Pool salt calculator: methodology

Method behind the Pool salt calculator · Last checked 2026-08-28

The formula

Salt in kilograms = pool litres × (target ppm − current ppm) ÷ 1,000,000. One ppm is one milligram per litre, so raising 50,000 L by 1,000 ppm takes 50 kg — exactly the "50 kg per 50,000 litres" the AstralPool VX manual quotes, and the Zodiac LM3 manual's "25 kg raises 50,000 L by 500 ppm". Bags are rounded up.

Pool volume: rectangle = length × width × average depth × 1,000; round = 3.14 × (diameter ÷ 2)² × average depth × 1,000; oval = 3.14 × (length ÷ 2) × (width ÷ 2) × average depth × 1,000; kidney or freeform ≈ 0.85 × length × width × average depth × 1,000. Average depth = (shallow + deep) ÷ 2.

Brand targets used as presets

Brand / model	Preset target	Range in manual
AstralPool (eQ, VX, E-Series, Halo)	4,000 ppm	never below 3,000; E-Series 4,000–7,000
Zodiac (eXO iQ, TRi, LM3)	4,000 ppm	LM3 maximum safe 13,000
Davey EcoSalt2 standard	4,500 ppm	3,000–6,000
Davey EcoSalt2 L (low salt)	3,000 ppm	1,500–6,000
Waterco Hydrochlor / Electrochlor	5,500 ppm	4,000–6,000
Hayward AquaRite (AU)	3,200 ppm	2,700–3,400
Custom	your number	—

Things the calculator warns about

If the water is below about 16 °C the chlorinator's Low Salt message can be false (Halo manual: "this warning can be ignored" when a test shows salt is correct), so the tool asks for a tested reading, not the display. If the current reading is already above the target it tells you salt is not the problem and points to cell cleaning. If the reading is more than 1,000 ppm over target it explains that salt only leaves with water — partial drain and refill is the only way down.

Add salt at the shallow end with the pump running, brush it about, and retest after 24 hours; Davey's manual warns against tipping it into the skimmer box.

Test vectors (for the build)

Inputs	Result
50,000 L, 3,000 → 4,000 ppm, 20 kg bags	50 kg, 3 bags
8 × 4 m, 1.2/1.8 m rectangle (48,000 L), 2,200 → 4,000 ppm, 25 kg bags	86.4 kg, 4 bags
30,000 L, 4,500 → 4,000 ppm	0 kg — "no salt needed; over target"

Data table: salt-targets.csv (CC BY 4.0).

Sources and manuals checked

Data table: data/salt-targets.csv. Last checked 28 August 2026.