Enter your spa's gallons and readings — get the bromine granule dose in teaspoons, plus the startup bromide bank if you're on a fresh fill.
The bromine dose math
grams of bromine granules ≈ ppm rise × liters ÷ 960
bromide bank (startup) = 0.5 oz sodium bromide per 100 gallons
Bromine granules (BCDMH, the standard concentrate) deliver close to their full weight as available bromine, and bromine tests read about twice as high as chlorine for the same oxidizing power — which is why the spa target is 3–5 ppm where chlorine's is 1–5. The formula above lands you at target; the floater's slow feed keeps you there.
A worked example
A 350-gallon spa reading 1 ppm, aiming for 4: a 3 ppm rise needs about 4 grams of granules — around three-quarters of a teaspoon, sprinkled in with the jets running. On a fresh fill, the same spa first gets 1.75 oz of sodium bromide (the bank), then a shock to activate it into that first bromine reading.
The routine that keeps it simple
Fill → bromide bank → shock to activate → tablets in the floater → test twice a week and correct with granules using this page. Shock weekly (the shock calculator covers it — MPS works great on bromine spas and reactivates the bank). And because the bank depletes only by splash-out and drains, the quarterly drain and refill is when you re-dose it.
Frequently asked questions
Bromine or chlorine for a hot tub?
Bromine holds up better in hot water, stays effective across a wider pH range, and smells gentler — the trade is higher cost and no protection from sunlight (fine under a cover). Chlorine is cheaper and familiar. Neither is wrong; bromine just suits the always-covered, always-hot spa environment particularly well.
What is a bromide bank and why do I need one?
A starter dose of sodium bromide dissolved at fill time (about half an ounce per 100 gallons). It doesn't sanitize by itself — it's a reserve that any oxidizer (shock, or the bromine granules themselves) converts into active bromine. Skip the bank and your first weeks of readings will be erratic.
Bromine tablets or granules?
Both work and many spas use both: tablets in a floater provide the slow baseline feed (roughly one tablet per 300 gallons, opening adjusted to hold your level), granules make the quick corrections when a test reads low. This calculator doses the granules; the floater is set-and-check.
Can I switch my hot tub from chlorine to bromine?
Chlorine to bromine: yes, easily — add the bromide bank and start dosing bromine; leftover chlorine just converts bromide into bromine. Bromine to chlorine is the hard direction: bromide in the water keeps turning your chlorine into bromine, so that switch requires a full drain and refill.