Hot Tub Alkalinity Calculator
Total alkalinity (TA) is the first number to fix in any spa 鈥?it buffers pH so it stops swinging. Target range: 80鈥?20 ppm. The fixer is ordinary grocery-store baking soda.
How the math works
- Baking soda (lbs) = 螖ppm 脳 Gallons 脳 8.34 梅 1,000,000 梅 0.832
- Rule of thumb: ~1 tablespoon per 100 gallons raises TA about 5 ppm 鈥?in a spa, doses are tablespoon-sized, not bag-sized
- Example: raising a 350-gal spa from 50 to 100 ppm needs 鈮?0.79 lbs 鈮?24 tablespoons (1.5 cups)
Spa-specific dosing rules
- Dose in quarters. Add a quarter of the calculated amount, jets on 10 minutes, retest. In 300 gallons, TA moves fast.
- TA first, pH second 鈥?always. Fixing TA usually moves pH; fixing pH barely moves TA. Correct order saves you a loop.
- Skip "spa alkalinity increaser" at the store 鈥?it's baking soda in a smaller, pricier jug.
- High TA in a spa is common (fill water + dissolving tablets). Lower it with small shots of muriatic acid, then aerate with jets to rebalance pH.
Frequently asked questions
How much baking soda raises TA 10 ppm in a 350 gallon hot tub?
About 2.6 tablespoons (roughly 0.55 oz). Dose a quarter first, retest after 10 minutes of jet circulation.
Why does my pH keep swinging even with good TA?
If TA is in range and pH still swings, look at aeration: air blowers and powerful jets drive pH up by off-gassing CO鈧? Hot tubs need lower TA (50鈥?0 ppm) than pools to hold pH steady.
Can I use the pool alkalinity calculator numbers for my spa?
The formula is identical 鈥?this calculator just repackages the result in tablespoons for spa-sized volumes, and adds the dose-in-quarters guidance that matters more when your pool holds 300 gallons.