Pool Liquid Chlorine Dosing Calculator (Gallons and Target FC)
Chlorine dosing is a ratio problem tied to a second, less obvious variable: cyanuric acid. CYA protects chlorine from ultraviolet breakdown but also binds it, so the free chlorine level that sanitises a pool with 30 ppm CYA is dangerously low in a pool with 80 ppm. Any dose calculation without a CYA reading is guesswork.
Add chlorine after sunset with the pump running — UV destroys unstabilised chlorine within hours in direct sunlight.
| Raise FC by | Liquid 10% (fl oz) | Liquid 12.5% (fl oz) | Cal-hypo 65% (oz) | Bleach 6% (fl oz) |
|---|---|---|---|---|
| 1 ppm | 192 | 154 | 19.5 | 320 |
| 2 ppm | 384 | 307 | 39.0 | 640 |
| 3 ppm | 576 | 461 | 58.5 | 960 |
| 5 ppm | 960 | 768 | 97.5 | 1,600 |
| 10 ppm | 1,920 | 1,536 | 195.0 | 3,200 |
- 1.
Pool volume = 15000 gallons - 2.
Current free chlorine = 1 ppm - 3.
Target free chlorine = 4 ppm - 4.
Chlorine product = 10 - 5.
Cyanuric acid (stabiliser) = 40 ppm - 6.
Liquid chlorine to add = 576 fl oz - 7.
FC increase = 3.0 ppm - 8.
Ideal FC for your CYA = 4.6 ppm - 9.
SLAM / shock level = 16.0 ppm
How to Use This Calculator
1. Enter your details
Use the labelled fields and units that match your situation.
2. Calculate instantly
The result, visual comparison, and working update as values change.
3. Review your result
Check assumptions, then copy, print, or share the calculation.
The FC to CYA relationship
The widely used rule keeps free chlorine at roughly seven and a half percent of the CYA level as an absolute minimum and eleven to twelve percent as a target. At CYA 40 that is 3 ppm minimum and 4.6 ppm target; at CYA 80 it is 6 ppm minimum and 9 ppm target. Pools that keep 'the usual 3 ppm' regardless of CYA are the ones that go green every August.
Which product to use when
Liquid chlorine adds no CYA and no calcium, which makes it the default for routine dosing. Cal-hypo adds calcium hardness, useful in soft-water areas and problematic in hard-water ones. Dichlor and trichlor tablets add CYA with every dose, which is convenient early in a season and the main cause of runaway stabiliser levels by the end of it.
Timing and circulation
Add chlorine in the evening with the pump running, pouring slowly around the deep end perimeter. Daytime dosing in an unstabilised or lightly stabilised pool can lose half the addition to UV within a few hours. Retest no sooner than fifteen minutes of full circulation.
Target chlorine levels by cyanuric acid
| CYA (ppm) | Minimum FC | Target FC | SLAM / shock FC | Notes |
|---|---|---|---|---|
| 20 | 1.5 | 2.5 | 8 | Low protection, fast UV loss |
| 30 | 2.0 | 3.5 | 12 | Common indoor / spring level |
| 40 | 3.0 | 4.5 | 16 | Typical outdoor target |
| 50 | 3.5 | 6.0 | 20 | Sunny climates |
| 70 | 5.0 | 8.0 | 28 | High — consider partial drain |
| 90 | 7.0 | 10.0 | 36 | Too high for most owners |
Frequently Asked Questions
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: pool volume 11700 gallons, chlorine product "Liquid chlorine 10% (pool grade)"
On the lower / more conservative end. Add chlorine after sunset with the pump running — UV destroys unstabilised chlorine within hours in direct sunlight.
Example 2: pool volume 15000 gallons, chlorine product "Liquid chlorine 10% (pool grade)"
A typical middle-of-the-road setup. Add chlorine after sunset with the pump running — UV destroys unstabilised chlorine within hours in direct sunlight.
Example 3: pool volume 19500 gallons, chlorine product "Liquid chlorine 12.5%"
On the higher / more demanding end. Add chlorine after sunset with the pump running — UV destroys unstabilised chlorine within hours in direct sunlight.
Quick answers about the Pool Chlorine Dosing
What exactly does the Pool Chlorine Dosing work out?
Chlorine dosing is a ratio problem tied to a second, less obvious variable: cyanuric acid. You enter pool volume, current free chlorine, target free chlorine and chlorine product (plus 1 more optional details) and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 5 fields: pool volume, current free chlorine, target free chlorine, chlorine product and cyanuric acid (stabiliser). Nothing else is needed and nothing is stored.
How is it calculated — the FC to CYA relationship?
The widely used rule keeps free chlorine at roughly seven and a half percent of the CYA level as an absolute minimum and eleven to twelve percent as a target. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for pool Chlorine Dosing?
Liquid chlorine adds no CYA and no calcium, which makes it the default for routine dosing. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Target chlorine levels by cyanuric acid" table on this page tell me?
It is the reference range this tool works against — 6 rows from "20" (1.5) up to "90" (7.0). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which chlorine product should I pick?
The dropdown offers 5 choices — Liquid chlorine 10% (pool grade), Liquid chlorine 12.5%, Household bleach 6%, Cal-hypo granular 65% and Dichlor granular 56%. Pick the one that matches your real situation rather than the one you would like to be true; chlorine product usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Do I have to press a button or reload the page to see the result?
No. Pool Chlorine Dosing runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Pool Chlorine Dosing leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
Roughly 10. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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