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Pool Dilution Calculator — Lower CYA, Calcium, Salt & TDS
Calculate exactly how many gallons to drain and refill to lower CYA,
calcium hardness, salt, or TDS to your target level.
Works for any pool size. No sign-ups. Instant results.
💡 Quick answer
Gallons to drain = Pool volume × (1 − Target ÷ Current)
Example: 20,000 gal pool, CYA 80 ppm → target 40 ppm: drain 10,000 gallons (50%)
Enter your numbers below for the exact figure.
💡 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:
Example calculation
A 20,000 gallon pool with CYA at 90 ppm. Target: 40 ppm. Fill water CYA: 0 ppm.
| Input | Value |
|---|---|
| Pool volume | 20,000 gallons |
| Current CYA | 90 ppm |
| Target CYA | 40 ppm |
| Fill water CYA | 0 ppm |
| Gallons to drain | 11,111 gallons (55.6%) |
| CYA after refill | 40 ppm |
Formula: 20,000 × (1 − 40 ÷ 90) = 11,111 gallons. Drain and replace with fresh water, then retest and rebalance all chemistry.
CYA drain and refill reference chart
How much of a 20,000 gallon pool to drain to reach common CYA targets from elevated starting levels. Scale proportionally for other pool sizes.
| Current CYA | Target 30 ppm | Target 40 ppm | Target 50 ppm |
|---|---|---|---|
| 60 ppm | 10,000 gal (50%) | 6,667 gal (33%) | 4,000 gal (20%) |
| 70 ppm | 11,429 gal (57%) | 8,571 gal (43%) | 5,714 gal (29%) |
| 80 ppm | 12,500 gal (63%) | 10,000 gal (50%) | 7,500 gal (38%) |
| 90 ppm | 13,333 gal (67%) | 11,111 gal (56%) | 8,889 gal (44%) |
| 100 ppm | 14,000 gal (70%) | 12,000 gal (60%) | 10,000 gal (50%) |
| 120 ppm | 15,000 gal (75%) | 13,333 gal (67%) | 11,667 gal (58%) |
Based on a 20,000 gallon pool with 0 ppm CYA in fill water. Use the calculator above for your exact pool size and fill water CYA. After refilling, use our CYA/FC calculator to confirm your new chlorine targets.
When a partial drain is the only fix
CYA above 80 ppm
CYA above 80 ppm means the minimum free chlorine needed to prevent algae climbs above 6 ppm. Maintaining that level daily in a large pool becomes expensive and inconsistent. More critically, the SLAM target at CYA 90 ppm is 36 ppm FC — an impractical amount requiring large volumes of liquid chlorine. Draining to bring CYA back to 30–50 ppm is the correct fix.
💡 Switching to liquid chlorine or cal-hypo after the drain stops CYA from accumulating again. Trichlor tablets and dichlor granules both add CYA with every dose.
Calcium hardness above 400 ppm
High calcium causes scaling on pool surfaces, inside equipment, and on salt cells. Scale buildup reduces heater efficiency, clogs filters, and degrades salt cell plates. Unlike CYA, calcium hardness can be slightly managed with sequestering agents, but a partial drain is the only permanent solution. Target 200–400 ppm.
Salt above 4,000 ppm (saltwater pools)
Salt chlorine generators typically operate best at 2,700–3,400 ppm. Excess salt above 4,000 ppm can cause corrosion to metal fittings, ladders, and equipment. It also tastes noticeably salty. A partial drain and refill with fresh water is the only way to lower salt concentration.
TDS above 2,000–3,000 ppm (non-salt pools)
High TDS causes water to look dull or hazy even when all other chemistry is in range. It can reduce chemical effectiveness and accelerate equipment wear. If TDS is elevated and all other parameters are balanced, a partial drain and refill is the recommended solution.
How to safely do a partial pool drain
Before draining
☐ Test and record all current chemical levels — CYA, calcium, alkalinity, pH, FC
☐ Use this calculator to confirm the exact gallons to drain
☐ Check local water authority rules — some require pool water to be dechlorinated before
discharge to storm drains or lawns
☐ For fiberglass or vinyl liner pools: do not fully drain without professional guidance —
shells can shift, float, or crack
☐ Drain during mild weather — avoid draining in extreme heat, which can blister exposed surfaces
Draining and refilling
☐ Use a submersible pump or backwash port to drain the calculated volume
☐ Keep the pump running during draining to circulate the remaining water
☐ Refill immediately — do not leave the pool partially empty overnight if avoidable
☐ Add fill water slowly if possible, pointing the hose away from pool walls
☐ For large drains (50%+), consider draining and refilling in two stages over two days
After refilling
☐ Test all parameters — alkalinity, pH, calcium hardness, CYA, and free chlorine
☐ Rebalance in order: alkalinity first, then pH, then calcium, then CYA, then shock
☐ Use our pool opening calculator to calculate all
doses in one step
☐ Run the pump for 24 hours after rebalancing before retesting
Why dilution is the only fix for some pool problems
🚫 No chemical removes CYA
There is no product that breaks down or removes cyanuric acid from pool water. Any product claiming to "eliminate" CYA chemically is not supported by pool chemistry science. Dilution is the only proven method.
🚫 No chemical removes calcium
Sequestering agents prevent calcium from precipitating out and scaling surfaces, but they do not remove dissolved calcium from the water. The only way to reduce calcium hardness is to replace a portion of the pool water.
✅ Partial drains are safe and effective
You rarely need to fully empty a pool. Draining 30–50% dilutes the problem parameter by the same percentage, preserves most of the water chemistry, and is safe for all pool types when done correctly.
📈 Multiple partial drains work
If one partial drain does not reach the target, you can do a second after refilling and retesting. Two 40% drains reduce CYA by 64% total — often more practical than a single 64% drain.
Pool drain and dilution calculator FAQs
How do I lower CYA in my pool?
The only way to lower CYA is to drain a portion of the pool and refill with fresh water. No chemical removes CYA. Use the calculator above to find the exact gallons to drain for your pool size and current/target levels. After refilling, switch to liquid chlorine or cal-hypo to prevent CYA from accumulating again.
How much water do I need to drain to lower CYA?
Use the formula: gallons to drain = pool volume × (1 − target CYA ÷ current CYA). To lower CYA from 80 to 40 ppm in a 20,000 gallon pool: 20,000 × (1 − 40 ÷ 80) = 10,000 gallons. The calculator handles this instantly for any values you enter.
Can I do multiple smaller partial drains instead of one large one?
Yes, and this is often the preferred approach. Two 33% drains reduce a parameter by 55% total — often achieving the same result as a single 55% drain with less disruption to the pool. Each drain dilutes the remaining concentration, so a second drain is calculated using the post-refill level as the new starting point. Use this calculator again after the first refill.
How do I lower calcium hardness in my pool?
Like CYA, the only reliable way to lower calcium hardness is to drain and refill with lower-calcium fresh water. Use the calculator above with "Calcium Hardness" selected. Note that if your fill water itself has high calcium, the reduction will be partial — enter your tap water calcium in the fill water field for an accurate result. After refilling, use the calcium hardness calculator to verify levels and add calcium chloride if needed.
Do I need to rebalance my pool after a partial drain?
Yes, always. Draining removes not just the target parameter but all dissolved chemistry proportionally. Alkalinity, pH, calcium hardness, CYA, and chlorine will all shift. After refilling, test everything and rebalance in the correct order: alkalinity → pH → calcium → CYA → chlorine. Our pool opening calculator walks through the full sequence with exact doses.
Is it safe to drain a fiberglass or vinyl pool?
Partial drains (up to 50%) are generally safe for fiberglass and vinyl pools under normal conditions. Never fully drain a fiberglass pool — the shell can pop out of the ground due to hydrostatic pressure from groundwater. Never drain a vinyl liner pool in cold weather — the liner can shrink and crack. When in doubt, consult the pool manufacturer or a local pool professional before draining more than 30%.
What if my CYA is still too high after one partial drain?
Refill, retest, and run the calculator again using the new (lower) CYA reading as the starting point for a second drain. Repeat until you reach the target range of 30–50 ppm. This is more practical than trying to calculate a single drain for very high CYA levels (above 150 ppm), where the required drain volume could exceed the pool's capacity.
Does backwashing lower CYA?
Only very slowly. A typical backwash cycle removes 150–250 gallons — roughly 1–1.5% of a 20,000 gallon pool. Over an entire season, accumulated backwash losses might reduce CYA by 5–10 ppm. This is not a meaningful CYA reduction strategy. A targeted partial drain is far more efficient.