To close a pool correctly for winter, you need to balance pH (7.2-7.6), total alkalinity (80-120 ppm), and calcium hardness (175-225 ppm), then add a stain and scale preventer, shock the pool at 2 lbs per 10,000 gallons, and finish with a winterizing algaecide added 24 hours after shocking. Get the order wrong or skip a step and you are rolling the dice on a green, stained, or corroded pool come spring. Here is every chemical, every number, and the right sequence to do this once and do it right.
Why Does Pool Closing Chemistry Actually Matter?
Most pool owners think closing is about pulling a cover over the water and walking away until May. What is actually happening is that your pool water sits untreated for four to six months, sometimes longer. Whatever is in that water when you close – low pH, suspended metals, algae spores – has all winter to do damage to your plaster, liner, equipment, and water quality. A 15-minute chemistry session before closing prevents hours of work in spring.
Water temperature also matters more than most people realize. Close your pool while water is still above 60 degrees Fahrenheit and you are giving algae a warm, dark environment to bloom all winter under the cover. Wait until the water is consistently below 60 F, even if that means closing a little later than the neighbors.
Step 1: Balance the Water Before Anything Else
Every closing chemical works better – or only works at all – in balanced water. Test your water a week before closing, not the day of. That gives you time to adjust alkalinity and pH across a few days without sending levels swinging.
- pH: Target 7.2 to 7.6. Lower pH is more corrosive to metal and equipment over a long winter. Higher pH causes scale buildup and reduces the effectiveness of your closing shock.
- Total Alkalinity: Target 80 to 120 ppm. Alkalinity buffers pH, so if alkalinity is off, pH will drift all winter with nothing to stabilize it.
- Calcium Hardness: Target 175 to 225 ppm. Water that is too soft will pull calcium from plaster and grout to satisfy itself. Water that is too hard will deposit scale on surfaces and equipment.
- Cyanuric Acid (stabilizer): No specific closing target, but drain and dilute if your CYA is over 80 ppm. High CYA makes your closing shock less effective than you think.
Step 2: Add a Stain and Scale Preventer
This step gets skipped constantly, and it is the reason a lot of pools open with brown or rust-colored stains on the walls in spring. A stain and scale preventer (also called a sequestrant or metal chelator) binds dissolved metals like iron, copper, and manganese in the water and keeps them from oxidizing and plating onto surfaces during the long off-season. Add it before you shock, because shocking water that already has dissolved metals in it can immediately oxidize those metals and stain the pool.
Dose is typically 1 quart per 10,000 gallons, but check your product label. Pour it in with the pump running, near a return jet, and give it a few hours to circulate before moving to the next step. If your pool had any staining during the season, this is the step that earns its money over winter.
Step 3: Shock the Pool
Use calcium hypochlorite (cal-hypo) shock for closing. Dose is 2 lbs per 10,000 gallons as a starting point. If you dealt with recurring algae during the summer – like the kind described in this breakdown of that stubborn green tint that keeps coming back – double that dose to 4 lbs per 10,000 gallons. The goal is to push your free chlorine level to 10-12 ppm or higher before you button everything up.
Always shock after dark. Cal-hypo exposed to direct sunlight burns off fast. Pre-dissolve shock in a bucket of water before adding to the pool, and pour it around the perimeter with the pump running. Let the pump circulate the water overnight – do not cover the pool yet. AquaDoc makes a cal-hypo shock that pool owners use specifically at closing because it dissolves clean without leaving residue that clouds water under a cover.
Step 4: Add Algaecide Last – Not With the Shock
This is the most common sequencing mistake. Algaecide added at the same time as a high-dose chlorine shock gets oxidized and destroyed before it ever gets a chance to work. Wait a full 24 hours after shocking, with the pump still running, before adding algaecide.
Use a winterizing or “non-foaming” algaecide formulated for long-term protection, not a summer maintenance dose. A 60% polyquat algaecide is effective and will not stain your pool if copper levels are low. Dose it per label – typically 1-2 quarts per 10,000 gallons for a winter application. This is the chemical that does the slow, quiet work of keeping your pool water clear for the next several months.
Optional But Worth It: Enzymes and Winter Floaters
A winterizing enzyme product breaks down non-living organic material – sunscreen, body oils, pollen – that chlorine alone cannot oxidize efficiently. It prevents the waterline ring that tends to show up by spring. Add it at the same time as the algaecide, and it works passively all winter long.
Some pool owners also drop in a winter floater loaded with a slow-release oxidizer or algae-fighting compound. These make sense for pools in milder climates where the water never truly freezes and algae pressure continues through winter, or for any pool where extra insurance feels worth the few extra dollars.
The Closing Day Sequence, in Order
- Test water and adjust pH, alkalinity, and calcium hardness. Give each adjustment 24-48 hours before retesting.
- Clean the pool: brush walls, vacuum the floor, clean out skimmer baskets. Do not cover a dirty pool.
- Add stain and scale preventer. Circulate for 2-4 hours.
- Add shock after dark. Run pump overnight. Do not cover.
- Wait 24 hours, then add algaecide and enzymes with the pump running.
- Lower the water level per your cover type requirements (mesh covers typically want water lower than solid covers).
- Blow out and plug the lines, install the cover, remove and store equipment.
Frequently Asked Questions
What chemicals do I need to close my pool for winter?
You need chlorine shock, algaecide, a stain and scale preventer, and a winterizing enzyme product at minimum. Balance pH (7.2-7.6), alkalinity (80-120 ppm), and calcium hardness (175-225 ppm) before adding any of them. Some pool owners also use a winter floater with a slow-release oxidizer.
What order should I add pool closing chemicals?
Balance your water chemistry first (pH, alkalinity, calcium hardness), then add stain and scale preventer, then shock the pool, then wait 24 hours and add algaecide last. Adding algaecide at the same time as shock destroys it before it can work.
How much shock do I add when closing a pool?
Use 2 lbs of calcium hypochlorite shock per 10,000 gallons as a baseline for closing. If your pool had algae problems during the season, double that dose. Shock after dark and let the pump run overnight before covering.
Can I close my pool without algaecide?
Technically yes, but skipping algaecide significantly raises the risk of opening to a green pool in spring. A good winterizing algaecide works slowly over months and costs far less than a spring algae treatment.
When is it safe to close my pool for the winter?
Close your pool when water temperatures consistently stay below 60 degrees Fahrenheit. Closing while water is warmer than that encourages algae growth under the cover all winter long.
The real payoff of getting closing chemistry right is a 30-minute pool opening in spring instead of a three-day algae recovery project. Do the chemistry work now, take your time with the sequence, and the cover will come off in April to clear blue water waiting for you. Pool service professionals will tell you the same thing: the pools that open clean are almost always the ones that were closed clean.