Hot Tub Chemical Calculator
How much chlorine, bromine, or shock to add to your spa. Pick the task and enter your water volume to get the dose. The dichlor and sodium bromide amounts are derived from the standard mass balance and cross-checked against established spa-care sources.
Dichlor to add
0.3 oz
Granular dichlor, the usual spa chlorine. It adds cyanuric acid over time, which is one reason spa water gets drained and refilled every few months.
Spa volumes are small, so doses are tiny and easy to overshoot. Add in stages with the jets running to mix, then retest before adding more. Get your real water volume from the hot tub volume calculator first, since every dose depends on it.
Start from your real water volume
Spa doses are tiny, so an accurate volume matters more than in a pool. Work out your gallons with the hot tub volume calculator first. Deciding between sanitizers? See bromine vs chlorine, and the hot tub maintenance guide covers the full routine. For pH and alkalinity, the pH and alkalinity calculators work for spa volumes too.
Frequently asked questions
How much bromine do I add to my hot tub?
To start a bromine spa, add sodium bromide to establish the bromide reserve: about half an ounce per 100 gallons gives a 30 ppm reserve, which this calculator scales to your volume. That is not a sanitizer by itself; you then shock the water to activate bromine and maintain 3 to 5 ppm with tablets in a floater, roughly one 1-inch tablet per 100 gallons.
How much chlorine do I add to a hot tub?
Spas usually use granular dichlor. The amount depends on your water volume and how far free chlorine needs to rise, which this calculator works out. Because spa volumes are tiny, the doses are small, so add in stages and retest. Dichlor adds cyanuric acid over time, a reason spa water is refreshed every few months.
How much shock does a hot tub need?
A typical non-chlorine shock (MPS) dose is about 1.5 to 2 ounces per 500 gallons, used weekly and after heavy use to clear body oils and, in a bromine spa, to reactivate bromine. Product strength varies, so follow the label. A dose of dichlor can also shock a chlorine spa.
What is a bromide reserve?
Bromine spas work from a reserve of bromide ions in the water, established once with sodium bromide. An oxidizer, your shock, converts some of that reserve into active bromine, the part that sanitizes. You rebuild the reserve with sodium bromide each time you drain and refill the spa.
Chlorine or bromine for a hot tub?
Both work. Chlorine, as dichlor, is cheaper and faster. Bromine is more stable in hot water and milder smelling, which many spa owners prefer. The bromine vs chlorine comparison covers the choice. Pick one and stay with it rather than switching back and forth.
Sodium bromide dosing from the mass balance (NaBr is 77.7% bromide; half an ounce per 100 gallons gives a 30 ppm reserve), matching Leisure Time, SpaDepot, and Caldera guidance. Dichlor uses the same cross-checked chlorine math as the pool chlorine calculator. Non-chlorine shock at the commonly published 1.5 to 2 oz per 500 gallons. Follow your product labels and retest.