Home Pool Dilution Calculator
Pool Dilution Calculator — Drain & Refill by Test Level
Calculate how much water to replace to lower CYA, calcium hardness, salt or TDS, including the concentration already present in your fill water.
💡 Dilution formula
Replacement fraction = (Current − Target) ÷ (Current − Fill water)
If fill water contains none of the parameter, this simplifies to 1 − Target ÷ Current.
Example: 20,000 gal, CYA 80 → 40 ppm, fill CYA 0 → replace 50% = 10,000 gallons.
💡 Test your water before and after draining with a reliable test kit. Accurate readings are essential for calculating the right drain volume.
Before and after draining — test accurately:
- Liquid reagent test kit — measure CYA, calcium, and chlorine accurately before draining
- Taylor K-2006 FAS-DPD kit — required if you plan to SLAM after the drain
Example calculation with fill water
Pool volume: 20,000 gallons · Current CYA: 80 ppm · Target: 40 ppm · Fill-water CYA: 0 ppm.
Replacement fraction: (80 − 40) ÷ (80 − 0) = 0.50, so the estimate is 10,000 gallons.
If the replacement water already contained 20 ppm CYA, the same target would require a larger fraction: (80 − 40) ÷ (80 − 20) = 66.7%.
CYA dilution reference when fill-water CYA is zero
| Current CYA | Target CYA | Water replaced |
|---|---|---|
| 60 ppm | 40 ppm | 33.3% |
| 80 ppm | 40 ppm | 50.0% |
| 100 ppm | 50 ppm | 50.0% |
| 120 ppm | 50 ppm | 58.3% |
This table assumes replacement water has 0 ppm CYA. Use the calculator above when fill water contains a measurable amount of the parameter being reduced.
One partial drain vs. several smaller water replacements
The calculator's one-step result assumes the measured pool water is replaced with fill water at the concentration you entered and then mixed. If structural or site limits require smaller stages, you can repeat the process: replace a safe amount, circulate thoroughly, retest, then calculate the next stage from the new measured level.
Several smaller drain/refill cycles usually use more total water than one equivalent replacement because each later drain removes some of the fresh water added during earlier stages. That does not make staged replacement wrong; it simply means you should recalculate from the new test result instead of adding the original percentages together.
Example with non-zero fill water
Suppose calcium hardness is 500 ppm, the target is 300 ppm and the fill water is 200 ppm. The required replacement fraction is (500 − 300) ÷ (500 − 200) = 66.7%. Ignoring the 200 ppm calcium in the refill water would underestimate how much replacement is required.
After the refill
Circulate and retest the parameter you were lowering, then retest chemistry that may have changed with the source water, especially pH, alkalinity and calcium hardness. Salt and CYA may also need rechecking when those were the reason for dilution.
When dilution is useful
Dilution is a direct way to reduce dissolved substances that do not have a practical in-pool removal method, including excessive CYA, calcium hardness, salt or TDS in many situations.
It is not a treatment for suspended algae or cloudy particles. If the water is green because of algae, diagnose and sanitize the water rather than assuming a drain is the first step.
How to plan a partial pool drain safely
A mathematically correct drain fraction is not automatically structurally safe. Groundwater pressure, pool construction, liner/shell type, local discharge rules and site conditions can limit how far the water level may be lowered.
Pentair warns against draining pools where high groundwater can cause an empty shell to lift, and specifically notes that fiberglass or in-ground vinyl pools should not be left empty. Before a large water replacement, check your builder/manufacturer guidance and local restrictions; consult a pool professional when conditions are uncertain.
If a large replacement must be split into stages, retest after each refill. Multiple sequential partial drains are less water-efficient than one equivalent replacement because each later drain removes some newly added water.
Why dilution is used for some pool problems
💧 CYA
Water replacement directly lowers CYA when the replacement water contains less CYA than the pool.
🪨 Calcium hardness
Dilution can lower calcium only when the fill-water calcium concentration is below the pool and below the target you want to reach.
🧂 Salt / TDS
The same concentration balance applies to excess salt or TDS when replacement water is lower in that parameter.
⚠️ Drain safety is separate
A 30%, 50% or larger replacement is not automatically safe for every pool. Check construction and groundwater constraints first.
Pool drain and dilution calculator FAQs
How do I calculate how much pool water to drain?
Use replacement fraction = (current level − target level) ÷ (current level − fill-water level), then multiply by pool volume. The simpler formula 1 − target/current is valid only when the replacement water contains essentially zero of the substance being reduced.
How do I lower CYA in a pool?
Partial water replacement is a direct way to lower CYA. Measure the current CYA and, if relevant, the fill-water CYA, then calculate the replacement fraction and retest after refill and circulation.
Can I lower calcium hardness by draining and refilling?
Yes, if the replacement water has lower calcium hardness than the pool. If fill-water calcium is at or above your target, simple dilution cannot reach that target.
Can I lower salt with a partial drain?
Yes, provided the fill water has a lower salt concentration than the pool. Use the current, target and fill-water salinity rather than assuming the fill is salt-free.
Is it safe to drain half of an in-ground pool?
Not automatically. Structural and groundwater risk depends on pool construction and site conditions. Manufacturer guidance can prohibit leaving fiberglass or in-ground vinyl pools empty, and high groundwater can create hydrostatic risk. Check before lowering the water level substantially.
Why can't the calculator reach my target?
If the fill-water concentration is at or above the target, simple drain-and-refill dilution cannot reach that target. Use a lower-concentration source if appropriate or choose a realistic target above the fill-water level.
Can I use several smaller partial drains instead of one large drain?
Yes when structural, groundwater, discharge or equipment constraints require stages. Replace a safe amount, circulate, retest, and calculate again from the new measured level. Several smaller cycles usually use more total water because later drains remove some previously added fill water.
Do I need to rebalance the pool after a partial drain?
Yes. Fill water can change pH, alkalinity, calcium hardness and other dissolved material. After the refill is fully mixed, retest the parameter you were lowering and the other chemistry affected by the source water before adding more chemicals.
Does backwashing lower CYA?
Backwashing lowers CYA only indirectly by removing pool water that is then replaced with lower-CYA fill water. The reduction is proportional to the net water replacement, so ordinary backwashing is usually much smaller than a deliberate partial drain for a large CYA reduction.
How much water should I drain to lower CYA?
Use the concentration-balance formula with your measured current CYA, target CYA and fill-water CYA. If fill water has 0 ppm CYA, lowering 80 ppm to 40 ppm requires replacing 50% of the water mathematically; structural and groundwater limits still determine whether that can be done in one stage.
What if CYA or calcium is still too high after one partial drain?
Circulate the refill water thoroughly and retest instead of repeating the old percentage blindly. Use the new measured level and the current fill-water level in the calculator to plan the next stage.