CYA Drain and Refill Calculator
Find exactly how much water to swap to bring stabilizer, calcium, salt, or TDS back into range.
Free. No signup. Works on your phone at the pool.
Test your hose water
Plaster pools: keep it at or under 50%
Drain and refill
Check hydrostatic valve and groundwater before draining a plaster or fiberglass pool. Retest after the refill circulates for a few hours.
Log this reading to the customer's record
Pool Service Log calculates LSI on every stop, saves each reading and dose with a GPS timestamp and photos, and texts the homeowner a service report. When a plaster warranty claim shows up next spring, you have the history.
No credit card. Keep your current billing software.
Frequently asked questions
How do I lower CYA in a pool?▼
Cyanuric acid does not break down or get consumed in normal service, so draining and refilling with fresh water is the only practical method. The percentage to drain is (current minus target) divided by (current minus fill water level).
Is it safe to drain half the pool?▼
Most plaster and fiberglass pools tolerate a 50% or smaller partial drain. Deeper drains risk the shell floating if groundwater is high. Set the max drain per cycle and the calculator splits the job into safe cycles.
Why do multiple partial drains need more total water?▼
Each refill mixes fresh water into water that has already been diluted, so the second drain removes less CYA per gallon than the first. Two 50% drains remove 75%, not 100%.
What CYA should I target?▼
30 to 50 ppm for chlorine pools and 60 to 80 ppm for salt pools. Above 100 ppm, chlorine gets sluggish and you need higher free chlorine to prevent algae.
More free pool tools
LSI Calculator
Langelier Saturation Index with CYA-corrected alkalinity. Corrosive, balanced, or scaling in one tap.
Chemical Dose Calculator
Exact ounces of chlorine, muriatic acid, soda ash, baking soda, calcium, stabilizer, and salt.
Pool Volume Calculator
Gallons for rectangle, round, oval, and kidney pools from a tape measure.