Total alkalinity is the one water chemistry number most new hot tub owners skip over, and it is the one that quietly causes the most problems. Once you understand what it does, the target range makes immediate sense, and keeping the water balanced becomes a lot easier.
What is total alkalinity?
Total alkalinity measures how much alkaline material is dissolved in your water, expressed in parts per million (ppm). In practical terms, it acts as a buffer for pH. When alkalinity is in the right range, your pH stays relatively stable even after adding chemicals, introducing fresh water, or soaking. When alkalinity is off, pH becomes unpredictable and difficult to hold.
Think of alkalinity as the shock absorber. pH is what you feel on your skin and what the sanitizer needs to work properly. Alkalinity is what stops pH from swinging wildly every time something changes in the water.
The target range: 80–120 ppm
The recommended total alkalinity for a hot tub is 80 to 120 ppm, with 100 ppm being a comfortable midpoint. Most test strips include alkalinity alongside pH, so you can check both at the same time.
| Reading | What it means | What to do |
|---|---|---|
| Below 80 ppm | Low alkalinity — pH will swing unpredictably | Add alkalinity increaser (sodium bicarbonate) |
| 80–120 ppm | Target range — pH is buffered and stable | Nothing needed |
| Above 120 ppm | High alkalinity — pH climbs and resists adjustment | Add pH decreaser (sodium bisulphate) in small doses |
What happens when alkalinity is too low
Low alkalinity removes the buffer that keeps pH stable. The result is pH bounce — the pH swings up and down rapidly, sometimes several points in a single day. This creates a cascade of problems:
- Eye and skin irritation. Water that is too acidic stings eyes and strips oils from skin. Even short soaks become uncomfortable.
- Ineffective sanitizer. Chlorine works best between pH 7.2 and 7.8. When pH drops below that range, chlorine loses most of its effectiveness — you are adding it but it is not doing the job.
- Equipment corrosion. Acidic water attacks metal fittings, pump seals, and heater elements over time. This is the kind of damage that is expensive and entirely preventable.
- Difficult balancing. When you try to fix pH with low alkalinity, the adjustment overshoots and then swings the other way. You end up chasing the balance back and forth instead of landing it.
What happens when alkalinity is too high
High alkalinity pushes pH upward and makes it resistant to correction. Above 120 ppm you will find:
- Persistently high pH. The water becomes basic, which reduces the effectiveness of chlorine and can cause mild eye irritation on the opposite end of the scale.
- Cloudy water and scaling. High alkalinity, especially combined with high calcium hardness, leads to calcium carbonate deposits on the shell, jets, and equipment. The water may turn hazy white-grey.
- Sluggish pH adjustment. Lowering pH when alkalinity is very high requires large doses of pH decreaser and multiple rounds of treatment.
Always adjust alkalinity before you adjust pH. Because alkalinity buffers pH, getting alkalinity into range first often brings pH close to its target with minimal or no additional adjustment. Fixing pH first, when alkalinity is off, is working against the chemistry.
How to adjust alkalinity
Raising low alkalinity
Add sodium bicarbonate (sold as alkalinity increaser or total alkalinity up). Dissolve it in a bucket of warm water first, then pour it around the perimeter of the tub with the jets running. Wait 30 minutes, then test. Add more in small increments rather than overdosing in one go.
Lowering high alkalinity
Add sodium bisulphate (pH decreaser) in small doses with the pump running. This also lowers pH, so test both after each addition. Because both parameters move together, it takes a few rounds of small adjustments to land on the right spot without overcorrecting.
How often should you check it?
Test alkalinity every time you test the rest of your water — weekly is a good cadence for most owners. It tends to be more stable than pH from week to week, so you will not always need to act on it. After a full water change or after heavy bather loads, a test is especially worthwhile because both situations can shift the balance quickly.
The short version
Total alkalinity is the buffer that keeps your pH stable. Keep it between 80 and 120 ppm, adjust it before you touch the pH, and the rest of water balancing becomes significantly easier. Low alkalinity causes pH swings, irritation, and corrosion. High alkalinity causes cloudy water, scaling, and pH that resists correction. Test it weekly alongside everything else and you will rarely have to deal with either problem. For a full overview of water chemistry in one place, see our complete water balance guide.