When draining is the only fix
Most pool readings can be pushed up or down with a chemical. A few can’t, because nothing practical takes them out of the water:
- Stabilizer (CYA) doesn’t break down in sunlight or burn off with chlorine. It only leaves with water.
- Salt doesn’t evaporate. Topping up evaporation adds water but no salt.
- Calcium hardness climbs as hard fill water evaporates and leaves its minerals behind.
- Total dissolved solids build up from every chemical you add.
For all four, you replace some of the pool water with fresh water that has less of the problem in it.
The formula
Share to drain = (current − target) ÷ (current − fill water)
If your fill water has none of the substance, this is simply (current − target) ÷ current. If it does, you have to drain more, because the refill brings some back.Draining in several smaller rounds reaches the same level but uses more water in total. For n equal rounds:
Share each round = 1 − ((target − fill) ÷ (current − fill))1/n
Worked examples
Lower CYA from 120 to 50 ppm in a 15,000 gallon pool
- Share = (120 − 50) ÷ 120 = 58.3%, about 8,750 gallons
- Split into 3 rounds instead: drain 25.3% each time, 3,796 gallons per round, 11,389 gallons in total
Lower calcium from 600 to 400 ppm when tap water has 100 ppm
- Share = (600 − 400) ÷ (600 − 100) = 40%
- Without accounting for the tap water you would have drained only 33%, and ended up at 433 ppm instead of 400.
Where CYA ends up after one drain
Drain safely
- In-ground pools and groundwater. An empty or half-empty shell can be pushed up by groundwater pressure after heavy rain. Avoid big drains when the ground is wet, and talk to a local pool pro before draining most of an in-ground pool.
- Vinyl liners. A liner that dries out can shrink and wrinkle. Several partial drains that keep the liner covered are the safer route.
- Above-ground pools. Walls rely on the water for support. Drain in stages rather than all at once.
- Where the water goes. Many towns don’t allow chlorinated or salty water into storm drains. Let chlorine drop to zero first, and check local rules before you drain.
Use the pool volume calculator first if you’re not sure how many gallons you have. The percentage doesn’t depend on it, but the gallons and the inches to lower do.
Frequently asked questions
How much water should I drain to lower CYA?
Divide the drop you need by the current level. To go from 120 to 50 ppm, drain 58% of the water, assuming your fill water has no CYA, which tap water usually doesn’t.
Can I lower CYA without draining?
Some products claim to break CYA down biologically, with mixed results. Draining and refilling is the method that always works.
Is it better to do one big drain or several small ones?
One big drain uses the least water. Three smaller rounds to the same target used 11,389 gallons in the example above versus 8,750 for one drain. Small rounds are safer for vinyl liners and above-ground pools.
How do I measure how far to lower the water?
Multiply the share to drain by the average depth. The calculator shows it in inches or millimetres, assuming fairly straight walls.
Does rain lower stabilizer or salt?
Only if water leaves the pool. Rain that raises the level and then overflows, or that you pump off, carries some CYA and salt out with it.
How long after refilling can I retest?
Run the pump for a full turnover, usually 6 to 8 hours, so the new water is fully mixed. Then retest everything, because the fresh water also dilutes chlorine, alkalinity and calcium.
Sources
- Cyanuric Acid (fact sheet 6). Oregon Health Authority.
- Model Aquatic Health Code, 5th edition (2024). CDC.
- Pool Water Efficiency (WaterSense guide EPA-832-F-22-015), September 2022. U.S. Environmental Protection Agency.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.