Cal Hypo vs. Liquid vs. Dichlor: Pool Shock Compared

If you've stood in front of a wall of shock products and wondered why three bags look identical but cost wildly different amounts, this guide is for you. We wrote it for residential pool owners who already know they need to shock, but aren't sure which chlorine type fits their pool, their weather, and their current cyanuric acid level. We'll cover the four chlorine products people actually use to shock a pool, the trade-offs that matter, and a clear decision matrix so you don't end up bleaching your liner or stalling your sanitizer for a week.

A quick note on terminology before we dive in. "Shock" just means raising free chlorine high enough (typically 10 ppm or more) to break apart chloramines and kill organic buildup. The product doesn't have to say "shock" on the bag. What matters is the active ingredient, the strength, and what else it puts into your water.

The short version is below. The honest breakdown, including why we're skeptical of enzyme-based reducers, follows after.

• For a typical 20,000-gallon residential pool:

  • Cal hypo (calcium hypochlorite, 65 to 78% available chlorine): Best all-around granular shock for vinyl, fiberglass, and plaster pools with normal to low calcium hardness. Roughly $4 to $9 per pound.

  • Liquid chlorine (sodium hypochlorite, 10 to 12.5%): Best for pools with already-high CYA or high calcium, and for owners who shock weekly. Roughly $4 to $8 per gallon. Loses strength fast on the shelf.

  • Dichlor (sodium dichloro-s-triazinetrione, 56 to 62%): A stabilized granular shock. Convenient, but adds cyanuric acid every dose. Use sparingly.

  • Trichlor (trichloro-s-triazinetrione, 90%): Sold mostly as slow-dissolve tabs. Not a true shock product despite some labeling. Acidic and stabilized.

  
  

  The skeptical take: Most homeowners are better off rotating cal hypo and liquid chlorine. Trichlor tabs are fine as a feeder sanitizer in some pools, but using stabilized chlorine to "shock" is the single fastest way to lock your pool into a high-CYA problem.

Chlorine in your pool comes in two forms. "Free chlorine" is the active sanitizer, the stuff that kills bacteria and oxidizes contaminants. "Combined chlorine" is what's left over after free chlorine has reacted with sweat, sunscreen, urine, and other organics. Combined chlorine is a much weaker sanitizer, and it's what produces the harsh chlorine smell people associate with pools.

Shocking is the process of raising free chlorine high enough to break those combined chlorine molecules apart and convert them back to harmless byproducts. The technical name is breakpoint chlorination. Below breakpoint, you're just making the chloramine problem worse. Above it, the chloramines clear out and your sanitizer recovers.

The rule of thumb is to raise free chlorine to roughly 10 times your combined chlorine reading. For most residential pools, that lands somewhere between 10 and 30 ppm. Hold it there until combined chlorine drops back under 0.5 ppm.

Any of the four chlorine products on the wall can hit that level. The question is which one fits your water chemistry without creating a new problem.

Calcium Hypochlorite (Cal Hypo)

Granular or briquette form. Typically 65 to 78% available chlorine. Unstabilized, meaning it adds no cyanuric acid to your water.

The byproduct is calcium. Every pound of cal hypo adds roughly 0.6 to 0.8 ppm of calcium hardness per 10,000 gallons. That's a feature in vinyl and fiberglass pools with naturally low calcium and a problem in plaster pools that already test above 350 ppm CH.

Cal hypo has a pH around 12, so it pushes pH up briefly before settling out. It also produces some insoluble residue, which is why most manufacturers tell you to pre-dissolve it in a bucket of water before pouring it in.

Liquid Chlorine (Sodium Hypochlorite)

The same chemistry as household bleach, just stronger. Pool-grade liquid is 10 to 12.5% available chlorine. Bleach off the grocery shelf is 5 to 8.25% and usually not cost-competitive once you account for the dilution.

Liquid chlorine has a pH around 13, but the net pH impact on your pool is close to neutral after the chlorine consumes its hydroxide ions during sanitization. There's no calcium, no stabilizer, no residue. You're adding chlorine and a small amount of salt.

The catch is shelf life. Liquid chlorine starts degrading the day it's manufactured. Stored in heat, it can lose 20 to 50% of its strength in 60 to 90 days. Buy what you'll use in a month, not a season.

Dichlor (Sodium Dichloro-s-Triazinetrione)

A granular, fully soluble, stabilized chlorine. 56 to 62% available chlorine. pH around 6.7, close to neutral.

Dichlor is the granular cousin of trichlor. Each pound adds chlorine and cyanuric acid in roughly a 2-to-1 ratio. Convenient, fast-dissolving, and easy on liners.

The cost is CYA accumulation. Add a pound of dichlor per week to a 20,000-gallon pool, and you'll add roughly 4 to 5 ppm of CYA per pound. Over a season, that adds up to a stabilizer problem we've covered separately.

Trichlor (Trichloro-s-Triazinetrione)

90% available chlorine, the highest of any common pool product. Sold mostly as 1-inch and 3-inch tabs, occasionally as granular. pH around 2.8, strongly acidic. Heavily stabilized.

Trichlor is engineered to dissolve slowly through a floater, feeder, or skimmer. As a "shock" product it's a poor fit. It dissolves too slowly to hit a fast oxidation target, it lowers pH aggressively, and every dose loads more CYA into the water. Some homeowners do reach for trichlor granular as a shock anyway. We'd generally tell them not to.

• CYA is the most under-discussed variable in shock selection. The higher your CYA, the more chlorine you need to reach an effective sanitizer level.

  • CYA 0 to 30 ppm: Any product works. Dichlor or trichlor tabs are fine for slow build-up.

  • CYA 30 to 50 ppm (the sweet spot for most outdoor pools): Use cal hypo or liquid for shock. Avoid dichlor as the primary shock to keep CYA from climbing.

  • CYA 50 to 80 ppm: Slow down or stop using stabilized chlorine. Cal hypo and liquid only.

  • CYA above 80 ppm: You're already in chlorine-lock territory. Liquid chlorine for shocking, and start planning a partial drain to bring CYA back into range. We cover that in our CYA reduction guide.

Hot, Sunny, Heavy Use

Liquid chlorine and cal hypo, used frequently. UV degrades chlorine fast. Maintain CYA at 30 to 50 ppm and shock weekly. Don't drift toward dichlor just because it's convenient.

Cooler Climates, Light Use

Cal hypo monthly is usually enough. If you only swim a few months a year, the cost of a stabilized routine isn't worth the CYA buildup.

After a Storm or Heavy Bather Load

Cal hypo or liquid for a sharp, fast spike to break chloramines. Dichlor is too slow and adds CYA you don't need.

Indoor Pools

Liquid chlorine, almost always. No UV means no need for stabilizer, and adding CYA indoors through dichlor or trichlor is one of the most common chemistry mistakes we see.

This is the step most articles skip. Lowering CYA once is fine. Doing it every year because you didn't change your sanitizer plan is a waste of water and money.

The honest fix:

• Switch from trichlor tabs to liquid chlorine or cal hypo for in-season sanitization. Liquid chlorine adds no CYA. Cal hypo adds no CYA but does add calcium hardness, so it's a better fit for soft-water regions.

• Use trichlor only for short stretches, like when you're going on vacation and need slow-dissolving tabs.

• Stabilize manually once at season open, with pure cyanuric acid granular stabilizer, to the level you actually want.

• Avoid dichlor shock if your CYA is already mid-range or higher. Use cal hypo or liquid chlorine for shock instead.

If you're on a salt system, you do not need trichlor tabs at all under normal conditions. Adding them as a "boost" is what pushes saltwater pool CYA into the danger zone.

Sticker price misleads here. What matters is the cost per ppm of free chlorine delivered to your pool.

To raise a 10,000-gallon pool by 10 ppm free chlorine, you need approximately:

• Cal hypo (68%): about 19 to 21 oz, or roughly $5 to $9 per shock.

• Liquid chlorine (12.5%): about 1 gallon, or roughly $4 to $8.

• Dichlor (56%): about 24 oz, or roughly $7 to $10.

• Trichlor granular (90%): about 14.5 oz, or roughly $6 to $10, plus the long-term cost of locked-up CYA.

Liquid is usually cheapest per ppm if you buy fresh. However, keep in mind the waste involved with the plastic jugs. Cal hypo is a close second and stores far better. The stabilized products look competitive on the bag but cost more in downstream water chemistry headaches.

Always follow manufacture guidlines and best practices. These are oxidizers, not casual yard chemicals.

• Cal hypo: Stores 1 to 3 years sealed in a cool, dry place. Keep away from acids, organics, and trichlor. Mixing cal hypo and trichlor in the same scoop has caused fires and explosions.

• Liquid chlorine: Stores 3 to 6 months at best. Degrades faster in heat and sun.

• Dichlor and trichlor: Stable for 2 to 5 years sealed. Same hard rule, never store or scoop near cal hypo.

If you only remember one thing: use a dedicated scoop for each product, and don't mix containers.

For most residential pool owners we work with, the right answer is a rotation of cal hypo for routine weekly shocking and liquid chlorine for spike events (after storms, parties, or a chloramine smell). Skip stabilized shock as a routine product. Use trichlor tabs only if your CYA is well-controlled and you understand what they're adding.

If you find yourself reaching for dichlor every week, that's usually a sign the pool is under-circulated, under-sanitized between shocks, or both. The shock isn't the fix. The baseline chlorination is.

The "best" pool shock is the one that matches your pool surface, your current CYA, and your weather. For most homeowners, that means cal hypo as the default granular shock, liquid chlorine for fast spikes or high-CYA pools, dichlor sparingly, and trichlor reserved for slow-dissolve sanitizing rather than shocking.

If you're not sure what your pool needs this week, take a water sample and send us your test numbers. We'll tell you straight which product to use, not the one with the highest margin.

Shop pool shock or browse all pool chemicals. Want a recommendation for your specific pool? Contact our team with your gallons, surface type, and current CYA reading, and we'll spec a shocking routine that fits.

• The Ultimate Guide to Cloudy Pool Water -- If your water looks dull or hazy after adjusting pH, this is your next stop.

• The Ultimate Guide to a Green Pool -- High pH that lets algae take hold leads here.

• How to Raise Chlorine Levels in Your Pool -- Because pH and chlorine work together, not separately.

• Pool Test Strips: The Owner's Way for Quick Checks -- How to get accurate readings every time.

• The Ultimate Guide to Pool Maintenance -- The full picture, for when you want to understand the whole system.

• Saltwater vs. chlorine pool 5-year cost breakdown -- Industry trends and where cost is headed. 

• Pool clarifier vs. flocculant -- if cloudy water came along with the CYA issue

• How to lower cyanuric acid in a pool -- what to do if your CYA levels get to high.