Chlorine vs. hydrogen peroxide for a cold plunge: the dosing math (and the one mix you must never make)

A practical guide for DIY chest-freezer, Ice Barrel, and chiller plunges.

Almost every cold plunge that isn't drained after each use runs on one of two sanitizers: chlorine or hydrogen peroxide. Both work. They are dosed completely differently, measured on different scales, and — this is the part people get wrong — you cannot run both in the same water. Here's how to pick one and dose it correctly.

Never mix chlorine and hydrogen peroxide in the same plunge.

The two react and cancel each other out: peroxide rapidly consumes free chlorine, so you end up with no active sanitizer and a tub you think is protected. If you want to switch from one to the other, drain and refill completely first — don't dose the new chemical on top of the old.

Which should you use?

There's no universal winner — it's a trade-off:

Chlorine — cheap, fast-acting, and the most studied sanitizer there is. Holds a measurable residual for days. The downside is the smell and the fact that cold water makes it easy to over- or under-shoot if you're eyeballing it.
Hydrogen peroxide — no chlorine smell, gentler on skin, popular with the wellness crowd. The downside is that it degrades faster (especially without a chiller), so you re-dose and change water more often.

Pick based on how much you mind the smell versus how often you want to top up. Then dose to the right range below and confirm with test strips — never by eye.

Target ranges

Parameter	Target	Notes
Free chlorine (FC)	1.0–3.0 ppm	Lower steady-state than a hot tub is fine — cold water suppresses bacterial growth.
Hydrogen peroxide	50–100 ppm	Per Plunge.com guidance for cold plunge water.
pH	7.2–7.6	Cold water shows pH drift more visibly, so keep the window tight.

The chlorine dosing math

Free-chlorine dosing scales with the product's concentration. Using the standard pool/spa constants, to raise free chlorine by 1 ppm per 10,000 gallons:

Product	Amount per 10k gal per 1 ppm
Dichlor (56%)	2.4 oz
Cal-hypo (68%)	2.0 oz
Liquid chlorine (12.5%)	10 fl oz

A cold plunge is tiny next to a pool, so the actual doses are small. For a typical 100-gallon plunge (1/100th of 10k gal), raising FC by 1 ppm with dichlor is about 0.024 oz — well under a gram. This is exactly why measuring by eye fails: the difference between "right" and "way too much" is a fraction of a teaspoon. Use a small scale or the per-volume math, then re-test.

After each session, plan on roughly 0.3 ppm of free-chlorine demand per person — far lower than a hot tub because sessions are short and the water is cold.

The hydrogen peroxide dosing math

Common household peroxide is 3%, which is 30,000 ppm. The clean rule of thumb:

1 mL of 3% peroxide per liter of water raises the concentration by ~30 ppm.

So to bring a fresh fill into the 50–100 ppm range, aim for roughly 2.5 mL of 3% per liter. For a 100-gallon (~378 L) plunge, that's about 1 liter of 3% peroxide to reach ~75 ppm.

If you use 35% food-grade peroxide instead, it's about 11.7× more concentrated than 3% — so you need roughly 1/12th the volume. At that strength it's caustic: wear gloves and eye protection, add it to the water (never water to it), and keep it away from skin and clothing.

After each session, budget about 2 ppm of peroxide demand per person and top up to keep within range.

Frost does this math for you

Enter your plunge volume, pick chlorine or peroxide, and Frost gives you the exact dose for your test reading — with a hard "do not mix" interlock built into the chemistry. No subscription, no ads, works offline. It also handles after-session top-ups, water-change cadence, and a "water looking off?" diagnostic.

Get Frost on the App Store

Sources

Always confirm with your own test strips and follow the safety guidance on your chemical's label. This article is general guidance, not a substitute for testing your own water.