How much water to drain to lower CYA, calcium or salt

Replacing a fraction f of the water lowers a level from C to C × (1 − f) + fill × f. To halve CYA with fill water that has none, replace 50% of the water.

Drain and refill calculator

50%

of the water to replace: 7,500 gal

Drain 7,500 gal and refill to bring cyanuric acid (stabilizer) from 100 ppm to 50 ppm.

In two equal rounds: drain and refill 29% (4,393 gal) twice, testing in between.
With a hose running while the pool overflows, you would need about 10,397 gal of fresh water (0.7 times the volume), because new water mixes with old.
Draining an in-ground pool completely can damage it when groundwater is high, and vinyl liners can shift if drained low. Check with a local pool professional before a deep drain.
After refilling, retest everything and rebalance pH, alkalinity and chlorine.

Save this test and get reminded when it is time to retest.

Pool Calc for iPhoneComing soon to the App Store

The formula

Fraction to replace = (current − target) ÷ (current − fill water). If the fill water already contains some of what you are trying to lower, such as calcium in hard tap water, you need to replace more, and you can never get below the fill water’s level.

Two smaller drains or one big one

Two partial drains reach the same target with a little more water in total, but each is safer for the pool. Draining with a hose running at the same time (overflow) wastes the most water, because fresh water mixes with old water before it leaves.

Questions

How much water do I drain to lower CYA from 100 to 50?

Half the water, if your fill water has no CYA.

Is it safe to drain my pool?

Partial drains usually are. Fully draining an in-ground pool can damage it when groundwater is high, and vinyl liners can shift. Check locally first.