Every fall, the shock aisle at the pool store turns into a decision you didn’t plan for: trichlor on the left, cal-hypo on the right, and a closing checklist in your hand that just says “shock the pool.” If you’ve been standing there wondering whether it actually matters which one you grab – and how much of it you need – you’re not alone.
For a 20,000-gallon inground pool, the standard winter closing dose is 2 lbs of 73% cal-hypo shock, added the evening before you cover the pool. Trichlor can work in a pinch, but it comes with trade-offs that make it the weaker choice for a long winter shutdown. This article walks through the exact amounts for each type, why the differences matter, how to time the dose correctly, and the mistakes that send people back to the store in the spring with a swamp instead of a pool.
Cal-Hypo vs. Trichlor: What You’re Actually Choosing Between
These are not the same product with different labels. Cal-hypo (calcium hypochlorite) is an unstabilized shock – it delivers a fast, aggressive chlorine hit with no added cyanuric acid (CYA). Trichlor (trichloroisocyanuric acid) is a stabilized chlorine that dissolves slowly and adds CYA to the water every time you use it. That distinction matters a lot when you’re closing a pool that won’t be actively managed for five or six months.
- Cal-Hypo: Fast-dissolving, high available chlorine (65-73%), no CYA impact, ideal for super-chlorination at closing. The downside is it raises calcium hardness slightly and reacts violently if mixed with trichlor directly – never combine the two in a bucket.
- Trichlor: Slow-dissolving, typically 90% available chlorine in tablet or granular form, adds CYA to every dose. At closing, that extra CYA sits in your water all winter doing nothing useful except potentially pushing you over the 100 ppm threshold by spring.
- Liquid Chlorine (sodium hypochlorite): Often the cleanest option for closing – no CYA, no calcium increase, immediate impact – but you need about 2 gallons of 12.5% liquid chlorine per 10,000 gallons to hit the same chlorine level as 1 lb of 73% cal-hypo, so it gets heavy and expensive fast.
For most 20,000-gallon inground pool closings, cal-hypo is the practical winner. It hits fast, dissipates before spring, and doesn’t mess with your CYA heading into the next season.
Exact Doses for a 20,000-Gallon Pool
Here’s the worked math. A standard closing super-chlorination targets 10 to 15 ppm of free chlorine. One pound of 73% cal-hypo raises a 10,000-gallon pool by roughly 7 ppm. For 20,000 gallons, that means:
- Normal conditions (water is clear, balanced chemistry): 2 lbs of 73% cal-hypo. That gets you to approximately 7 ppm in 20,000 gallons, which gives you a solid head start before the cover goes on.
- Borderline conditions (slight haze, early algae, high bather load was heavy this season): 3 lbs of 73% cal-hypo to push closer to 10 ppm.
- Visibly green or problem water: 4 lbs (two separate 2-lb doses, 12 hours apart) plus balance correction before closing. If you’re closing a green pool, the shock dose alone won’t save you – closing over an algae problem just freezes the problem in place.
If you insist on using trichlor granules for closing, the dose for 20,000 gallons is 3 to 4 lbs, given that trichlor granules are around 90% available chlorine. But budget an extra day for it to fully dissolve and distribute, and test CYA before covering – if you’ve been using trichlor tabs all summer, your CYA may already be near or above 80 ppm.
| Shock Type | Available Chlorine | Dose for 20,000 Gal (Normal) | Adds CYA? | Speed |
|---|---|---|---|---|
| Cal-Hypo (73%) | 73% | 2 lbs | No | Fast (hours) |
| Trichlor Granular (90%) | 90% | 3-4 lbs | Yes | Slow (1-2 days) |
| Liquid Chlorine (12.5%) | 12.5% | 4 gallons | No | Immediate |
Why the Differences Matter at Closing Specifically
During the swim season, a little extra CYA from trichlor tabs isn’t a crisis – you’re testing and adjusting constantly. At closing, you’re walking away for months. CYA doesn’t dissipate over winter; it just sits there. If you add another 20-30 ppm at closing via trichlor shock, you may open in spring to CYA levels above 100 ppm, which effectively neuters your chlorine’s ability to sanitize. That’s one of the reasons most pool professionals default to cal-hypo or liquid chlorine for the final shock of the season, as industry closing guides consistently recommend.
Calcium hardness is the other variable. Cal-hypo does add some calcium – about 7-9 ppm per pound in 20,000 gallons. If your calcium hardness is already at the high end (above 350 ppm), that small bump matters more. Test before closing and, if you’re already high, liquid chlorine is the cleaner path.
Step-by-Step: How to Shock a 20,000-Gallon Pool at Closing
- Balance the water first. Get pH to 7.4-7.6, alkalinity to 80-120 ppm, and calcium hardness to 200-400 ppm before adding any shock. Shocking out-of-balance water wastes chlorine and can damage plaster or liners. Many pool owners get this wrong by shocking first and balancing second – it’s an expensive mistake.
- Test your current free chlorine. If it’s already at 3+ ppm, you need less shock. If it’s near zero, start at the full 2-lb dose for a clean pool.
- Pre-dissolve cal-hypo before adding. Fill a 5-gallon bucket with pool water, add the shock slowly while stirring, then pour the solution around the perimeter of the pool with the pump running. Never dump granular cal-hypo directly onto a vinyl liner.
- Run the pump for at least 8 hours. Why 8 hours? That’s roughly one full turnover for most 20,000-gallon pools with a properly sized pump, ensuring the shock distributes evenly through every corner and return line.
- Test free chlorine before covering. Target 1-3 ppm when the cover goes on. Above 5 ppm risks bleaching a vinyl liner or degrading a mesh safety cover.
- Add algaecide after chlorine drops. Wait until free chlorine is below 3 ppm before adding your winter algaecide – high chlorine will oxidize most algaecide formulas on contact, wasting the product entirely.
For a full picture of what goes under the cover before you close up, the complete inground pool winterizing process is worth a read alongside the dosing details here. And once that cover is on, remember it becomes its own maintenance project – winter covers collect more debris and contamination than most owners expect.
Common Mistakes That Ruin a Pool Over Winter
- Shocking too early: If you shock five days before closing and the sun and residual organic load burn off that chlorine, you’re covering a pool with near-zero sanitizer. Shock the night before closing, not the weekend before.
- Covering at high chlorine: Pulling a mesh or solid cover onto water above 5 ppm free chlorine is a liner or cover warranty voider. Test the morning of closing.
- Mixing trichlor and cal-hypo: These two products react violently when combined in concentrated form. Use one or the other, never both in the same application window without thoroughly rinsing equipment.
- Skipping water testing entirely: Eyeballing the water and assuming it’s “good enough” is how people open in spring to stained plaster, cloudy water, or a liner that’s shifted. Test strips at minimum, a proper liquid kit for accuracy. AquaDoc’s closing kit includes pre-measured shock and a test strip pack specifically calibrated for winterizing ranges – one less thing to guess at when you’re rushing to beat the first frost.
- Adding algaecide before chlorine drops: Shock first, cover last, algaecide in the middle. The order matters. Experienced pool service techs follow this sequence every single time for good reason.
If you want to see how chemical volumes scale across different pool sizes, the post on 12×24 inground pool gallons and chemical dosing gives a solid framework for understanding why volume is the only number that really drives your dose decisions.
Frequently Asked Questions
How much cal-hypo shock do I need to close a 20,000-gallon pool?
Use 2 lbs of 73% cal-hypo shock for a standard closing on clear, balanced water. If the water is hazy or you’ve had algae problems this season, increase to 3 lbs. For visibly green water, use 4 lbs in two separate doses and address the underlying chemistry before covering.
Can I use trichlor shock to winterize a pool?
Yes, but it’s not the ideal choice. Trichlor adds cyanuric acid with every dose, and that CYA accumulates over a winter with no way to manage it. For a 20,000-gallon pool, you’d need 3 to 4 lbs of trichlor granules to hit a comparable chlorine level. Most pool pros prefer cal-hypo or liquid chlorine at closing specifically to avoid inflating CYA into spring.
Should I shock before or after adding algaecide when closing a pool?
Always shock first. High chlorine concentrations break down most algaecides on contact, rendering them useless. After shocking, run the pump for 8 hours, then test free chlorine. When it drops to 1-3 ppm – typically 24 to 48 hours later – add your winter algaecide.
What chlorine level should I have before putting the pool cover on for winter?
Aim for 1 to 3 ppm of free chlorine at the moment the cover goes on. Above 5 ppm risks bleaching vinyl liners and degrading mesh safety covers. Shock the evening before closing, run the pump overnight, and test in the morning before you cover.
How much shock do you need to winterize a pool per 10,000 gallons?
The baseline is 1 lb of 73% cal-hypo per 10,000 gallons under normal conditions. A 20,000-gallon pool needs 2 lbs. Double the dose to 2 lbs per 10,000 gallons if algae is present or water is visibly off, and resolve the water quality issue before covering rather than hoping the shock does all the work under the cover.
The short answer: close a 20,000-gallon inground pool with 2 lbs of 73% cal-hypo the night before covering, test in the morning to confirm free chlorine is in the 1-3 ppm range, and add your algaecide last. That’s the sequence that keeps problems from waiting for you in April.