Pool Calculators

Pool Closing Chemical Calculator —
Your Winterizing Shopping List

Enter your pool size and current chemistry to get an exact closing chemical shopping list — shock dose, pH, alkalinity, calcium, and algaecide amounts in the right order.

What chemicals do you need to close a pool?

A proper pool closing uses five categories of chemicals — some mandatory, some conditional on your current water readings:

1. Pool shock — raise FC to shock level (CYA × 40%, minimum 10 ppm) to destroy any remaining algae and bacteria before the pump shuts down for winter.
2. pH adjustment — muriatic acid if pH is above 7.6, soda ash if below 7.2. Acidic water over winter etches plaster and corrodes metal equipment. Basic water causes calcium carbonate scaling on tile lines and surfaces.
3. Alkalinity adjustment — baking soda if TA is below 80 ppm. Low alkalinity allows pH to crash over winter, accelerating surface damage even with no swimmers and no pump running.
4. Calcium hardness — calcium chloride if CH is below the minimum for your surface type. Soft water pulls calcium from plaster surfaces and can cause pitting or etching over a long winter.
5. Winter algaecide — 60% polyquat applied 24–48 hours after shocking. Not a substitute for shock; works alongside it to prevent algae growth through cold months when there is no active chlorine residual.

What order do you add closing chemicals?

1. Baking soda (if TA is low) — adjust alkalinity first; it buffers all the adjustments that follow.
2. Muriatic acid or soda ash (if pH is out of range) — after alkalinity is set, adjust pH. Pre-dissolve soda ash in a bucket of water before adding.
3. Calcium chloride (if CH is low) — pre-dissolve in a bucket of water (add water first, then calcium chloride — the reaction is exothermic). Add slowly with the pump running.
4. Pool shock — add last, with the pump running. Broadcast granular shock around the perimeter, or dilute in a bucket of water first. Do not pour directly into the skimmer.
5. Winter algaecide — 24–48 hours after shocking, once FC has dropped below 5 ppm. High chlorine degrades algaecide rapidly, so timing matters.

What should CYA be at pool closing?

Ideally 30–50 ppm. CYA does not degrade over winter — whatever CYA you close with is what you open with in spring. If CYA is above 70–80 ppm at closing, plan for a partial drain at spring opening. High CYA over winter means your minimum FC target next season will be elevated from day one, and continued tablet use will push CYA even higher.

Frequently asked questions

How much shock do I need to close my pool?

Shock target at closing is CYA × 40%, with a floor of 10 ppm FC. Subtract your current FC from that target to find how much you actually need to add. For a pool with CYA 50 ppm and current FC of 3 ppm: shock target is 20 ppm, so you need to raise FC by 17 ppm. The calculator above does this math based on your pool volume.

Can I use liquid chlorine instead of cal-hypo for closing shock?

Yes — liquid chlorine (12.5% sodium hypochlorite) is an excellent closing shock. It adds no CYA and no calcium. The dose is roughly 1.6× higher by volume compared to cal-hypo by weight for the same FC increase, but there are no compatibility issues and it dissolves instantly with no residue.

Do I need to balance pool chemicals before closing if I live in a mild climate?

Yes. Mild-climate pools often get reduced maintenance over winter — not zero maintenance — and unbalanced water still damages surfaces and equipment during that period. If water temperatures stay above 50°F, algae can still grow. A closing shock dose plus balanced chemistry provides a buffer for months of reduced attention.

How long after adding closing chemicals can I cover the pool?

Run the pump for at least 24 hours after shocking before covering. Many pool owners add algaecide the morning after shocking, then cover that evening. Test FC before covering if possible — it should be below 5 ppm to avoid degrading the cover material from prolonged high-chlorine contact.

Does pH matter when closing a pool?

More than most people realize. Closing at pH 7.2–7.6 protects surfaces through winter. Acidic water (pH below 7.0) aggressively attacks plaster, etches grout, and corrodes equipment even with no pump running. Basic water (pH above 8.0) causes calcium to precipitate as calcium carbonate scale on tile lines, inside plumbing, and on heater elements. Both problems are harder to correct in spring than they are to prevent in fall.