Free tool No. 2
How much chlorine to add
With CYA at 40 ppm, keep free chlorine between 5 and 7 ppm and never below 3. Raising a 50,000-litre pool from 2 to 6 ppm takes 1.5 L of 12.5% liquid chlorine. Put in your own numbers for your dose.
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Worked example
A 50,000-litre pool at CYA 40, with FC 2 ppm and 12.5% liquid chlorine.
- 1
Find the target from CYA
CYA 40, chlorine added by hand: minimum 3 ppm, target 5 to 7 ppm, aim for 6
- 2
Work out the increase
6 ppm goal − 2 ppm now = 4 ppm
- 3
Convert the volume
50,000 L ÷ 3.785 = 13,209 US gal
- 4
Dose the 12.5% chlorine
FC 2 → 6 ppm: 4 ppm × 13,209 gal ÷ 977.9 = 54 fl oz
Add 1.5 L of 12.5% liquid chlorine.
Pour it in front of a return jet with the pump running, then retest after 30 minutes.
Chlorine doses per 10,000 litres
| Raise FC by | Liquid chlorine 12.5% | Cal-hypo 65% | Dichlor 56% | Trichlor 90% (tablets) |
|---|---|---|---|---|
| 1 ppm | 80 mL | 15 g | 20 g | 10 g |
| 2 ppm | 150 mL | 30 g | 35 g | 20 g |
| 5 ppm | 400 mL | 80 g | 90 g | 55 g |
| 10 ppm | 800 mL | 160 g | 180 g | 110 g |
Trichlor is for slow, steady dosing in a floater or feeder, not for raising chlorine quickly. Dichlor and trichlor add stabilizer (CYA) with every dose; cal-hypo adds calcium.
Free chlorine by CYA
| CYA | Minimum | Target | Salt cell target | Shock (SLAM) |
|---|---|---|---|---|
| 0 (none) | 2 | 3–5 | 3–5 | 10 |
| 30 | 2 | 4–6 | 3–6 | 12 |
| 40 | 3 | 5–7 | 3–7 | 16 |
| 50 | 4 | 6–8 | 3–8 | 20 |
| 60 | 5 | 7–9 | 4–9 | 24 |
| 70 | 5 | 8–10 | 5–10 | 28 |
| 80 | 6 | 9–11 | 6–11 | 31 |
| 90 | 7 | 10–12 | 6–12 | 35 |
Salt cells produce chlorine all day, so they can run lower in the range. Shock (SLAM) is 40% of CYA.
How to add chlorine to a pool
- 1
Test free chlorine and CYA
Use a drop-count kit if you can; strips are rougher for both readings. CYA sets how much chlorine the pool needs.
- 2
Find your target
Look up your CYA in the chart, or let the calculator do it. Aim for the middle of the target range.
- 3
Measure the dose
Measure the amount the calculator gives for your product. Check the strength on the label; bleach loses strength as it ages.
- 4
Add it with the pump running
Pour liquid chlorine slowly in front of a return jet. Pre-dissolve granules in a bucket of pool water. Never mix chlorine with acid.
- 5
Retest
Retest after 30 minutes of circulation, and again the next evening to see how much the sun used.
How we calculate
Targets come from the FC/CYA relationship: stabilizer protects chlorine from sunlight but also slows it down, so the more CYA a pool has, the more free chlorine it needs. The minimum, target and shock levels follow the published chart used by pool-chemistry communities.
Doses come from each product’s available chlorine: ppm × gallons ÷ a product factor, the same formula as the Pool Math calculator, then rounded to an amount you can measure. This page runs the same chemistry engine as the app, so the numbers match.
Questions
What should my free chlorine be?
It depends on your CYA. At CYA 40, keep FC between 5 and 7 ppm and never below 3. The chart on this page lists every common CYA level.
How much bleach equals liquid chlorine?
Unscented household bleach is the same chemical as pool liquid chlorine, just weaker (6% or 8.25% against 10% or 12.5%). Pick your strength in the calculator and it scales the amount.
Should I use tablets or liquid chlorine?
Tablets (trichlor) are convenient but add CYA and lower pH with every dose, so CYA climbs all season. Liquid chlorine adds nothing but chlorine and salt. Many owners use liquid for daily dosing and keep tablets for holidays.
When should I shock (SLAM) the pool?
When you see algae, cloudy water, or combined chlorine above 0.5 ppm. Shock means holding FC at 40% of your CYA (16 ppm at CYA 40) until the water clears and passes an overnight chlorine loss test.
How long after adding chlorine can I swim?
Once it has circulated for about 30 minutes and FC is back within the target range. At shock level, wait until FC drops below the shock level for your CYA.