Plan your pool's winter closing chemicals. Calculate algaecide and shock amounts to ensure a clear opening in the spring. Taking the time to balance your water before the winter freeze prevents stains and saves you hundreds on startup chemicals next year.
Before you calculate closing chemicals, fully balance the water while the pool is still warm enough for chemistry to react, then enter your pool volume in gallons or liters along with current readings for pH, total alkalinity, calcium hardness, and free chlorine. The tool uses these to size your final shock dose and winter algaecide. Aim to enter water that is already in range: pH near 7.4 to 7.6, total alkalinity 80 to 120 ppm, and calcium hardness 200 to 400 ppm, because balanced water stored all winter resists staining, scaling, and etching. Also note your water temperature, since closing is most effective once it holds consistently in the cool range where algae growth stalls. If you use a stabilized chlorine, record your cyanuric acid level too. Accurate volume is the master input; an over- or under-stated gallon count throws off every dose, so use a verified volume from a shape-based measurement rather than a builder's round number.
Two doses dominate a close. For a non-chlorine or chlorine shock, the breakpoint target is roughly 10 ppm free chlorine. Using 12.5% liquid chlorine, the rule of thumb is about 10 fluid ounces per 10,000 gallons to raise FC by 1 ppm, so a 20,000-gallon pool needing 10 ppm requires 10 × 2 × 10 = about 200 fluid ounces, near 1.5 gallons (≈5.9 liters). For a long-life polymeric (polyquat) algaecide dosed at roughly 6 ounces per 10,000 gallons, that same 20,000-gallon pool needs about 12 fluid ounces (≈355 mL). Add winterizing chemicals only after the shock chlorine has dropped back below 3 ppm, because high chlorine degrades many algaecides on contact. Run the pump several hours after shocking to circulate, then lower the water below the skimmer per your plumbing plan. Recheck pH one last time and nudge it to about 7.5 before the cover goes on.
Accuracy and sequencing protect both your wallet and your surfaces: water closed out of balance can etch plaster, stain from metals, or bloom into a green swamp that costs far more to clear in spring than a correct close costs in fall. The most frequent mistake is adding algaecide and shock at the same time, which wastes the algaecide; always stagger them and let chlorine fall first. Never mix chemicals in the same bucket or pour different sanitizers together, as this can release toxic gas or cause a violent reaction. Add liquids slowly into water, never water into chemical, and keep dry shock and acid in separate, dedicated scoops. Wear eye protection and gloves, dose in a ventilated area, and store leftovers sealed and frost-free. Because cold water reacts slowly, give each addition time to circulate before testing again, and do a final confirming test of pH and chlorine before sealing the cover so the pool overwinters in genuinely balanced, protected condition.
Wait until the water temperature is consistently below 60°F (15°C). Algae struggles to grow in cold water, ensuring the pool stays clear until spring.
Never drain a pool completely! You only need to lower the water level below the skimmer and return lines to allow for proper plumbing blow-out and plugging.
Yes. A specialized winter algaecide or a long-lasting 'polyquat' helps provide an extra layer of protection as the chlorine levels naturally dissipate over the long winter months.