How much chlorine to add to a hot tub
In a 400-gallon hot tub, 8 g of 56% dichlor (about 1½ tsp) raises chlorine from 1 to 4 ppm. Keep it at 3–4 ppm, or up to the limit on your 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 (6 sections)
Per 1 ppm of free chlorine (FC) in 400 gallons, that is 2.7 g (about ½ tsp) of 56% dichlor granules, or 24 mL (0.8 fl oz, about 5 teaspoons) of 6% household bleach. The target for a hot tub is 3–4 ppm. The CDC sets 3 ppm as the minimum for hot tubs, and many spa chlorine labels set 4 ppm as the highest level for soaking. Your label's number is the one to use, and the calculator lets you enter it. If you know the exact percentage on your label, type it into the calculator: bleach sold at 6%, 7.5%, and 8.25% differs by almost a third in dose. Always follow the product label.
Chart: 200, 250, 300, 350, 400, 450, 500, 600, 700 gallons
The chart shows how much product raises free chlorine by 1 ppm. Multiply by the number of ppm you need. Example: a 300-gallon tub at 1 ppm that you want at 4 ppm needs 3 ppm, so 3 × 2.0 g = about 6 g of 56% dichlor.
| Hot tub size | Dichlor 56% | Pool liquid chlorine 10% | Household bleach 6% |
|---|---|---|---|
| 200 gal | 0.05 oz (about ¼ tsp)1.4 g | 0.3 fl oz (1.5 tsp)7.6 mL | 0.4 fl oz (2.5 tsp)12 mL |
| 250 gal | 0.05 oz (about ¼ tsp)1.7 g | 0.3 fl oz (1.9 tsp)9.5 mL | 0.5 fl oz (3.1 tsp)15 mL |
| 300 gal | 0.05 oz (about ½ tsp)2.0 g | 0.4 fl oz (2.3 tsp)11 mL | 0.6 fl oz (3.7 tsp)18 mL |
| 350 gal | 0.1 oz (about ½ tsp)2.4 g | 0.4 fl oz (2.7 tsp)13 mL | 0.7 fl oz (4.3 tsp)21 mL |
| 400 gal | 0.1 oz (about ½ tsp)2.7 g | 0.5 fl oz (3.1 tsp)15 mL | 0.8 fl oz (4.9 tsp)24 mL |
| 450 gal | 0.1 oz (about ½ tsp)3.0 g | 0.6 fl oz (3.5 tsp)17 mL | 0.9 fl oz (5.6 tsp)27 mL |
| 500 gal | 0.1 oz (about ¾ tsp)3.4 g | 0.6 fl oz (3.8 tsp)19 mL | 1.0 fl oz30 mL |
| 600 gal | 0.15 oz (about ¾ tsp)4.1 g | 0.8 fl oz (4.6 tsp)23 mL | 1.2 fl oz37 mL |
| 700 gal | 0.15 oz (about 1 tsp)4.7 g | 0.9 fl oz (5.4 tsp)26 mL | 1.4 fl oz43 mL |
Multiply by how many ppm you need. 1 tsp = 1/6 fl oz. For dichlor, a teaspoon means a level teaspoon of granules (about 5 g); weigh it if you can. Dichlor also adds stabilizer, which builds up in hot tubs until you change the water.
Not sure of your gallons? Every number here scales with volume, so check your tub's real capacity before you trust any chart, including this one.
Dichlor vs liquid chlorine vs bleach in a hot tub
| Product | Pros | Cons |
|---|---|---|
| Dichlor granules (56% or 62%) | Dissolves fast, close to neutral pH, easy to store, long shelf life | Adds about 0.9 ppm stabilizer (CYA) per 1 ppm chlorine; the CDC recommends no CYA in hot tubs |
| Pool liquid chlorine (10% or 12.5%) | No CYA; cheap per ppm | Loses strength in storage, faster when warm; raises pH slightly; sold in large jugs |
| Plain household bleach (6% to 8.25%) | No CYA; sold everywhere in small bottles | Weaker than pool chlorine, so larger doses; must be unscented, with no thickeners or "splash-less" additives |
A common approach: use dichlor for the first week of a fresh fill, when there is no CYA yet, then switch to liquid chlorine or bleach. If you stay on dichlor, CYA climbs with every dose. Each 10 ppm of chlorine you add over time brings about 9 ppm of CYA, and the only way to remove it is to drain. That is one reason tubs on dichlor seem to "need more and more chlorine" toward the end of a fill.
Liquid chlorine loses strength as it sits: PHTA data show 12.5% chlorine losing half its strength in about 180 days at 77 °F and about 48 days at 95 °F. A jug kept in a hot shed may be far weaker than its label says. If your doses keep falling short, test 30 minutes after adding and compare the rise to what the calculator predicted.
After-soak dosing
Bathers use up chlorine fast in a small, hot body of water. The simplest routine is to test right after a soak and bring FC back to 3–4 ppm (or your label's limit). Then leave the cover open with the jets running for about 30 minutes while the dose mixes. Typical after-soak top-ups for +2 ppm:
| Tub size | Dichlor 56% | Liquid chlorine 10% | Bleach 6% |
|---|---|---|---|
| 300 gal | 4.1 g (0.15 oz) | 23 mL (0.8 fl oz, 4.6 tsp) | 37 mL (1.2 fl oz) |
| 400 gal | 5.4 g (0.2 oz) | 30 mL (1.0 fl oz) | 49 mL (1.6 fl oz) |
| 500 gal | 6.8 g (0.25 oz) | 38 mL (1.3 fl oz) | 61 mL (2.1 fl oz) |
How much you need depends on how many people soaked, for how long, and how hot the water was. Treat these as a starting point, and let your test decide. Soaking every day? Test before you get in as well: the CDC advises testing chlorine and pH at least twice a day when a pool or hot tub is in use. If chlorine is very low or the water smells strongly of chlorine, go to shocking the tub instead of topping up.
Chlorine not rising? Check CC and pH
You added chlorine and the reading barely moved. The usual causes, in order:
- Combined chlorine (CC) is high. If total chlorine (TC) is well above free chlorine, sweat, urine, and lotions are using up each dose. Test both: CC = TC − FC. At 0.5 ppm CC or more, shock the tub (the calculator on the shock page works out the dose).
- The product is weak. Old liquid chlorine or bleach can be well below its label strength (see above). Dichlor that got damp may have clumped and lost strength.
- The test is off. Strips fade with age and heat. Above about 10 ppm, some drop tests bleach out and read low or even zero.
- Biofilm in the plumbing. If chlorine disappears within hours in a tub nobody used, slime in the pipes may be using it up. A line cleaner at the next drain usually fixes this. See cloudy hot tub fixes.
- Very high CYA. Months of dichlor can push CYA so high that free chlorine stays weak. Drain and refill.
High pH does not stop the chlorine reading from rising, but it makes chlorine work less well: at pH 8.0 only about 20% of free chlorine is in the active form, compared with about 50% at pH 7.5 (CDC model code). Keep pH at 7.4–7.6. The general chlorine levels guide for pools explains why pools with stabilizer need different numbers than hot tubs.
Safety
- Never mix chlorine with acid, and never mix two chlorine products (for example, dichlor and liquid chlorine) in a cup or bucket. Add each directly to the water.
- Don't soak while FC is above the swim limit on your chlorine label (4 ppm on many labels). Wait with the cover off until it drops.
- Use only unscented bleach without additives. Store chlorine cool and dry, away from acids.
- If someone swallows chlorine 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
How much chlorine do you put in a hot tub?
Enough to hold 3–4 ppm free chlorine, or up to the soaking limit on your label. For a 400-gallon tub, every 1 ppm takes about 2.7 g of 56% dichlor, 15 mL of 10% liquid chlorine, or 24 mL of 6% bleach. After a soak, a top-up of 2 ppm is a typical start. Your test tells you the rest.
How many grams of chlorine for a hot tub?
For 56% dichlor, about 0.68 g per 100 gallons for each 1 ppm. For 62% dichlor, about 0.61 g. So a 500-gallon tub needs about 3.4 g of 56% dichlor per ppm. Use a kitchen scale; spoon measures of granules vary.
Can hot tub chlorine make you sick?
Too little chlorine is the bigger risk: germs such as the bacteria that cause "hot tub rash" grow in warm water when sanitizer runs low, which is why the CDC sets a 3 ppm minimum. Too much can irritate skin, eyes, and lungs, so stay out when FC is above the limit on your label (4 ppm on many labels). A strong "chlorine smell" usually means combined chlorine, not too much free chlorine.
Sources
- CDC, Preventing hot tub rash (Pseudomonas, biofilm): cdc.gov
- CDC, Home pool and hot tub water treatment and testing (3 ppm minimum, testing twice a day, no CYA in hot tubs): cdc.gov
- CDC, Model Aquatic Health Code 2024, Annex (active chlorine share by pH, spa label limit): cdc.gov (PDF)
- PHTA fact sheet, Sodium hypochlorite (strength loss in storage): phta.org
- PHTA fact sheet, Dichlor: phta.org
- Taylor Technologies, K-1005 instruction manual (combined chlorine): PDF