When to Dilute your Swimming Pool

When is it necessary to dilute your pool? Let this article be your guide to when (and how much) to dilute your pool.

Covered in this article:

What is dilution?

Dilution is the process of discarding some water and replacing it with fresh water. A total refill is also technically dilution...though it is more of a full reset. Dilution, in our terms, is when you drain (or pump out) a portion of your pool and replace it with fresh water. This could be tap water or rain/snow (like during the winter with a mesh cover). The result is diluted chemistry, where factors like calcium and cyanuric acid typically go down.

Dilution is one of the best solutions to combat accumulation.

pool dilution, drain pool, refill pool, pool hose, chemical accumulation in pools, orenda, shutterstock


How dilution impacts water chemistry

Dilution reduces total dissolved solids (TDS). TDS includes calcium hardness, cyanuric acid, metals, alkalinity, and more. The rate of reduction is directly proportional to the percentage of total water volume removed, minus the tap water chemistry.

Simply draining half the pool does not mean you have exactly half the TDS. You need to account for what the new water is bringing into the pool. That being said, rain and snow dilution contain zero TDS, as rain and snow are distilled water from the sky. Therefore it's pure dilution.

swimming pool dilution from rain


Exact dilution percentages can only be known if you: a) know exactly how much water the pool contains, and b) know exactly how much water you are removing.

Both of these are possible if using water meters to fill the pool and to measure what you are discarding. But apart from such exact measurements, estimation is the best any of us can do. The good news is that dilution does not have to be exact. Any dilution is going to help reduce TDS, so just get as close as you can to your expected target, and test when it's done to see how well you did.

When is dilution necessary?

Factors like pH and alkalinity can be raised and lowered with chemicals, but some factors can only be raised with chemicals, not lowered. Those factors require some form of dilution to be reduced. And we include reverse osmosis (R.O.) filtration in the category of a form of dilution because it puts the filtered water back into the pool.

Those chemistry factors that can only be reduced by dilution are listed below.

Chemistries Reduced by Dilution

Calcium Hardness (CH)

Calcium hardness is the amount of dissolved calcium carbonate (CaCO3) in your water. The most practical way to reduce it is by dilution. CH can also be reduced by reverse osmosis (R.O.) filtration. Or, if you want a mess, you can deliberately throw in soda ash to cloud the pool, precipitate calcium carbonate as dust, and vacuum the dust.

We strongly advise against the last option...it's messy and a headache. But hey, it removes calcium carbonate.

Cyanuric Acid (CYA)

CYA stabilizes chlorine by protecting it from degradation from sunlight. It also does not evaporate or go away naturally. Eventually, it can be broken down and oxidized by enough chlorine and enough time, but this generally takes many months to happen. And even when it does, CYA reduction is slow. So for practical purposes, dilution is still the best way to reduce CYA.

Total Dissolved Solids (TDS)

Anything else that contributes to TDS will also be reduced with dilution. These include metals, long-term byproducts from chemicals (such as algaecides), and salt.

Dilution does not always reduce

What happens if your tap water has a higher level of certain chemistry than the swimming pool does? Let's take phosphates for example. Say you use PR-10,000 and have below 500 ppb of phosphate in your water (because you're proactive and awesome). One day you decide to dilute the water to address another issue, like high CYA (again, because you're proactive and awesome).

Sure, the CYA will go down because there should be zero CYA in the tap water. But if your tap water contains higher phosphates than the pool has, your phosphate levels will rise from dilution.

This is also an issue with metals or high calcium hardness from well water. Whenever tap water has something really high in it (high alkalinity, high calcium, whatever), there's a chance those levels will rise in the pool.


Dilution means replacing whole water with new water. That new water can be from the tap, or from natural rain and snow. The impact of dilution depends on how much you dilute, and what the chemistry of the replacement water is. This is why pools that are winterized with mesh covers (or no covers) have a distinct advantage over pools that never dilute. Rain and snow are distilled water, so the pool benefits from pure dilution during the winter.

covered swimming pool covered in snow and ice

In general, dilution is the most guaranteed way to reduce chemistries like calcium hardness, CYA and TDS. If you have an accumulation problem, dilution may be the best solution.

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