The dilution math
dilution % = rain inches ÷ (average depth × 12 + rain inches)
Two inches of rain into five feet of water is about a 3% water change — noticeable on a test kit but not dramatic. The reason storms cause real trouble isn't the dilution: it's the chlorine demand from everything the rain washed in, arriving exactly when clouds stopped the sun from being your backup sanitizer... actually the reverse: the sun stopped burning chlorine but the debris load spiked. Test, don't guess.
A worked example
A tropical downpour drops 5 inches into a 5-ft-deep pool: ~7.7% dilution. CYA 40 becomes ~37, salt 3,200 becomes ~2,950 — the SWG may start grumbling. Meanwhile the chlorine reading after the storm is what actually decides your evening: dose back to target with the chlorine calculator and run the pump overnight to filter what washed in.
Storm checklist
After heavy rain: skim the debris first, check the water level (drain to mid-skimmer if over-full), test chlorine and pH same-day, dose, and run the full report card within a couple of days for the slow movers. Doing it in that order keeps one storm from becoming a green week.