tub gallons calculator. For alkalinity, see how much baking soda to raise alkalinity in a hot tub.
What you’ll need
- 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) - Dichlor granules (sodium dichlor)Budget price
Fast-dissolving stabilized chlorine, the usual hot tub sanitizer. Each dose adds cyanuric acid as well as chlorine.
See options on Amazon (opens in a new tab) - Bromine tablets (BCDMH)Budget price
The usual hot tub bromine for a floater or feeder. Bromine holds up better than chlorine in hot water.
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 should be in a hot tub?
The CDC recommends at least 3 ppm of free chlorine (or 4–8 ppm of bromine) and pH 7.0–7.8. Many spa labels aim for 3–5 ppm. Above 10 ppm, wait for it to fall before soaking.
Can I use pool chlorine tablets in a hot tub?
Not 3 inch pool tablets — they are trichlor, dissolve far too much for a few hundred gallons, and add stabilizer and acid. Use spa products sized for hot tubs.
How much chlorine granules for a 300 gallon hot tub?
To raise chlorine by 3 ppm, about 0.2 oz (6 g) of 56% dichlor. One spa label suggests ½ oz per 500 gallons — about 0.3 oz for 300 gallons — for a 2–3 ppm residual after use.
Does an inflatable hot tub need different chemicals?
No — the same sanitizer levels apply. Inflatable tubs are usually 175–350 gallons, so doses are smaller; check the volume in your manual and follow the label.
Related guides
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.