Pool Calculators

Pool Water Replacement Calculator —
How Much to Drain & Refill

Enter your current readings and targets to find the single drain-and-refill percentage that fixes CYA, calcium hardness, salt, copper, and nitrates in one operation.

When should you replace pool water?

Pool water accumulates dissolved solids over time that can only leave through dilution — draining and refilling. The most common triggers are:

• CYA above 80 ppm — chlorine becomes less and less effective as CYA climbs. Above 80–100 ppm, a pool can be chronically underchlorinated even with what appears to be adequate FC on a test. Switching from trichlor tablets to liquid chlorine stops the buildup, but the only way to lower it is dilution.
• Calcium hardness above 400–500 ppm — especially common in desert climates (Phoenix, Las Vegas, inland Southern California) where fill water is already hard and evaporation concentrates it further every week. High CH causes scale on tile lines, cloudy water, and salt cell fouling.
• Copper above 0.3 ppm — from copper-based algaecides or corroding copper equipment (heat exchangers, old fittings). Copper stains surfaces permanently if left too long at high levels.
• Nitrates above 25 ppm — from fertilizer runoff, heavy bather load, or decomposing organics. Nitrates fuel algae and increase chlorine demand.
• Before closing for winter — CYA doesn't break down over winter. Closing at 90 ppm means opening at 90 ppm, which sets a high FC minimum from day one of swim season.

How pool dilution math works

When you drain X% of your pool and refill with fresh water, every dissolved parameter that isn't in your fill water drops by X%. If you drain 40% and refill, CYA drops 40%, copper drops 40%, and nitrates drop 40% — all in the same operation. The only exception is calcium hardness in hard-water climates, where your fill water contains calcium and partially replaces what you removed.

The formula for a simple dilution (fill water level = 0):

Drain % = (current − target) ÷ current × 100

For calcium hardness when fill water has significant CH:

Drain % = (current − target) ÷ (current − fill water CH) × 100

Staged drains for large reductions

Draining more than 50% of a pool in one operation is impractical for most residential pools — it risks liner flotation on vinyl pools and structural stress on inground shells. If the required drain exceeds 50%, a two-stage approach achieves the same result: drain 50% and refill, then recalculate and drain the remaining fraction. Each 50% drain halves every elevated parameter.

Calcium hardness and hard fill water

In cities with hard municipal water — Phoenix, Scottsdale, Las Vegas, Riverside, and most of the Mountain West — your fill water may already contain 200–350 ppm calcium hardness. This matters because every gallon you add to top up evaporation brings more calcium with it, and draining has a reduced effect: you're replacing pool water with water that's nearly as hard.

If your fill water CH is 300 ppm and your target is 250 ppm, no amount of draining will reach 250 ppm — you can only dilute toward your fill water level, not below it. In this case, reverse osmosis (which removes dissolved minerals without discarding water) is the more effective option. Enter your fill water CH in the calculator above to see the adjusted drain requirement for your specific conditions.

What to do after draining

1. Rebalance before shocking. Add alkalinity first (baking soda if TA is low), then pH (muriatic acid or soda ash), then calcium if CH dropped below target. Fresh fill water typically has low pH and alkalinity.
2. Add stabilizer if CYA dropped too far. If draining brought CYA below 30 ppm, add cyanuric acid to bring it back to 30–50 ppm before heavy swim use.
3. Shock the pool. Fresh water dilutes your existing chlorine. Shock to CYA × 40% (shock level) to start the refilled pool clean. Use the shock calculator for an exact dose.
4. Retest all parameters after 24 hours with the pump running. Fresh and refill water take time to mix fully.