Need help lowering calcium hardness level


New member
Jul 9, 2007
My CH level is 510 ppm, and I want to lower it to about 250 ppm by partial draining and refilling using water with 100 ppm. My inground plaster pool has about 20,000 gallons.

Can someone help me estimate how many gallons I should drain to achieve my CH goal with the refill water?


Mod Squad
TFP Expert
Apr 4, 2007
SW Indiana
Somewhere between 1/2 (~300ppm) and 3/4 (~200ppm) water replacement will get you there, but that's only if you do it all at once. Smaller drain and refill cycles will increase the amount of replacement needed.

chem geek

LifeTime Supporter
TFP Expert
Mar 28, 2007
San Rafael, CA USA
The formula for continuous dilution is:

Percent Required = -100% * ln(1 - (% Dilution / 100%))

To achieve a 25% dilution requires -100% * ln(1 - 25%/100%) = 28.8% of the total pool water volume being replaced.
To achieve a 50% dilution requires -100% * ln(1 - 50%/100%) = 69.3%.
To achieve a 75% dilution requires -100% * ln(1 - 75%/100%) = 139%.

So you can see that continuous dilution becomes very inefficient to produce significant dilution. Other methods such as using a large plastic sheet covering the pool, draining from underneath and filling from the top, or using large silage bags draining from outside and filling the inside, are more effective and do not waste water.

TECHNICAL DERIVATION (ignore if not interested)

This comes from the fact that continuous dilution is essentially from a small partial incremental dilution done repeatedly

Diluted Fraction Remaining = (1 - f)^n

where "f" is the fraction of new water and "n" is the number of times this is repeated so "n*f" is the total amount of water replaced. This can be rewritten as

Diluted Fraction Remaining = ( (1 + 1/x)^x )^(-n*f)

where x=-1/f and if we let x go to infinity (f goes to zero) for smaller and smaller water replacement done more and more frequently, then this becomes

Diluted Fraction Remaining = e^(-n*f) = e^(-(Total Water Volume Replaced Through Continuous Dilution))

so solving for total water volume we get

Total Water Volume Replaced Through Continuous Dilution = -ln(Diluted Fraction Remaining)

The % Dilution is 100% * (1 - Diluted Fraction Remaining).