Salt Water Pool Dilution Calculator

Salt level too high? Find how much water to drain and replace with fresh water to bring it back to your target.

Dilution formula

Salt doesn't evaporate and isn't used up by the chlorine generator, so the only way to lower salinity is to replace salty water with fresh water. Assuming you drain and then refill:

Drain % = (Current − Target) ÷ (Current − Refill ppm) × 100

With tap water at about 0 ppm this simplifies to (1 − Target ÷ Current) × 100.

Example

Your 18,000-gallon pool tests at 4,200 ppm and your manual recommends 3,200 ppm. Drain % = (4,200 − 3,200) ÷ 4,200 = 23.8%. Drain about 4,300 gallons — roughly a quarter of the pool, which in many pools means lowering the level by about a quarter of the average depth — then refill.

Before you drain

• Retest first. A single high reading from a generator display can be caused by temperature or a dirty cell. Confirm with a separate test kit.
• Don't over-drain. Draining too far can damage some pools: vinyl liners can shrink or shift, and in-ground shells can be affected by high groundwater. Many pool professionals advise draining in stages. If you need to remove a large share of the water, ask a local pool professional.
• Check local rules. Many areas regulate where salt water may be discharged, because salt can harm lawns, plants and waterways.
• Stage it. Draining and refilling partly, mixing, and retesting avoids overshooting below your target.
• Rebalance. Fresh water changes pH, alkalinity, calcium hardness and stabilizer (CYA), so retest them after refilling.

Need to raise salt instead? Use the pool salt calculator.