How to raise alkalinity in a hot tub
When you need to know how to raise alkalinity in hot tub water, baking soda is the easy answer. Keep adding sodium bicarbonate, whether plain or labeled as alkalinity increaser, until your strip shows 80 to 120 ppm.
Select your tub and today's alkalinity reading to get the dose.
- Range for total alkalinity
- 80 to 120 ppm
- Baking soda amount per 100 gallons for every 10 ppm
- 6 g (0.22 oz)
- Target level for the dose
- 100 ppm
- Let time pass before dosing again
- A wait of 6 hours
Should you choose baking soda or an alkalinity increaser?
Either option works. Frog notes an increaser works if sodium bicarbonate is the main ingredient, since that raises alkalinity.
Kitchen baking soda is this exact chemical. Hot Spring sells this chemical as pH/Alkalinity Up. Its other name is sodium hydrogen carbonate.
How much sodium bicarbonate should you add for your tub size?
Per 1,000 gallons (3,785 litres), use 0.14 lb (64 g) for every 10 ppm per the CPO dosage tables. Each dose raises the reading to about 100 ppm.
| Tub size | 70 ppm | 60 ppm | 40 ppm | 20 ppm | 0 ppm |
|---|---|---|---|---|---|
| 180 gal (670 L) | 1.2 oz (34 g) | 1.6 oz (45 g) | 2.4 oz (67 g) | 3.2 oz (90 g) | 4 oz (112 g) |
| 250 gal (930 L) | 1.6 oz (47 g) | 2.2 oz (62 g) | 3.3 oz (94 g) | 4.4 oz (125 g) | 5.5 oz (156 g) |
| 290 gal (1,100 L) | 1.9 oz (54 g) | 2.6 oz (72 g) | 3.8 oz (109 g) | 5.1 oz (145 g) | 6.4 oz (181 g) |
| 350 gal (1,330 L) | 2.3 oz (65 g) | 3.1 oz (87 g) | 4.6 oz (131 g) | 6.2 oz (175 g) | 7.7 oz (218 g) |
| 500 gal (1,900 L) | 3.3 oz (94 g) | 4.4 oz (125 g) | 6.6 oz (187 g) | 8.8 oz (249 g) | 11 oz (312 g) |
Similar figures appear elsewhere. Lay-Z-Spa suggests 0.6 oz (18 g) for every 10 ppm in 264 gallons (1,000 litres).
For the increaser Leslie's itself sells, its chart gives a higher figure: 300 gallons (1,136 litres) takes 1.5 oz (43 g). If your label says something else, follow that.
Hot tub alkalinity too low: pH bounce and corrosion
Alkalinity acts as a buffer to prevent pH from jumping. pH can swing from one test to the next once you're under 80 ppm, and Frog calls that pH bounce.
Those swings can corrode or scale parts, says the Hot Spring manual. Frog adds that low alkalinity also pulls pH down, leading to sore eyes and itchy skin.
If pH refuses to stay steady, test alkalinity before you try pH up or pH down products.
Baking soda compared with soda ash
Both substances raise alkalinity, yet they change pH by very different amounts. A study by CPO compares the two.
| Sodium bicarbonate (baking soda) | Sodium carbonate (soda ash, pH up) | |
|---|---|---|
| Main job | Raises alkalinity | Raises pH |
| Effect on pH | Small lift | Big lift |
| Alkalinity per pound | Less | About 60% more |
| Use it when | Alkalinity is low and pH is fine | Both pH and alkalinity are low |
| Watch out | Costs more per pound | Can cloud water that is high in hardness |
Data from: CPO dosage study. See cloudy hot tub water if your water turns hazy. Use the pH up guide if only pH is low.
How to raise alkalinity in hot tub water, step by step
- Start by testing. Check the alkalinity pad using the test strip guide and record the pH as well.
- Measure out the dose. Consult the chart or use the chemical calculator for other sizes.
- Put it in while the jets are on. Sprinkle it slowly into the water.
- Let the jets run 15 minutes before you test. Leslie's waits those 15 minutes before adding any other chemical.
- Pause before you add more. Frog recommends waiting at least 6 hours, then testing again.
- Check your pH level next. When alkalinity hits 80 to 120 ppm, change pH to between 7.4 and 7.6.
Add chemicals one by one, and never mix baking soda with chlorine or pH down in the same cup. Allow each treatment to circulate before adding the next one.
Common questions
How much baking soda goes into a hot tub to raise alkalinity?
Per 100 gallons, use about 0.22 oz (6 g) for each 10 ppm. To bring a 350 gallon tub from 60 ppm up to 100 ppm, you need about 3.1 oz (87 g).
Will increasing alkalinity also raise pH?
Slightly. The pH rise from baking soda is small, so retest pH when alkalinity is in range and adjust it next.
What level should hot tub alkalinity be?
Guides typically suggest 80 to 120 ppm. Check your manual since some brands, like Hot Spring, pick ranges such as 125 to 150 ppm.
Why does my alkalinity keep falling?
Rainwater, sweat, and dirt carried in by soakers are listed by Lay-Z-Spa. As Frog advises, test twice a week and add small doses to top it up.