How much chlorine to add to a hot tub?
Spa doses are kitchen-spoon sized, which is the whole difficulty: a heaped teaspoon instead of a level one is a 40% overdose, and there's no 15,000 gallons to hide it in. The doses below are computed from the products' available chlorine — the same engine the pool calculators use, on a tub's worth of water:
| Tub | +1 ppm FC | +3 ppm FC | +5 ppm FC |
|---|---|---|---|
| 250 gal | ½ tsp dichlor (1.7 g) 0.4 fl oz of 8.25% bleach dichlor adds 0.9 ppm CYA | 1¼ tsp dichlor (5.1 g) 1.2 fl oz of 8.25% bleach dichlor adds 2.7 ppm CYA | 2 tsp dichlor (8.5 g) 1.9 fl oz of 8.25% bleach dichlor adds 4.6 ppm CYA |
| 350 gal | ½ tsp dichlor (2.4 g) 0.5 fl oz of 8.25% bleach dichlor adds 0.9 ppm CYA | 1½ tsp dichlor (7.1 g) 1.6 fl oz of 8.25% bleach dichlor adds 2.7 ppm CYA | 2¾ tsp dichlor (11.9 g) 2.7 fl oz of 8.25% bleach dichlor adds 4.6 ppm CYA |
| 450 gal | ¾ tsp dichlor (3.1 g) 0.7 fl oz of 8.25% bleach dichlor adds 0.9 ppm CYA | 2 tsp dichlor (9.2 g) 2.1 fl oz of 8.25% bleach dichlor adds 2.7 ppm CYA | 1¼ tbsp dichlor (15.3 g) 3.5 fl oz of 8.25% bleach dichlor adds 4.6 ppm CYA |
Level spoons of dichlor at a bulk density of 0.9 g/mL (the manufacturers' figure for granules — a $10 kitchen scale beats any spoon); bleach by volume at 8.25% trade strength. 6% bleach needs 37% more; 12.5% liquid chlorine a third less. Per gallon the math is the pool math, so the pool chlorine calculator agrees with every cell.
What to aim for depends on stabilizer, exactly as in a pool: chlorine's strength is set by the FC-to-CYA ratio, not the FC number alone. A spa runs a little CYA — 20–40 ppm — because unstabilized chlorine in hot water is harsh on skin and burns off fast, and at 30 ppm the target is 4 ppm with 2.5 as the floor. The chlorine/CYA guide explains why; the table below shows what happens when the CYA keeps climbing.
The hot tub industry's "1–3 ppm free chlorine" assumes no stabilizer and a test right before you get in. At 104 °F with two people sweating, a tub at 2 ppm is at zero in twenty minutes — and that's the water you're sitting in. Running 4–6 ppm over 30 ppm CYA gives the same active chlorine as a bare 1–2 ppm, without the harshness, and with enough reserve to finish the soak sanitized. Above 8 ppm, wait for it to fall before getting in.
Dichlor or bleach? The stabilizer problem in small water
Dichlor is the spa chlorine everyone starts with — granular, dissolves instantly, pH-neutral — and it carries 0.91 ppm of cyanuric acid with every ppm of chlorine. In a pool that's a slow season-long creep. In a tub that uses 5 ppm a day it is this:
| Daily FC from dichlor | After 1 week | After 2 weeks | After 4 weeks | After 8 weeks | Switch to bleach on day |
|---|---|---|---|---|---|
| 3 ppm/day | CYA 19 FC target 4 | CYA 38 FC target 4.5 | CYA 76 FC target 9 | CYA 153 past workable — drain | 11 |
| 5 ppm/day | CYA 32 FC target 4 | CYA 64 FC target 7.5 | CYA 127 past workable — drain | CYA 255 past workable — drain | 7 |
| 7 ppm/day | CYA 45 FC target 5 | CYA 89 FC target 10.5 | CYA 178 past workable — drain | CYA 357 past workable — drain | 5 |
CYA from 0.91 ppm per ppm of FC, from dichlor's formula weight; FC target from the same FC/CYA relationship the pool calculators use. Nothing removes CYA except draining water. The last column is the day CYA passes 30 ppm.
At 5 ppm a day, dichlor takes a fresh fill to 30 ppm CYA on day 7 and to 137 ppm by the end of the month, where the FC target is 15.5 ppm and climbing. The fix is to use dichlor only until the CYA is where you want it, then switch to plain unscented bleach — which brings chlorine and nothing else — for the rest of the fill. The calculator counts it down: give it your CYA and it tells you the day. Bleach pushes pH up slowly, so expect a little acid; that trade is cheap against a drain every five weeks.
How much bromine to add to a hot tub?
Bromine is a two-part system, and most dosing advice only mentions one part. The reservoir is sodium bromide, dosed once on a fresh fill to about 30 ppm of bromide — it sanitizes nothing by itself. An oxidizer turns bromide into active bromine: BCDMH tablets or granules bring their own bromine and an oxidizer, while MPS (non-chlorine shock) and even dichlor simply convert what's in the bank. Used bromine falls back to bromide and is re-activated by the next shock — which is why a bromine spa's weekly MPS is not optional, and why bromine "goes down slowly": it isn't leaving, it's waiting.
| Tub | Bromide bank, fresh fill | Activate to 4 ppm (MPS) | +3 ppm from BCDMH | +3 ppm from MPS, bank present |
|---|---|---|---|---|
| 200 gal | 1 oz sodium bromide | 0.2 oz (1¼ tsp) | 3.5 g (0.1 oz) | 0.2 oz (¾ tsp) |
| 300 gal | 1.5 oz sodium bromide | 0.4 oz (1¾ tsp) | 5.3 g (0.2 oz) | 0.3 oz (1¼ tsp) |
| 400 gal | 2.1 oz sodium bromide | 0.5 oz (2¼ tsp) | 7 g (0.2 oz) | 0.4 oz (1¾ tsp) |
| 500 gal | 2.6 oz sodium bromide | 0.6 oz (2¾ tsp) | 8.8 g (0.3 oz) | 0.4 oz (2¼ tsp) |
Bromine as Br2. Sodium bromide at 77.7% bromide for a 30 ppm bank — it lands on the labels' "½ oz per 100 gallons". BCDMH at 98% purity, 66% available bromine; MPS at the 42.8% potassium peroxymonosulfate on spa-shock labels. Hydantoin blends (some tablets) run about 10% stronger per gram than pure BCDMH.
Tablets in a floating feeder are a rate, not a dose — the label's "one 1-inch tablet per 100 gallons" is what holds 2–4 ppm against normal use, and you tune it with the feeder's vents and a test strip. The BCDMH column above is for granules, or for topping up quickly after a heavy soak.
A DPD test measures oxidizing power and prints it in the sanitizer's own units. A bromine molecule weighs 2.25× a chlorine molecule and does the same job, so "4 ppm bromine" is the sanitizing equivalent of about 1.8 ppm chlorine, and a 3–6 ppm bromine range is the 1.3–2.7 ppm chlorine range in disguise. That's also why a bromine spa never needs CYA: bromine isn't protected by stabilizer, and it doesn't need to be under a cover. What it can't do is take sun — an uncovered outdoor spa in daylight is a chlorine spa's job.
How much shock after a soak?
"Shock weekly" is pool advice transplanted. A hot tub's sanitizer demand arrives in lumps — each soak — and the honest rhythm is to replace what each one used, right after. The demand is roughly 9 g of chlorine per person-hour at 104 °F (Trouble Free Pool's rule of thumb), which in a tub is a lot of ppm:
| Soak | Chlorine used | In 250 gal | In 350 gal | In 450 gal | Replace it with |
|---|---|---|---|---|---|
| 1 person, 30 min | 4.5 g | 4.8 ppm | 3.4 ppm | 2.6 ppm | 1¾ tsp dichlor 1.8 fl oz 8.25% bleach 1¼ tbsp MPS |
| 2 people, 1 hour | 18 g | 19 ppm | 13.6 ppm | 10.6 ppm | 2½ tbsp dichlor 7.4 fl oz 8.25% bleach 5 tbsp MPS |
| 4 people, 1 hour | 36 g | 38 ppm | 27.2 ppm | 21.1 ppm | 4¾ tbsp dichlor 14.8 fl oz 8.25% bleach 10¼ tbsp MPS |
Chlorine-equivalent demand; MPS sized to the same oxidizing power at the label strength above. The dose is per soak, not per gallon — the same people leave the same grams behind whatever the tub, which is why a small tub swings so hard. In a bromine spa the MPS line is the dose; it regenerates bromine from the bank. Demand varies with temperature, how clean people are and how long the cover was off; test 20 minutes after dosing and adjust the habit, not the table.
Two people for an hour in a 350-gallon tub is about 13.6 ppm — a day's target gone in one evening, which is why a spa left "at 3 ppm" on Friday is at zero by the time anyone tests it on Sunday. Dose after the soak with the cover off and the jets on for twenty minutes, and the water is right for the next one. Non-chlorine MPS lets you get back in after 15 minutes and doesn't raise the chlorine reading, so it's the choice when the tub is in daily use; dichlor or bleach after a soak is the choice when the reading was low to begin with.
Hot tub chemical levels
| Reading | Range | Aim for | Why the spa number differs from the pool's |
|---|---|---|---|
| Free chlorine | 2.5–8 ppm at CYA 30 | 4 ppm | Set by the FC/CYA ratio; sized to survive a soak |
| Bromine | 3–6 ppm | 4 ppm | = 1.3–2.7 ppm chlorine-equivalent; no CYA |
| CYA (chlorine spas) | 20–40 ppm | 30 ppm | Lower than a pool: no sun, and it climbs fast on dichlor |
| pH | 7.2–7.8 | 7.5 | Same as a pool; jets push it up, so it drifts high |
| Total alkalinity | 50–80 ppm | 60 ppm | Lower than the "80–120" on the bottle — aeration turns high TA into a weekly pH fight |
| Calcium hardness | 130–250 ppm | 150 ppm | Acrylic shell, hot water: enough to stop foaming, low enough not to scale the heater |
FC from the same FC/CYA relationship as the pool tools; TA and CH follow Trouble Free Pool's spa guidance rather than the industry card. pH doses use the pH calculator with your tub's volume — the buffer chemistry is identical, and at 350 gallons the acid dose is a capful.
How often should you drain a hot tub?
Two clocks run at once. The industry's rule for bather load — gallons ÷ 3 ÷ bathers per day — is a proxy for the dissolved solids, sweat and soap that accumulate in water nobody filters out:
| Tub | 1 bather/day | 2 bathers/day | 4 bathers/day |
|---|---|---|---|
| 250 gal | 83 days | 42 days | 21 days |
| 350 gal | 117 days | 58 days | 29 days |
| 450 gal | 150 days | 75 days | 38 days |
The APSP/PHTA water-replacement interval. Halve it for a tub that gets used without showering first; it's a ceiling, not a schedule.
The second clock is stabilizer, and on dichlor alone it's faster: from the 30 ppm switch point, another 15 days at 5 ppm a day puts CYA past 100 and the water needs replacing regardless of how clean it looks. Switching to bleach stops that clock entirely, and a bromine spa never starts it. When you do drain, the refill is cheap — a few dollars of water, priced in the cost to fill calculator — and the start-up sequence is the same every time: fill, balance TA and pH, then either the bromide bank plus an activating shock, or dichlor to 30 ppm CYA and bleach from there.