Pool Salinity Calculator: Raise or Lower Salt
Move your pool from its current salinity to your target: this tool tells you either how much salt to add to raise it or how many gallons to drain and refill to lower it.
Calculator
Drain and refill about 4,000 gallons (20% of the pool) to lower salinity from 4,000 to 3,200 ppm.
Salinity drifts over a season. It creeps up a little as splash-out is topped off with more salted water and as backwash concentrates what is left; it drops when rain overflows the pool or when you drain and refill with fresh water. This tool handles both directions from a single pair of readings.
If your target is above the current reading, the answer is a weight of salt to add. If your target is below it, salt cannot be removed chemically — the only reliable way down is to replace part of the water, so the tool returns the gallons to drain and refill instead.
Formula
To raise salinity (target above current) it is the same solute weight as the Salt Calculator:
salt (lb) = gallons × (target − current) × 0.00000834
To lower salinity (target below current) you dilute by swapping out a fraction of the water:
drain (gal) = gallons × (1 − target ÷ current)
The dilution formula works because draining a share of the pool and refilling with fresh water scales the salinity down by exactly that share. Draining 20% of the water, for instance, removes 20% of the dissolved salt.
Worked example
Lowering: a 20,000-gallon pool reads 4,000 ppm and you want 3,200 ppm:
20,000 × (1 − 3,200 ÷ 4,000) = 20,000 × 0.20 = 4,000 gal
So you drain and refill about 4,000 gallons — one-fifth of the pool. Raising works the other way: to lift the same pool from 2,800 to 3,200 ppm you add 20,000 × 400 × 0.00000834 = 66.7 lb of salt.
Why salinity moves, and how to hold it
Because salt does not evaporate, a well-sealed pool that is topped up with fresh water will slowly fall in salinity as evaporation removes water and leaves the salt behind — then rises again if that make-up water is itself salted. Heavy backwashing and draining after storms are the biggest downward swings. In practice most owners nudge the level a little in spring and leave it alone.
When you need to lower salinity, drain from the bottom (through the waste setting or a submersible pump), not the surface, and refill slowly so you do not stir up the water you are trying to remove. Re-test after a full turnover: a single dilution rarely lands exactly on target, so plan on one or two smaller passes rather than a single large drain, which stresses the liner and wastes water.
Very high salinity is worth chasing down because it can corrode ladders, light rings and heater components and can push a salt cell into a fault state. Very low salinity simply means the generator makes less chlorine, so the fix there is the Salt Calculator. Either way, always trust a manual test over the cell display before you act.
How you measure matters. The number on a salt cell is a conductivity estimate that drifts as the cell ages and scales, so it can read high or low even when the water has not changed. A drop-based salinity test kit or a fresh lab strip is the tiebreaker. When the cell and your test disagree, act on the test, then clean or recalibrate the cell — chasing a phantom high or low reading is the most common reason owners over-drain or over-dose. Log the two numbers a couple of times a season and you will quickly learn how far your particular cell tends to wander.
Frequently asked questions
How do I lower the salt level in my pool?
You cannot remove dissolved salt chemically — you dilute it. Drain a portion of the water and refill with fresh water. To go from 4,000 to 3,200 ppm in a 20,000-gallon pool you replace about 4,000 gallons (20%). Do it in one or two passes and re-test between them.
My salinity keeps rising — why?
Evaporation removes water but leaves salt behind, so if you top up with water that already contains salt, the concentration climbs over time. It also rises when backwashing concentrates the remaining water. Small seasonal dilution keeps it in check.
Is it better to add salt or drain to hit my target?
It depends on which side of the target you are on. If your reading is below target, add salt (cheap and simple). If it is above target, the only reliable route is to drain and refill, because there is no additive that removes salt from water.
How much water do I drain to cut salinity in half?
Halving the salinity means replacing half the water: drain (gal) = gallons × (1 − 0.5) = 50% of the pool. The formula generalizes — to reach any lower target, replace the fraction (1 − target ÷ current) of the volume.
Does temperature affect my salinity reading?
Yes. Many salt cells read lower in cold water, so a pool that looks slightly low on a cool morning can be fine once it warms. Confirm with a manual test at a stable temperature before adding salt or draining.