Should I adjust CYA (stabilizer) before closing?

For outdoor pools, you do not need to add stabilizer at closing — CYA remains in the water over winter and will still be there in spring. However, if CYA is above 80 ppm, consider a partial drain before closing to bring it down to 40–50 ppm. This makes spring opening easier since you won't need to drain to manage high CYA. If CYA is below 30 ppm, you can leave it as-is and add stabilizer during spring opening.

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Pool Winterizing Calculator — Above Ground & Inground Closing Chemicals

Q: What chemicals do I need to winterize my pool?

To winterize a pool you need: (1) shock — raise free chlorine to 10–20 ppm the night before closing, (2) algaecide — a winter-specific algaecide dose added after shocking, (3) pH adjuster — bring pH to 7.2–7.6 before closing, (4) alkalinity increaser — raise alkalinity to 100–120 ppm for winter stability, and (5) clarifier — optional but helps drop any fine particles before covering. For above ground pools, also remove and store equipment before adding any closing chemicals.

Q: What is the correct order to add winterizing chemicals?

Add closing chemicals in this order: (1) Adjust total alkalinity first — raise to 100–120 ppm with baking soda if needed. (2) Adjust pH to 7.2–7.6. (3) Adjust calcium hardness if below 200 ppm (plaster pools). (4) Add clarifier and run pump for 4 hours. (5) Shock the pool — add at dusk, run pump overnight. (6) Add winter algaecide the following morning, at least 8 hours after shocking. Never add shock and algaecide together — chlorine destroys algaecide on contact.

Q: When should I close my pool for winter?

Close your pool when water temperatures consistently fall below 60°F (15°C). Algae cannot grow below this temperature, so waiting until the water is cold means you need less shock and algaecide. In most northern US climates this is late September through October. Closing too early (while water is still warm) means algae can establish under the cover despite the closing chemicals. For above ground pools, close before the first hard freeze to protect equipment and plumbing.

Q: Do I need algaecide to close my pool?

Yes, for most pools. A winter-grade algaecide (60% or higher concentration) provides protection through the entire winter after chlorine levels drop. Pools in warmer climates that experience mild winters, or pools that are closed when water temperature is still above 60°F, benefit most from algaecide. Pools in cold northern climates where water freezes solid can sometimes skip algaecide if properly shocked, but it is still recommended as insurance.

Q: How much shock do I need to close my pool?

For a clear pool at closing, add 2 lbs of calcium hypochlorite (cal-hypo) shock per 10,000 gallons the night before closing to raise free chlorine to approximately 20 ppm. For cloudy water, double the dose to 4 lbs per 10,000 gallons. Never add the pool cover until chlorine drops below 5 ppm — typically 24–48 hours after shocking. Do not use trichlor or dichlor as closing shock as they add CYA and alter pH.

Q: What is different about closing an above ground vs inground pool?

Above ground pools: lower the water level 4–6 inches below the return jets, remove and store all fittings, ladders, and skimmer baskets, add a skimmer plug or gizzmo, blow out the plumbing lines with a shop vac, and use an air pillow under the winter cover to handle ice expansion. Inground pools: lower water 12–18 inches below the tile line (or to skimmer level for mesh covers), blow out all plumbing lines with a compressor, plug all returns and skimmer with expansion plugs, drain the pump, filter, heater, and chlorinator, and add pool antifreeze to plumbing lines in freeze-prone climates.

Q: Can I use regular shock to winterize my pool?

Yes, calcium hypochlorite (cal-hypo) granular shock or liquid chlorine are both suitable for closing shock. Do not use trichlor-based shock (stabilized shock pucks or granules) — these add cyanuric acid with every dose and lower pH, creating imbalanced water over winter. Avoid dichlor as well for the same reason. Cal-hypo shock is the standard recommendation for pool closing because it adds no CYA and has a neutral to slightly basic pH effect.

Calculate the exact chemicals needed to close your pool for winter. Enter your pool size and current water readings to get a complete closing chemical plan — shock, algaecide, pH, alkalinity, and calcium hardness — in the correct order. No sign-ups. Instant results.

💡 Quick answer — pool closing chemical order

Correct closing order: Alkalinity → pH → Calcium → Clarifier → Shock (dusk) → Algaecide (next morning)
Closing shock dose: 2 lbs cal-hypo per 10,000 gal (clear water) | 4 lbs per 10,000 gal (cloudy)
When to close: When water temperature drops below 60°F (15°C) — algae cannot grow below this threshold
⚠️ Never add shock and algaecide at the same time — chlorine destroys algaecide on contact
Enter your pool details below for a complete, personalised closing chemical plan.

Pool Winterizing Calculator

Pool type Pool volume (gallons)

Calculate your pool volume →

Pool surface Current water condition

Current test readings (optional — skip any you haven't tested):

Total alkalinity (ppm) Ideal at closing: 100–120 ppm

Skip — haven't tested alkalinity

pH level Ideal at closing: 7.2–7.6

Skip — haven't tested pH

Calcium hardness (ppm) Ideal at closing: 200–400 ppm (plaster)

Skip — haven't tested calcium

CYA / Stabilizer (ppm) Ideal at closing: 40–60 ppm

Skip — haven't tested CYA

Include winter algaecide? Climate zone

JavaScript is required for the interactive calculator. Use the dosage charts below for quick reference, or enable JavaScript for a personalised closing plan.

⚠️ Do not add the pool cover until free chlorine drops below 5 ppm — typically 24–48 hours after the closing shock.

Shop your closing chemicals:

Pool closing chemical dosage chart by pool size

Quick reference for closing shock and algaecide doses based on pool size. These assume starting from 0 ppm free chlorine — adjust if your pool currently has residual chlorine.

Pool size	Closing shock (clear water)	Closing shock (cloudy water)	Winter algaecide (60% concentration)
10,000 gal	2 lbs cal-hypo	4 lbs cal-hypo	16 fl oz
15,000 gal	3 lbs cal-hypo	6 lbs cal-hypo	24 fl oz
20,000 gal	4 lbs cal-hypo	8 lbs cal-hypo	32 fl oz (1 qt)
25,000 gal	5 lbs cal-hypo	10 lbs cal-hypo	40 fl oz
30,000 gal	6 lbs cal-hypo	12 lbs cal-hypo	48 fl oz (1.5 qt)
40,000 gal	8 lbs cal-hypo	16 lbs cal-hypo	64 fl oz (2 qt)

Based on 65% calcium hypochlorite shock to raise FC to ~20 ppm. Algaecide dosing based on 60% concentration winter algaecide at 1.6 fl oz per 1,000 gallons. Always add algaecide at least 8 hours after shocking — never simultaneously.

Above ground pool winterizing chemicals & steps

Above ground pools require a slightly different approach to winterizing than inground pools. Because the plumbing is exposed and the pool shell is more vulnerable to freeze damage, the equipment removal steps are as important as the chemistry.

Above ground pool closing — chemical steps

The chemical steps are identical to inground pools: balance alkalinity and pH first, then add clarifier, then shock at dusk, then algaecide the following morning. However, above ground pools need particular attention to lowering the water level 4–6 inches below the return jets before adding closing chemicals — this prevents water from sitting in the fittings over winter. Use our alkalinity calculator and pH calculator for exact doses based on your pool size.

Above ground pool closing — equipment steps

After adding chemicals and waiting for chlorine to drop below 5 ppm:
☐ Remove and store the ladder, skimmer basket, and return jet fittings
☐ Install a skimmer plug (Gizzmo) or freeze-resistant skimmer cover
☐ Disconnect and drain the pump, filter hoses, and filter
☐ Store pump and filter indoors — freezing destroys seals and cracking filter tanks
☐ Place an air pillow (pool pillow) in the center of the pool before covering
☐ Secure the winter cover tightly with cable and winch — gaps allow debris in
☐ In freeze climates: add pool antifreeze to any plumbing that cannot be fully drained

💨 Air pillow — why it matters

An air pillow placed under the winter cover absorbs ice expansion as the surface water freezes. Without it, ice expansion pushes outward against the pool walls, which can permanently deform an above ground pool frame.

🔌 Store equipment inside

Above ground pool pumps and filters are not freeze-rated. Even a single freeze cycle cracks filter housings and destroys pump seals. Always store indoors — do not leave equipment in a cold garage or shed if temperatures drop below freezing.

Inground pool winterizing chemicals & steps

Inground pools require more preparation than above ground pools — specifically blowing out and plugging all plumbing lines to prevent freeze damage. In freeze climates, a burst return line or cracked skimmer can cost $500–$2,000+ to repair. Getting the closing chemistry and equipment steps right protects that investment.

Inground pool water level

Solid winter covers: Lower water 12–18 inches below the coping (tile line). This prevents ice from contacting the tile and cracking grout.
Mesh winter covers: Lower water to just below the skimmer opening. Mesh covers allow rainwater and snowmelt to pass through, so water level rises over winter — start lower.

Inground pool plumbing preparation

☐ Blow out all return lines, skimmer lines, and main drain lines with a compressor (do not use a shop vac — insufficient pressure)
☐ Install rubber expansion plugs in all returns and skimmer openings immediately after blowing out
☐ Remove and store the pump basket, filter grids, and D.E. powder (D.E. filters)
☐ Drain the pump, filter, heater, and salt chlorinator completely
☐ Add pool-grade antifreeze (propylene glycol, not automotive antifreeze) to any lines that cannot be fully blown out

💡 If you have a salt chlorine generator: turn it off and remove the salt cell before closing. Store the cell indoors — freeze cycles degrade the titanium plates. Do not reinstall until water temperature is above 60°F in spring.

When to close your pool for winter

The single most common pool closing mistake is closing too early, while the water is still warm. Algae grows rapidly in water above 60°F — a pool closed in September with warm water and a full dose of algaecide can still turn green by October if the algaecide breaks down while water temperatures remain high.

Water temperature	Algae risk	Recommendation
Above 70°F	🚫 High	Do not close — keep pool active and chemically balanced
60–70°F	⚠️ Moderate	Can close if temperatures are falling — double algaecide dose
Below 60°F	✅ Low	Ideal closing window — algae cannot establish below this threshold
Below 50°F	✅ Minimal	Close promptly — protect equipment from first freeze

In most northern US climates (USDA zones 5–7), the ideal closing window is late September through October. Pools in zones 8–9 (Southeast US, Pacific Northwest) can often wait until November. Always watch the 10-day forecast — aim to close 2–3 days before the first hard freeze.

Managing CYA (stabilizer) at pool closing

Unlike chlorine, CYA does not break down or evaporate over winter — it remains in the water at exactly the level you left it. This has two practical implications for pool closing:

If CYA is too high (above 80 ppm) at closing: Do a partial drain and refill before closing to bring CYA down to 40–60 ppm. This is easier and cheaper to do now than in spring when you're also balancing every other parameter. Use our pool dilution calculator to find the exact drain volume.

If CYA is in range (30–60 ppm): Do nothing — it will be there for spring opening. You will not need to add stabilizer at spring opening.

If CYA is low (below 30 ppm): You can either add a small amount before closing or wait until spring opening. Adding at closing means the stabilizer is in place when you shock — which is slightly more efficient. However, since there is no UV exposure over winter, CYA provides no active benefit during the off-season.

Use our stabilizer calculator for exact CYA dosing amounts and our CYA/FC calculator to confirm your spring chlorine targets based on whatever CYA level you close with.

Pool closing chemicals — what not to use

🚫 Trichlor tablets as closing shock

Trichlor tablets are 90% chlorine but add cyanuric acid with every dose and lower pH. Using them as closing shock leaves your pool with elevated CYA and low pH over winter — both make spring opening harder.

🚫 Dichlor as closing shock

Dichlor also adds CYA. A closing dose of dichlor into a 20,000 gallon pool adds 8+ oz of CYA — enough to raise CYA by 3–5 ppm. Over several seasons this accumulates. Use cal-hypo or liquid chlorine instead.

🚫 Shock and algaecide together

Chlorine oxidizes and destroys algaecide molecules on contact. Adding them together wastes money and provides no algae protection. Always wait 8–24 hours after shocking before adding algaecide.

🚫 Automotive antifreeze

Automotive antifreeze (ethylene glycol) is toxic to pets and wildlife and will contaminate pool water. Always use pool-grade propylene glycol antifreeze — non-toxic and specifically formulated for pool plumbing lines.

What to expect when opening a properly winterized pool

A pool closed correctly — balanced chemistry, proper shock dose, winter algaecide, and a secure cover — should open in spring with minimal effort. Realistic expectations based on closing quality:

Closing quality	Expected spring condition	Opening effort
Excellent — all chemistry balanced, winter algaecide used, secure cover	Clear or slightly cloudy — slight colour tint normal	Light shock, pH/alkalinity adjustment, 1–2 days to swim-ready
Good — chemistry mostly balanced, algaecide used, good cover	Slightly cloudy, possible light tinting	Standard shock, chemistry rebalance, 2–3 days to swim-ready
Fair — chemistry off, algaecide skipped, cover gaps	Cloudy, light green possible	Heavy shock or SLAM method, 3–7 days to swim-ready
Poor — no closing chemistry, uncovered or poor cover	Green — established algae	SLAM method required — 5–14 days to swim-ready

For spring opening, use our Pool Opening Calculator — it generates a full startup chemical plan based on your current test readings after removing the cover. If the pool opens green, use our SLAM Calculator for a systematic algae elimination plan.

Why closing pool chemistry matters more than opening chemistry

Most pool owners spend more time thinking about spring opening than fall closing — but the reverse is more logical. The chemistry decisions you make at closing determine the condition of your pool for the entire off-season and directly control how easy or difficult spring opening will be. A well-closed pool that opens clear in April represents a few hours of careful closing work the previous October. A pool that opens green in May represents days of SLAM treatment, filter cleaning, and chemical expense — all of which could have been avoided with proper winterizing.

The most critical closing chemistry decisions are alkalinity and pH balance, shock dose, and timing of algaecide relative to shock. Alkalinity set to 100–120 ppm at closing acts as a chemical buffer for the entire off-season — it prevents pH from drifting to damaging extremes in either direction while the pool sits without active circulation. Shock at 20 ppm eliminates any residual bacteria and algae spores, giving the pool a clean chemical slate. Winter algaecide, added 8–24 hours after shock so it isn't immediately destroyed by chlorine, provides the long-duration protection needed over 5–7 months of stagnant water.

For above ground pools specifically, the stakes are higher because the equipment is more exposed. A pump left connected through a freeze cycle can crack the housing and destroy the motor — a $200–500 repair that proper winterizing would have prevented entirely. For inground pools, the risk is in the plumbing: a burst return line that went undetected over winter can flood the equipment pad and cost thousands to excavate and repair.

The approach in this calculator reflects standard industry winterizing protocols: balance the water chemistry in the correct sequence, use calcium hypochlorite for closing shock (not stabilized chlorine products), apply winter-grade algaecide at the correct time and concentration, and complete all equipment steps before the first hard freeze. Follow the output of this calculator and your pool will be in the best possible condition when you lift the cover next spring.

Pool winterizing calculator FAQs

What chemicals do I need to winterize my pool?

The essential closing chemicals are: shock (calcium hypochlorite, to raise FC to 20 ppm the night before closing), winter algaecide (60%+ concentration, added 8–24 hours after shock), pH adjuster (muriatic acid or soda ash, to reach 7.2–7.6), and alkalinity increaser (baking soda, to reach 100–120 ppm). For plaster pools, also check calcium hardness and raise to 200–400 ppm if low. Clarifier is optional but helps the filter drop any fine particles before you cover the pool.

What is the correct order to add winterizing chemicals?

Always follow this sequence: (1) Alkalinity — raise to 100–120 ppm if needed, wait 6 hours. (2) pH — adjust to 7.2–7.6, wait 4 hours. (3) Calcium hardness — raise if below 200 ppm (plaster pools), wait 4 hours. (4) Clarifier — add and run pump 4 hours. (5) Shock — add at dusk, run pump overnight. (6) Algaecide — add the following morning, at least 8 hours after shock. Never add shock and algaecide at the same time — chlorine destroys algaecide on contact and you lose your winter protection.

When should I close my pool for winter?

Close when water temperature consistently stays below 60°F (15°C). Algae cannot grow below this threshold, so waiting for genuinely cold water reduces the amount of shock and algaecide needed and improves how well the closing chemicals last through winter. Closing too early — while water is still above 60°F — risks algae establishing under the cover even with a full algaecide dose.

Do I need algaecide to close my pool?

Yes for most pools. A winter-grade algaecide (60% or higher concentration, not the standard 10% maintenance algaecide) provides multi-month protection as chlorine levels naturally drop over winter. The one exception is pools in very cold climates where water freezes solid — algae cannot survive in frozen water, so algaecide has less value. However, even in freeze climates, algaecide is still recommended for the autumn period before the pool fully freezes.

How much shock do I need to close my pool?

For a clear pool, add 2 lbs of calcium hypochlorite (65%) per 10,000 gallons at closing to raise FC to approximately 20 ppm. For cloudy water, use 4 lbs per 10,000 gallons. Do not add the pool cover until FC drops below 5 ppm — usually 24–48 hours after shocking. See the closing dose chart above for all pool sizes. Use our shock calculator for other shock types (liquid chlorine, dichlor).

Should I adjust CYA before closing my pool?

Check your CYA level and act accordingly. If above 80 ppm: do a partial drain and refill before closing — use our dilution calculator for the exact drain volume. If between 30–60 ppm: do nothing, it will still be there in spring. If below 30 ppm: you can add a small amount before closing or wait until spring opening — either approach works fine.

What is different about closing an above ground vs inground pool?

The chemistry steps are identical. The equipment steps differ significantly. Above ground pools require removing and storing the pump and filter indoors (they will be destroyed by a single freeze cycle), lowering water 4–6 inches below the returns, installing a skimmer plug, and using an air pillow under the cover. Inground pools require blowing out all plumbing lines with a compressor, installing expansion plugs, and adding antifreeze to any lines that cannot be fully blown dry. Both pool types need water level adjusted before adding closing chemicals.

Can I use regular shock to winterize my pool?

Yes — calcium hypochlorite (cal-hypo) is the correct choice for closing shock. It raises chlorine quickly with no CYA or calcium side effects that would complicate your spring opening. Do not use trichlor-based shock (stabilized shock packets or granules), as these add cyanuric acid that accumulates over winter. Avoid dichlor for the same reason. If you only have liquid chlorine available, that works too — just scale the volume using our shock calculator.