Trichlor vs. Dichlor vs. Cal-Hypo: Which Pool Chlorine Is Right for You?
Pool Care Guides They're all "chlorine," so does it matter which you buy? It does. Each type has a different...
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Pool Care Guides
Three-inch tablets are the easiest way to keep a pool sanitized — but "toss one in and forget it" is how pools go cloudy. Here's exactly how many to use, where to put them, and what to avoid.
If you want clear water without babysitting your pool every day, 3-inch chlorine tablets are the workhorse that gets you there. They dissolve slowly and steadily, so a single tablet can sanitize for the better part of a week. But a few small habits separate people whose water stays glass-clear from people who end up fighting cloudy or green water — and almost all of it comes down to how you dose and where you place the tablets.
What are 3-inch chlorine tablets (trichlor)?
Most 3-inch tablets are made of trichlor (trichloroisocyanuric acid). Two things make them the default choice for routine sanitizing:
- High strength: trichlor is about 90% available chlorine, so a little goes a long way.
- Stabilized: each tablet contains cyanuric acid, the "sunscreen" that keeps UV rays from burning off your chlorine during the day.
They're compressed hard and dissolve slowly, which is exactly what you want for steady, hands-off sanitizing. Two things to keep in mind: trichlor is mildly acidic, so it nudges pH down over time, and it slowly adds cyanuric acid to the water — both easy to manage once you know to watch for them.
How many 3-inch chlorine tablets does your pool need?
The simplest rule of thumb: use about one 3-inch tablet per 5,000 gallons of water, replaced weekly. Start there, then fine-tune based on your free chlorine reading (target 1–3 ppm).
| Pool volume | Tablets per week (starting point) | Notes |
|---|---|---|
| 5,000 gal | 1 tablet | Small above-ground / spa-adjacent |
| 10,000 gal | 2 tablets | Typical small in-ground |
| 15,000 gal | 3 tablets | Mid-size in-ground |
| 20,000 gal | 4 tablets | Average family pool |
| 25,000 gal | 5 tablets | Large in-ground |
Add a tablet during heat waves, after heavy rain, or following a crowded weekend — all of those burn through chlorine faster. Not sure of your pool's volume? Our homepage has a free pool volume & dosing calculator that turns your dimensions into a weekly tablet plan.
Quick sizing
Because each 3-inch tablet weighs about half a pound, a 5 lb bucket holds roughly 10 tablets, a 10 lb bucket about 20, and a 25 lb bucket about 50. A 10,000-gallon pool using 2 tablets a week goes through a 10 lb bucket in roughly 10 weeks.
Where to put chlorine tablets: floater vs. feeder vs. skimmer
Never drop tablets straight onto the pool floor — they'll bleach and damage the surface. Use one of these instead:
Floating dispenser (simplest)
An inexpensive floating chlorinator holds several tablets and releases chlorine as it drifts. Set the vents to control the dissolve rate. Keep it moving and away from the liner, steps, and skimmer, and pull it out if it bumps into swimmers.
Automatic chlorinator / inline feeder (most consistent)
An inline feeder installs on your equipment pad after the heater and lets you dial the exact feed rate. It's the most consistent option because it delivers a steady dose every time the pump runs — set it and check it weekly.
Skimmer (not recommended)
Dropping tablets in the skimmer is tempting but hard on your equipment. When the pump shuts off, concentrated acidic water sits in the plumbing, heater, and pump and accelerates corrosion.
Avoid this
Don't run tablets through the skimmer, and never leave them sitting on the pool floor. Both concentrate acidic chlorine in one spot — one corrodes equipment, the other stains and damages your surface.
How to use 3-inch chlorine tablets, step by step
- Balance the water first. Get pH into the 7.4–7.6 range so your chlorine works efficiently.
- Calculate your dose. One tablet per 5,000 gallons as a starting point.
- Load your floater or feeder. Add the tablets — wear gloves and handle in a ventilated spot.
- Set the flow rate. Open the vents or dial partway, then adjust after you see results.
- Run the pump 8–12 hours a day. Circulation spreads the chlorine evenly.
- Retest in a day or two. Aim for free chlorine of 1–3 ppm and adjust the feed rate.
- Refill weekly. Top up whenever the dispenser runs low so sanitizing never lapses.
Stock up on 3-inch tabletsCommercial-strength trichlor tabs in 5, 10, 25 and 40 lb buckets — the bigger the bucket, the lower the cost per pound.
Shop Chlorine Tablets
6 chlorine-tablet mistakes to avoid
- Using the skimmer as a chlorinator. As above, it corrodes equipment when the pump is off.
- Never testing. Guessing leaves you either cloudy and under-sanitized or harsh and over-chlorinated.
- Ignoring the pH drift. Trichlor is acidic — check pH weekly and bump it back up when needed.
- Letting stabilizer creep too high. Test cyanuric acid monthly; if chlorine stops holding, high CYA is often why.
- Handling and storing carelessly. Wear gloves, work in fresh air, and store tablets sealed in a cool, dry place away from other chemicals.
- Mixing tablet types in one feeder. Stick to one product — never combine different chlorine types.
Safety first
Never mix trichlor tablets with cal-hypo shock or any other chlorine type — combining them can ignite or release toxic gas. Add each product to the pool water separately, and store different chemicals apart from one another.
When tablets aren't enough
Tablets are built for steady, everyday sanitizing — not for rescuing water that's already cloudy or green. Those situations call for a strong dose of shock to reset your chlorine and clear the problem. If your water is hazy or turning green, follow our step-by-step guides on clearing cloudy pool water and getting rid of green algae.
Curious how trichlor tablets compare to other chlorine types, and when you'd reach for each? See trichlor vs. dichlor vs. cal-hypo. And to fold tablets into a full routine, start with our weekly pool maintenance schedule.
Frequently asked questions
How many 3-inch tablets does a 20,000-gallon pool need?
About four tablets a week as a starting point (one per 5,000 gallons). Add more in hot weather or after heavy use, and confirm free chlorine reads 1–3 ppm.
Can I put chlorine tablets in the skimmer?
It's not recommended. When the pump shuts off, concentrated acidic water sits in the skimmer, plumbing, heater, and pump and speeds up corrosion. A floater or inline chlorinator is safer.
How long does a 3-inch chlorine tablet last?
Usually about 5 to 7 days. Slow-dissolving trichlor is made for week-long sanitizing, though hot water, heavy bather loads, and higher dosing shorten each tablet's life.
Do chlorine tablets lower pH?
Yes — trichlor is acidic, so it gradually lowers pH and alkalinity. Test regularly and add a pH or alkalinity increaser to stay in the 7.4–7.6 range.
Are 3-inch tablets better than 1-inch tablets?
For most in-ground and larger above-ground pools, yes: they dissolve slower, sanitize longer, and need fewer refills. The 1-inch size suits small pools and spas where a full 3-inch tablet would over-chlorinate.
This guide is general educational information. Tablet needs vary by pool size, surface, and local conditions — always follow the directions on each product label and test your water before dosing.
