CYA Calculator
Estimate how much cyanuric acid stabilizer to add—or what fraction of pool water to replace—to move toward your tested CYA target.
Calculate your CYA adjustment
Choose a direction. Add stabilizer only when your target is higher; estimate water replacement when it is lower.
Enter values to calculate.
Estimates assume even mixing; verify with a water test after circulation.
How to use this CYA calculator
1. Test and measure
Use a suitable cyanuric-acid test and enter your pool's volume in US gallons. CYA test readings have uncertainty; avoid making large corrections from a questionable reading.
2. Choose the direction
Choose Raise CYA when the target exceeds the current reading. Choose Lower CYA to estimate replacement water when the reading is too high.
3. Check the result
For additions, confirm the product's actual cyanuric acid percentage and directions. For dilution, check replacement-water CYA and local draining restrictions before acting.
Understanding the calculations
Raising CYA: the mass-balance method
One part per million in water is approximately one milligram per liter. A US gallon is approximately 3.785 liters, so a 10,000-gallon pool requires about 0.08345 lb of pure CYA per 1 ppm increase. A 20 ppm increase in that pool therefore takes about 1.67 lb of pure CYA in ideal conditions. For a product that is less than 100% CYA, divide the pure-CYA requirement by its label purity fraction.
The result is a dry cyanuric acid mass estimate, not a universal liquid stabilizer dose. Liquid products can have different compositions and densities; follow their own labels. Do not confuse product purity with the percentage on chlorine products that contain stabilizer.
Lowering CYA: one-step dilution
Adding fresh water alone does not reliably lower CYA unless some existing water leaves the pool. In the simplified well-mixed replacement model, the final concentration equals the fraction of old water retained times its concentration plus the fraction of replacement water times its concentration. That is why the calculator asks for the CYA in your fill water.
Worked CYA examples
| Scenario | Starting values | Estimated result | Interpretation |
|---|---|---|---|
| Raise CYA | 10,000 gal · 20 → 40 ppm · 100% CYA | 1.67 lb (26.7 oz) dry CYA | Approximate mass to add, subject to product label |
| Raise with 95% product | 15,000 gal · 30 → 50 ppm · 95% CYA | 2.64 lb (42.2 oz) product | Higher mass offsets lower purity |
| Lower CYA | 15,000 gal · 100 → 50 ppm · fill water 0 ppm | 50% / 7,500 gal | Idealized single replacement |
| Lower with CYA in fill | 20,000 gal · 100 → 60 ppm · fill water 20 ppm | 50% / 10,000 gal | Fill-water CYA changes the fraction |
Choosing a sensible CYA target
There is no single target that fits every pool. Sanitizer type, manufacturer guidance, sunlight exposure, local regulations and the chlorine level you maintain all matter. In particular, higher CYA can reduce the effectiveness of a given free-chlorine concentration; it is not a substitute for routine testing and proper disinfection. Follow your pool-system manual and applicable public-health guidance rather than choosing a target solely from a generic chart.
For broader chemistry planning, use the pool chlorine calculator after verifying CYA and free chlorine with a suitable test. If you are correcting several water parameters, the chemical dose calculator can help keep those adjustments distinct.
Common mistakes and limitations
- Using an inaccurate pool volume. A 20% volume error causes approximately a 20% dry-CYA dose error. Verify dimensions with the pool volume calculator.
- Confusing weight and fluid ounces. The add-CYA result uses ounces by weight, not liquid volume.
- Assuming fresh water has zero CYA. The lowering model lets you enter the tested fill-water concentration.
- Using a stabilized chlorine product as pure CYA. The calculator does not model CYA added by tablets or dichlor products; see the pool shock calculator for a separate chlorine-dose context.
- Retesting too soon or overdosing. Follow product instructions for circulation and retesting; introduce additions conservatively rather than repeatedly dosing against a lagging test.
- Ignoring drain and refill time. The pool fill time calculator estimates how long a refill may take at measured hose flow.
Frequently asked questions
How many pounds of stabilizer raise CYA by 10 ppm?
In 10,000 US gallons, roughly 0.835 lb of pure cyanuric acid raises CYA by 10 ppm under an ideal mixing model. Adjust for product purity and confirm against the manufacturer's directions.
Can I lower CYA without replacing pool water?
Conventional CYA does not simply evaporate. Water replacement is a predictable dilution approach, although other treatments may be marketed for CYA reduction and their performance varies. Avoid assuming a treatment works without before-and-after testing.
Does topping off a pool lower CYA?
Only to the extent old water is displaced or removed. Topping off after evaporation generally replaces water that left without the dissolved CYA, so it is not equivalent to draining and refilling.
Why does the calculator reject my target?
Raise mode requires a target above current CYA. Lower mode requires a target below current CYA but above the replacement-water CYA; otherwise the chosen dilution cannot reach the target.
How long should I wait to retest after adding CYA?
Use the product's label instructions and your test kit guidance. Dissolution, mixing and test-method variability affect how quickly a stable reading appears. Avoid immediate repeat dosing.
Can I use this for liquid stabilizer?
Not directly. The raising result is dry product mass calculated from the cyanuric acid percentage by weight. Liquid stabilizers need a product-specific concentration and dosing conversion from the label.
Is a 50% drain always safe?
No. The percentage is only a concentration calculation. Pool structure, groundwater, local rules and discharge requirements can make a large drain unsafe; consult a qualified professional.
Related pool calculators and guides
Educational planning estimates only. Test pool water, follow chemical labels and local public-health guidance, and avoid mixing pool chemicals. Large water replacement can be unsafe without professional advice. All volume inputs are US gallons.