How to Test Cyanuric Acid (CYA) in a Pool
To test cyanuric acid (CYA, the stabilizer that shields chlorine from sunlight), mix equal parts pool water and CYA reagent, pour the cloudy mix into the view tube until the black dot at the bottom disappears, and read the number at that line. If the dot is already hidden at the top line, dilute the pool sample 1 to 1 with tap water and double the result.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (7 sections)
The CYA test is a turbidity test: you judge how cloudy a liquid is by eye. That makes it the least exact test in a typical kit, so how you read it matters. Most chlorine pools aim for 30–50 ppm (see what cyanuric acid does), and a misread by 20 ppm changes your chlorine target.
What you need
- A kit with CYA reagent (Taylor and Trouble Free Pool kits use reagent R-0013), a mixing bottle, and a view tube with a black dot at the bottom.
- A water sample from elbow depth, away from jets and skimmers.
- Tap water for dilution, if the first line hides the dot.
- Daylight. Bright light at your back, in the shade of your body.
Test strips also have a CYA pad, but the color steps are far apart (often 0, 30, 50, 100, 150 and 300 ppm). They are fine for a rough check and not for deciding to drain water. See how to read pool test strips and strips vs drops vs digital.
The turbidity test, step by step
These steps follow Trouble Free Pool's extended directions for its kits. Taylor's kit works the same way but uses its own bottle marks. Read your kit's sheet for the sample volume.
- Fill the mixing bottle to the lower mark with pool water. The marks depend on the kit: 7 mL, 7.5 mL or 15 mL of pool water.
- Add reagent to the upper mark. The mixture should be equal volumes of reagent and pool water.
- Cap the bottle and mix for 30 seconds by turning it over repeatedly. The mix turns cloudy if there is CYA.
- Pour the cloudy mix into the view tube up to the first line (marked 90 or 100).
- Stand outdoors with your back to the sun, hold the tube at waist level, and look down at the dot. Glance; do not stare.
- If you can still see the dot, pour to the next line and glance again. Repeat until the dot is gone.
- Read the number next to the line where the dot is gone. Then use your kit's rule (next section).
If the tube is full and the dot is still clearly visible, CYA is zero. If the dot is only partly hidden at the top, CYA is above zero but below the lowest line (20 or 30 ppm for most kits).
How to read the dot: the reading table
Kits do not use one rule. Taylor's instruction says to lower the liquid until the dot "just disappears" and read the tube at the liquid level. Trouble Free Pool defines the end point as the dot "completely obscured" and tells you to add 10 to the line number, to round up because the scale is not even between lines. Pick the rule that matches the sheet in your kit and use it every time.
| Line number at end point | Taylor rule: read the line | Trouble Free Pool rule: line + 10 |
|---|---|---|
| 30 | 30 ppm | 40 ppm |
| 40 | 40 ppm | 50 ppm |
| 50 | 50 ppm | 60 ppm |
| 60 | 60 ppm | 70 ppm |
| 70 | 70 ppm | 80 ppm |
| 80 | 80 ppm | 90 ppm |
| 90 | 90 ppm | 100 ppm |
The two rules describe different end points ("just disappears" and "completely obscured"), so the same tube can give lines that differ. Do not mix rules. Taylor's K-1720 reads 0, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90 and 100 ppm.
How close can you get? Trouble Free Pool states the precision of the test, done correctly, at about ±15 ppm up to 90 and ±30 ppm for 100 to 200, and says beginners tend to read higher than the true value. A "50" result can really be between 35 and 65. If your next step is costly, such as draining half the pool, run the test twice, ideally on different days.
Dilution for high CYA
When the dot is hidden at the first line (CYA is 90 or more), the tube cannot count higher. Dilute the sample so the reagent sees less CYA, test as usual, then multiply back.
- Fill the mixing bottle to the lower mark with pool water.
- Fill it to the upper mark with tap water, not reagent.
- Cap and mix for 30 seconds, then pour out half so the bottle is at the lower mark again.
- Run the test from the start. Multiply the final answer by 2.
This 1 to 1 method is Trouble Free Pool's. Another version on its forum mixes 10 mL of pool water with 20 mL of distilled water, tests that, and multiplies by 3. The multiplier is always (total parts) ÷ (parts of pool water).
| Pool water : added water | Multiply by | Example: tube reads 60 | Highest result (tube at 100) |
|---|---|---|---|
| 1 : 1 | 2 | 120 ppm | 200 ppm |
| 1 : 2 | 3 | 180 ppm | 300 ppm |
| 1 : 4 | 5 | 300 ppm | 500 ppm |
The multiplier is the same under either reading rule; apply your rule first, then multiply. Taylor says its K-1720 kit covers 0–500 ppm by dilution, which matches the 1 : 4 row. The errors multiply too: a ±15 ppm error on the tube becomes ±30 ppm after doubling, so a high result is an estimate, not a precise value. Trouble Free Pool uses tap water; if your tap water contains CYA, use distilled water instead.
To plan a drain from your result, the CYA drain calculator shows how much water to replace. To pick a chlorine level that matches your CYA, use pool chlorine levels.
Why readings go wrong
- Cold sample. Trouble Free Pool says to warm a sample below 60 °F to at least 60 °F, because the test is affected by temperature.
- Poor light. Strong indirect light with the sun at your back works best. Indoor light, even a bright kitchen, is dimmer, so readings are less reliable.
- Staring at the dot. Your eyes adjust and you pour too far. Look quickly. If you are unsure, pour the liquid back into the bottle and refill the tube.
- Cloudy or murky pool water. This rarely matters, but you can check: fill the tube with plain pool water. If you can see the dot clearly, the water is not changing the result.
- Wrong volumes. The sample and the reagent must be equal. Use the marks on your own bottle.
Test again after adding stabilizer or draining, but wait for the water to circulate for a few hours first. The pool stabilizer calculator gives the amount to add.
FAQ
How do you read a CYA test black dot?
Pour the cloudy mixture into the tube until the dot disappears, and glance down in daylight. Read the line at that level. Use your kit's rule: Taylor reads the line, while Trouble Free Pool adds 10 to round up.
How accurate is the CYA test?
About ±15 ppm up to 90 ppm and ±30 ppm from 100 to 200 ppm when done correctly, according to Trouble Free Pool. Beginners often read too high.
What do I do if my CYA is over 100?
Dilute the sample with tap water 1 to 1 and double the result, or use a larger dilution and multiply by that factor. Then use the drain calculator to see how much water to replace.
Are CYA test strips accurate?
They show only a few steps (0, 30, 50, 100, 150, 300 on many strips), so they cannot tell 40 from 60. Use them for a rough check and use the turbidity test before making decisions.
Do I need to test CYA in a hot tub?
No. The CDC recommends not using cyanuric acid, or chlorine products with it, in hot tubs.
Sources
- Trouble Free Pool, CYA test instructions (steps, +10 rule, dilution, precision, lighting, 60 °F): troublefreepool.com
- Trouble Free Pool forum, CYA dilution reality check (1 : 2 version): troublefreepool.com
- Taylor Technologies, Visual determination cyanuric acid K-1720 (reading steps, ranges, dilution to 500 ppm): taylortechnologies.com
- CDC, Home pool and hot tub water treatment and testing (no CYA in hot tubs): cdc.gov