Pool closing chemicals checklist: what to buy and how much
Closing a pool for winter is mostly a chemistry job. The water you put in at closing is what your pool will sit in for five or six months — no pump, no maintenance, no corrections. Getting the chemistry right once, before you cover it, protects your surface, plumbing, and equipment through everything winter throws at it. Here’s what to buy, how much, and the order to add it.
At a glance: what to buy
| Chemical | Generic name | Closing target | ~15,000 gal qty |
|---|---|---|---|
| Baking soda | Sodium bicarbonate | TA 60–90 ppm | 5 lbs |
| Muriatic acid | Hydrochloric acid | pH 7.4–7.8 | 1 gal |
| Soda ash | Sodium carbonate | pH < 7.4 only | 2 lbs |
| Calcium chloride | Hardness increaser | Plaster: 200–400 ppm Vinyl/fiberglass: 150–300 ppm | 5 lbs |
| Liquid chlorine | Sodium hypochlorite 10% | FC = CYA × 40% (min 10 ppm) | 2 gal |
| 60% polyquat | Winter algaecide | Per label for your pool size | 32 oz |
Test your water before buying — you may not need all of these. Add in order: alkalinity first, shock second-to-last, algaecide 24–48 hours after shock. For exact doses, use the pool closing calculator.
One thing you don’t need: CYA
Cyanuric acid does not degrade over winter. Whatever level you close with is what you’ll open with in spring. If your CYA is in the 30–50 ppm range, do nothing. If it’s above 70 ppm, plan for a partial drain at spring opening rather than trying to fix it now.
The closing chemicals
Sodium Bicarbonate Baking Soda
What it does: Raises total alkalinity. Alkalinity is the buffer that prevents pH from drifting. If alkalinity is too low going into winter, pH can crash over months of inactivity — and acidic water attacks plaster, corrodes metal, and etches grout without anyone noticing until spring.
Closing target: 60–90 ppm
How much to buy: 4–8 lbs depending on how far off you are. If your TA is already in range, skip it entirely. Use the baking soda calculator for the exact amount based on your current reading and pool volume.
Muriatic Acid Hydrochloric Acid
What it does: Lowers pH (and lowers alkalinity slightly). Most pools that have been running all summer need pH pulled down before closing — pH tends to drift upward as CO² off-gasses from the water.
Closing target pH: 7.4–7.8
How much to buy: 1 gallon covers most pH adjustments for pools up to 20,000 gallons. Use the pool pH calculator to get the exact dose.
Soda Ash Sodium Carbonate
What it does: Raises pH. If your water is running acidic at the end of the season (pH below 7.4), use soda ash to bring it up. It also raises alkalinity slightly, so it’s useful when both are low.
How much to buy: 2–3 lbs. Many pools won’t need it — test first. If pH is in range, skip it.
Calcium Chloride Hardness Increaser
What it does: Raises calcium hardness. This matters more at closing than most pool owners realize. Water that is too soft will pull calcium out of your plaster surface over the winter months — because chemistry always seeks equilibrium, and low-calcium water in contact with a calcium-rich surface will slowly dissolve it. Calcium hardness is one of the five factors in the Langelier Saturation Index, which determines whether your water is corrosive, balanced, or scale-forming.
Closing target: Plaster/concrete: 200–400 ppm • Vinyl/fiberglass: 150–300 ppm
How much to buy: 3–8 lbs depending on how low your hardness is. If your CH is already in range, skip it. Vinyl and fiberglass pools with CH above 150 ppm do not need this.
Liquid Chlorine Sodium Hypochlorite 10%
What it does: This is the closing shock. You're not targeting a maintenance level — you're putting a high dose of chlorine in to kill anything alive in the water before the pump shuts off for the season. That residual slowly dissipates through late fall, leaving the water protected through the early weeks of winter.
Shock target: CYA × 40%, with a minimum of 10 ppm regardless of CYA
How much to buy: 2–3 gallons of 10% liquid chlorine for a 15,000-gallon pool. Exact quantity depends on your CYA level and how much FC you're starting with. Use the pool shock calculator for your specific pool.
Why liquid chlorine: It dissolves instantly, adds no CYA, and doesn't affect calcium hardness. Cal-hypo (granular shock) also works but adds calcium — factor that in if you've already adjusted CH. For saltwater pools, liquid chlorine only: cal-hypo is incompatible with salt chlorinator housings.
60% Polyquat Algaecide Winter Algaecide
What it does: Suppresses algae growth during the period when the shock residual has dissipated and there’s no active chlorine in the water. Without it, algae can establish through late fall and early spring — not dramatically, but enough that you open to a slightly green pool even if you closed it perfectly clear.
How much to buy: Follow the label for your pool size. For a 15,000-gallon pool, most products call for 32–48 oz. Buy 60% polyquat specifically — it’s non-foaming and leaves no staining residue. Avoid copper-based algaecides: if pH drifts over winter and copper precipitates out of solution, it stains.
Critical timing: Add algaecide 24–48 hours after shocking, once FC drops below 5 ppm. High chlorine rapidly degrades polyquat — adding it right after shock wastes it entirely.
The order matters
Each chemical affects the others. Add them in the wrong order and you’re wasting product or creating false readings. Follow this sequence:
| Step | Chemical | Why This Order |
|---|---|---|
| 1 | Alkalinity (baking soda) | Stabilizes pH so your next adjustment holds; adjust this first every time |
| 2 | pH (muriatic acid or soda ash) | Now that TA is buffering, pH will hold where you put it |
| 3 | Calcium hardness | Doesn’t affect other readings; safe to add after pH |
| 4 | Shock (liquid chlorine) | Last chemistry step; high pH reduces chlorine effectiveness, so fix pH first |
| 5 | Algaecide (60% polyquat) | 24–48 hours after shock, once FC drops below 5 ppm — not before |
Run the pump for at least 30 minutes between each addition and retest before moving to the next step. After shocking, run the pump for at least 24 hours before lowering the water level and blowing lines.
How much to buy by pool size
These are estimates for a typical end-of-season pool where chemistry has been maintained — small adjustments needed, not a full rebuild. Adjust based on your actual test results.
| Chemical | 10,000 gal | 15,000 gal | 20,000 gal | Approx. cost |
|---|---|---|---|---|
| Baking soda | 3 lbs | 5 lbs | 7 lbs | $3–5 |
| Muriatic acid | 0.5 gal | 1 gal | 1.5 gal | $5–10 |
| Liquid chlorine (shock) | 1.5 gal | 2–2.5 gal | 3 gal | $12–18 |
| 60% polyquat algaecide | 24 oz | 32 oz | 40 oz | $15–25 |
| Calcium chloride (if needed) | 3 lbs | 5 lbs | 8 lbs | $6–10 |
| Total | $40–65 |
Buy generic, not branded
“Pool Shock”, “Winter Algaecide”, and “Alkalinity Up” at the pool store are the same chemicals as liquid chlorine, 60% polyquat, and baking soda from a hardware store — at two to three times the price. Every chemical on this list is available at Home Depot, Lowe’s, or a farm supply store.
Closing a saltwater pool (SWG)
A saltwater pool uses the same closing chemicals as a chlorine pool — balance pH and alkalinity, shock with liquid chlorine (not cal-hypo), add polyquat algaecide — with a few additional steps specific to the salt chlorinator:
- Shock with liquid chlorine only. Cal-hypo is incompatible with salt chlorinator housings.
- Remove and store the salt cell indoors. The titanium plates are fragile in freeze-thaw cycles. Clean the cell with a mild acid wash (1 part muriatic acid to 10 parts water) if calcium scale is visible before storing.
- Drain the cell housing and plug the union connections.
- Leave the salt in the water. Salt does not degrade or evaporate. Your salt level at spring opening will be close to what you closed with, adjusted only for any rainfall dilution.
What you don’t need
Pool stores will try to sell you winterizing kits, enzyme treatments, phosphate removers, stain preventers, and clarifiers. For most pools with maintained chemistry, these add cost without meaningfully changing the result. Get the six chemicals above and skip everything else. If you open in spring with problems despite balanced water, that’s when to investigate specialty treatments — not at closing.
Don’t add algaecide too early
The most common closing mistake is adding winter algaecide right after shocking. At high FC levels (the shock residual), chlorine rapidly degrades polyquat. Wait until FC drops below 5 ppm — typically 24–48 hours after shocking with the pump running. The algaecide label will usually remind you; the pool store selling you a kit often won’t.
Get the exact shopping list for your pool
PoolChem Tracker calculates your closing chemical doses based on your actual pool volume and test readings — so you buy what you need and nothing you don’t.
Common questions
How much liquid chlorine do I need to close my pool?
The closing shock target is CYA × 40%, with a minimum of 10 ppm. For a pool with 30 ppm CYA at 2 ppm FC, you need to raise about 10 ppm — roughly 1 gallon of 10% liquid chlorine per 10,000 gallons. For 50 ppm CYA, the target is 20 ppm and you may need closer to 2 gallons per 10,000 gallons. The pool shock calculator gives the exact amount based on your CYA and current FC.
Can I use cal-hypo instead of liquid chlorine?
Cal-hypo works for the shock, but it adds calcium to your water — about 0.7 lbs of calcium per 1 lb of 65% cal-hypo granules. Factor that into your calcium hardness calculation before using it, especially if CH is already near the upper end of the range. For saltwater pools: do not use cal-hypo; it can react with salt chlorinator housings. Use liquid chlorine only.
What is different about closing an above-ground pool?
The same six chemicals apply. Scale all doses to your pool’s actual volume (most above-ground pools hold 5,000–13,000 gallons). Vinyl liner pools should keep calcium hardness on the lower end (150–250 ppm) and skip calcium chloride if fill water already tests above 150 ppm. In most climates above-ground pools don’t need lines blown — draining the equipment and plugging the returns is sufficient.
Why is it important to balance chemistry before closing?
The chemistry you add at closing is the chemistry your pool sits in for 5–6 months with no corrections. Low pH and low alkalinity corrode plaster and metal over that entire period — quietly, continuously, invisibly under the cover. High calcium combined with high pH deposits scale inside heaters, on tile lines, and in plumbing. Neither type of damage shows up until you open in spring, by which point it has been happening all winter. See what is LSI for how all five parameters combine into a single balance measure.
Keep reading
- How to Close a Pool for Winter — the complete step-by-step guide: cleaning, blowing lines, equipment draining, cover installation
- Pool closing calculator — exact chemical doses based on your pool volume and current readings
- Pool shock calculator — CYA-based shock level and liquid chlorine dose
- Pool startup chemicals checklist — the spring companion: what to buy when reopening
- How to open your pool for summer — step-by-step spring opening process
- Pool opening calculator — chemical shopping list based on spring opening condition
- What is LSI? — why water balance protects your pool surface through winter
- How to balance pool water — the four-step sequence for getting all parameters in range