Verify labels, manuals, local code, and site conditions before acting.

Terms
Back to Playbooks

Water Exchange Calculator

Estimate water exchange for partial drain planning when you need to lower CYA, calcium hardness, salt, or metals and retest after refill.

Before you drain

Treat shell, drain, groundwater, and discharge rules as stop checks before you pump.

  • Manuals, local code, and site conditions win over Poolometer.
  • High groundwater or saturated soil can float a shell or push pool walls inward when the water level drops.
  • Use a qualified pro if the next step could change shell support or drain-cover safety.
  • Retest after refill and full mixing before planning another exchange.

Solve for the exchange percent

Start with the reading you have and the target you want.

Target exchange updates automatically

Target-based exchange

Start with the reading you want to reach.

Exchange percent
75.0%
Exchange volume
15,000 gallons
Projected reading
20.0 ppm
How to use this target

This is a planning estimate for a staged drain and refill, not a guarantee of the exact post-refill reading.

  • Example: dropping from 80 ppm to 20 ppm in a 20,000-gallon pool is a 75% exchange on ideal mixing.
  • Stop after the refill is mixed, then retest before planning a second exchange.
  • CYA, salt, calcium hardness, and metals in the source water can limit how far a refill helps.

Project a known exchange percent

Start with a planned drain/refill percentage and see the new reading.

Projection updates automatically

Percent-based projection

Start here when the exchange percentage is already planned.

Projected reading
60.0 ppm
Exchange volume
5,000 gallons
Reading drop
20.0 ppm
How to use this projection

Use the projected reading to decide whether the planned exchange is worth doing before you start pumping.

  • Example: a 25% exchange is a planning estimate, not a promise that the final reading lands exactly there.
  • Stop and retest after refill and full mixing before stacking another drain or refill.
  • Salt, CH, TDS, and metals in the fill water can blunt the gain from the exchange.

Assumptions

  • The replacement water is treated as a single source reading.
  • The calculator assumes the exchanged water mixes fully before you retest.
  • The math is linear for concentration-style readings only.

Important limits

  • Structural, groundwater, and drain risk varies by vessel and site.
  • Hydrostatic pressure can float a shell or bow/cave pool sides inward when the pool is drained too far.
  • Vinyl, fiberglass, and plaster do not share the same empty-shell tolerance.
  • Check local discharge, sewer, and storm-drain rules before pumping water out.

Source-water caveats

  • Metals in fill water can reintroduce the same stain risk you were trying to reduce.
  • High salt, CH, or TDS in the source water can limit how far a refill helps.
  • Staged exchange may not match a one-shot drain if the water mixes unevenly.