Same chemistry as pools at one-fortieth the volume — which flips the practical problem. Pool owners under-dose because jugs feel expensive; spa owners overshoot because "a scoop" is calibrated for pools. In 350 gallons, one heaping tablespoon of dichlor is a 10 ppm event.
A worked example
350-gallon spa at 1 ppm, target 4: three ppm of rise needs about 7 grams of dichlor — a level teaspoon and a half. With 6% bleach it's about 3.3 fl oz, two capfuls. That's the whole dose; measure it, because "a splash" in spa volumes is how tubs end up at 15 ppm.
The dichlor-then-bleach rhythm
Every gram of dichlor brings ~0.9 grams of CYA, and in a spa that accumulates within weeks. The community-standard method: use dichlor after a fresh fill until CYA reaches ~30 ppm (a few weeks of normal dosing), then switch to plain liquid chlorine for the rest of the water's life. You get the stabilizer's benefits without the endless climb — and the quarterly drain resets the clock. For the weekly oxidizer burn that clears the water rather than just sanitizing it, the hot tub shock calculator has the dose.
Frequently asked questions
How much chlorine do I put in a hot tub?
For a typical 350-gallon spa, raising free chlorine 1 ppm takes only about 0.7 grams of dichlor — roughly an eighth of a teaspoon. Small water moves fast, which is why teaspoon-precision matters in spas where cups work for pools.
Dichlor or bleach for a hot tub?
Dichlor is the spa standard: it's stabilized, gentle on pH, and easy to dose dry. The catch is it adds CYA with every dose, so many spa folks use the 'dichlor-then-bleach' method — dichlor until CYA reaches ~30, then switch to plain liquid chlorine.
Why does my hot tub chlorine disappear so fast?
Heat, aeration, and bather load in a tiny volume: one person in a 350-gallon spa is like 40 people in a pool. Hot water also off-gasses chlorine faster. Dosing after each soak, not on a calendar, is the rhythm that works.
How long after adding chlorine can I use the hot tub?
For a normal 1–3 ppm top-up with jets running, 20–30 minutes is typical; after heavy shocking, wait until it tests back at or below 5 ppm. Small volumes also settle much faster than pools.