We get this question more than any other. Sometimes the homeowner has been pouring chlorine into their pool for three weeks and watching the FC reading barely move. Sometimes they've burned through three buckets of cal-hypo and bleached a section of their plaster. Both are dosing problems, and both come from the same root issue: the dose on the back of the bucket is a generic number, not a number for your pool.
So here's how chlorine actually works.
Free chlorine vs. total chlorine vs. combined chlorine
If you only ever learn one piece of pool chemistry, learn this:
Free chlorine (FC) is the chlorine in your pool that's still available to sanitize.
Combined chlorine (CC) is chlorine that's already done its job — it bonded to ammonia, sweat, sunscreen, etc. — and is now mostly useless and a little smelly. (That "chlorine smell" people complain about is actually combined chlorine, not free chlorine.)
Total chlorine (TC) = FC + CC.
Cheap test strips and a lot of automatic readers only show you total chlorine. That can fool you into thinking you have plenty of sanitizer when in reality your free chlorine is at zero and you're swimming in spent product. Get a DPD or FAS-DPD kit. They cost $25–$60 and they pay for themselves in a single avoided algae bloom.
Your weekly target
For a typical Ellis County residential pool with stabilizer (CYA) at 30–50 ppm, target free chlorine somewhere in the 2–4 ppm range. If your CYA is higher — say 70 or 80 — your target FC needs to be higher, too. We'll get to that.
What "weekly" really means in Texas
Pool service textbooks were written in the Midwest. They are wrong about Texas.
In Ellis County summer heat, sun degrades chlorine fast. A pool that holds a clean 3 ppm on Saturday morning may be at 0.5 ppm by Wednesday — not because anything is wrong, but because UV ate it. So "how much chlorine should I add weekly" is really two questions:
How much do I need to add right now to get back to 3 ppm?
How much do I need on a slow drip (tablets, salt cell, liquid bleach) to keep it there?
The first one is what the Free Chlorine Dose Calculator is for.
The math
To raise FC by 1 ppm in 10,000 gallons you need roughly:
1.6 oz of liquid chlorine at 12.5% strength (about 1.5 cups), or
0.21 oz of cal-hypo at 73% (about 1 tablespoon), or
0.13 oz of dichlor at 56%
These numbers are roughly. Your pool isn't exactly 10,000 gallons, your bottle of liquid chlorine isn't exactly 12.5% (it degrades), and your starting FC isn't exactly zero. That's why we don't trust the back of the bucket — and why every dose recommendation we give you on this site is calculated for your gallon count and your current reading.
When the chlorine isn't holding
If you're adding chlorine on schedule and watching it disappear, the most likely culprits in this order are:
CYA is too low. Without stabilizer, North Texas sun will burn through chlorine in hours.
CYA is too high. Too much stabilizer "locks up" your chlorine and reduces its effectiveness, which makes you add more, which raises CYA further. Death spiral.
You have early algae. Algae eats chlorine before you can read it.
Your free chlorine is being measured as combined chlorine. Get a real test kit.
Your bottle of liquid chlorine is old. Liquid chlorine loses about 50% of its strength in three months on a hot garage shelf.
The lazy-but-correct weekly routine
For a 20,000-gallon Ellis County pool with proper CYA, this is what we'd tell a homeowner to do at minimum:
Saturday morning: test FC, pH, and TA. Dose chlorine to get FC back into the 2–4 range. Adjust pH if needed.
Wednesday evening: test FC. Top off if it dropped below 2 ppm.
Once a month: test CYA, calcium hardness, and salt (if applicable).
That's it. Two real tests a week and a calculator. The hard part isn't the chemistry — it's remembering to do it on a Saturday in August when it's 102 degrees and you'd rather be inside.
When to just have us do it
If you're dosing chlorine more than twice a week and it's still not holding, something else is wrong and adding more product won't fix it. That's exactly the call we're best at — diagnose what's actually happening (CYA creep, algae, bad pump turnover, a salt cell that's tired) and fix the root cause instead of chasing the FC reading.
A weekly service plan with A&M usually costs less per month than a homeowner spends on chlorine alone in summer. If you've burned through four 50-lb buckets of cal-hypo this season, run the math.
Run the Free Chlorine Dose Calculator or call Thomas directly at (214) 399-7347.
