How Much Chlorine to Add to Your Pool — Calculator
Enter your pool volume, current FC, and CYA-based target FC to get an exact dose. Not sure what target to use? Your target FC = CYA × 7.5% for most pools — see the FC/CYA chart.
Higher CYA requires higher free chlorine (FC) to stay effective.
Why liquid chlorine is the preferred choice
Liquid sodium hypochlorite (pool bleach) raises FC cleanly — it adds only chlorine and water. Cal-hypo adds calcium, which can cloud water if your calcium hardness is already high. Trichlor adds CYA with every dose, which gradually raises your FC minimum and can lock you into a high-CYA chemistry spiral.
For regular top-offs, liquid chlorine at 10% or 12.5% is the simplest and cheapest option over a season. Cal-hypo is useful if you prefer a granular product or if your calcium hardness is on the low side.
Add chlorine near a return jet
With the pump running, pour liquid chlorine near a return jet or around the perimeter of the pool — never in front of the skimmer, and never mixed with other chemicals. For granular products, pre-dissolve in a bucket of pool water before adding.
How the chlorine dose is calculated
All dose formulas work from the same principle: how many grams of available chlorine do you need to raise FC by the target amount, given your pool's water volume?
Concentrations used: Liquid 10% = 0.10 (density 1.074 g/mL), Liquid 12.5% = 0.125 (density 1.094 g/mL), Cal-Hypo 65% = 0.65, Cal-Hypo 73% = 0.73, Trichlor 90% = 0.90.
Worked example
A 15,000-gallon pool tests at FC 1.5 ppm. Target is 4.0 ppm — a rise of 2.5 ppm. Using 12.5% liquid chlorine:
2.5 × 15,000 × 3.785 ÷ 1,000 = 142 g of available chlorine needed. Divided by (1.094 density × 29.57 mL/fl oz × 0.125 concentration) = about 35 fl oz — a little over 1 quart. Add near a return jet with the pump running. Wait 30 minutes and retest before assuming the job is done.
Frequently asked questions
How much liquid chlorine raises FC by 1 ppm?
For a 10,000-gallon pool: about 13 fl oz of 10% chlorine, or 10 fl oz of 12.5%. For a 15,000-gallon pool: about 20 fl oz (10%) or 15 fl oz (12.5%). The exact amount scales linearly with pool volume and chlorine strength — use the calculator above for your specific numbers.
Should I use liquid chlorine or cal-hypo?
Liquid chlorine (sodium hypochlorite) is the preferred choice for routine dosing — it adds only chlorine and water, no calcium, no CYA. Cal-hypo is a reasonable alternative if your calcium hardness is below 200 ppm. Avoid trichlor for regular top-offs: it adds CYA with every dose and gradually raises your minimum FC target over the season.
What FC level should I be targeting?
Target FC depends on your CYA: minimum is CYA × 7.5% for standard pools (CYA × 5% for saltwater/SWG). At CYA 0–10, aim for 2–4 ppm. At CYA 50, aim for 5–7 ppm. At CYA 80, aim for 7–9 ppm. See the FC/CYA chart for a full table.
How often do I need to add chlorine?
Most pools need chlorine added every 2–4 days during swim season — daily during heat waves or heavy use. If FC drops more than 1–2 ppm per day without a clear cause, test your CYA level. Low CYA means sunlight is destroying chlorine faster than you're adding it.
Why does my pool chlorine drop so fast?
Fast FC loss usually has one of four causes: CYA too low (UV destroying unprotected chlorine), early algae growth consuming FC overnight, heavy bather load, or pH above 8.0 (reducing chlorine's effectiveness). Test CYA first — it's the most common culprit and the easiest to fix.
Log a reading. Get the exact dose.
PoolChem Tracker calculates your chlorine dose automatically from your current readings — no calculator required. It also tracks your history and tells you when to add more.
