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Drain / Refill to Lower CH

Calculate how much water to replace to reach your target Calcium Hardness (CH).

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If you don’t know your fill-water CH, test it — it matters a lot for this math.
Recommended water replacement:
Practical tips:
  • If replacement is large, consider multiple partial drains to reduce risk (liners, hydrostatic pressure, plaster stress).
  • After refilling, circulate several hours and retest CH.
  • If your fill water CH is high, CH will creep back up — CSI management may be a better long-term strategy.
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Safety note: Large drains can risk liner shifting/floating and structural issues. If unsure, consult a pro.

How the Drain and Refill Calculator Works

This calculator estimates how much pool water must be replaced to lower Calcium Hardness from the current level to a selected target. It uses the pool volume, current CH, target CH, and the CH of the replacement water.

The fill-water reading is essential. If the replacement water already contains significant calcium, a larger drain may be needed, and some targets may not be possible.

Why Calcium Hardness Builds Up

Calcium enters through fill water, calcium-based chlorine products, and some pool chemicals. Water evaporates, but calcium does not. As fresh water is added to replace evaporation, the calcium concentration can slowly climb.

Backwashing, splash-out, leaks, and draining remove calcium because they remove actual pool water.

When Water Replacement May Help

  • CH is too high to manage comfortably with pH and CSI control.
  • Scale is forming on surfaces, salt cells, or heaters.
  • Fill-water CH is lower than the pool’s current CH.
  • A partial drain can also lower CYA, salt, or other dissolved material.
  • Reverse osmosis is unavailable or impractical.

Common Drain and Refill Mistakes

  • Skipping the fill-water CH test.
  • Draining too much from a vinyl-liner pool.
  • Ignoring high groundwater or hydrostatic pressure.
  • Expecting one partial drain to reach an impossible target.
  • Failing to circulate and retest after refilling.

Frequently Asked Questions

Can high CH be lowered without draining?

CH cannot be reduced by ordinary pool chemicals. Reverse osmosis may be available in some areas. Otherwise, water replacement is the usual method.

Why can’t I reach a target below my fill-water CH?

Replacement water sets the practical lower limit. Refilling with water at 300 ppm CH cannot produce a pool at 200 ppm through dilution alone.

Should a large drain be split into smaller drains?

Often, yes. Multiple partial replacements can reduce risk, especially for vinyl liners, high groundwater, and pools with structural concerns.

What should I test after refilling?

Retest CH, pH, Total Alkalinity, CYA, salt, and sanitizer after the water has circulated thoroughly.

Pool Gal Pro Tip 💦

Test the hose water before you drain anything. Your replacement water may be the reason the calcium climbed in the first place.