Pool Chlorine Calculator
Don't know it? Use the pool volume calculator.
From the label. Changes with the product.
Liquid chlorine to add
1.44 qt
Add 1.44 qt of liquid chlorine (12.5%) to raise free chlorine from 1 to 4 ppm in 15,000 gallons.
- FC increase
- 3 ppm
- Product
- liquid chlorine at 12.5%
- Rate
- 10.2 fl oz per 10,000 gal per 1 ppm
- Never mix pool chemicals with each other, wet or dry.
- Always add chemicals to water, never water to chemicals.
- Follow the manufacturer's label. It overrides any calculator, including this one.
How it's calculated
Free chlorine is measured in parts per million (ppm), which for water is the same as milligrams per liter. So the amount of chlorine you need is simply the increase you want times the volume of water, divided by how much available chlorine the product carries.
- ΔFC: target free chlorine minus current free chlorine.
- Liquid strength is "trade percent": grams of available chlorine per 100 mL. 12.5% means 125 g per liter, or 3,697 mg in each fluid ounce.
- Dry strength is percent available chlorine by weight, so 65% cal-hypo delivers 0.65 oz of chlorine per ounce of product.
Side effects come from the chemistry of each product: dichlor and trichlor release cyanuric acid as they dissolve, and cal-hypo releases calcium. Per 1 ppm of chlorine, dichlor adds 0.91 ppm CYA, trichlor adds0.61 ppm CYA and cal-hypo adds 0.71 ppm calcium hardness (molar masses; the same ratios used by Trouble Free Pool's PoolMath). Household bleach labeled by weight actually carries slightly more available chlorine than its label suggests, so the calculator errs on the low side for bleach.
Worked example
A 15,000 gallon pool tests at 1 ppm free chlorine and you want 4 ppm, using 12.5% liquid chlorine:
- Increase needed: 4 − 1 = 3 ppm.
- Volume in liters: 15,000 × 3.785 = 56,781 L.
- Chlorine needed: 3 × 56,781 = 170,344 mg.
- Each fl oz of 12.5% carries 12.5 × 10 × 29.57 = 3,697 mg.
- Dose: 170,344 ÷ 3,697 = 46.1 fl oz, or about 0.36 gallons.
Pour it slowly in front of a return jet with the pump running, wait at least 30 minutes, and retest before adding more. If the pool loses a lot of chlorine overnight, your stabilizer may be low; check it with theCYA calculator.
Chlorine needed per 10,000 gallons for +1 ppm
| Product | Amount | Also adds |
|---|---|---|
| Liquid chlorine 12.5% | 10.2 fl oz | none |
| Liquid chlorine 10% | 12.8 fl oz | none |
| Household bleach 8.25% | 15.5 fl oz | none |
| Household bleach 6% | 21.3 fl oz | none |
| Cal-hypo 65% | 2.05 oz | +0.71 ppm CH |
| Cal-hypo 73% | 1.83 oz | +0.71 ppm CH |
| Dichlor 56% | 2.38 oz | +0.91 ppm CYA |
| Trichlor 90% | 1.48 oz | +0.61 ppm CYA |
Typical label strengths sold in the US. Liquid chlorine strength is 'trade percent' (available chlorine by volume); dry products are % available chlorine by weight. Scale linearly: 20,000 gallons needs twice as much, +3 ppm three times as much.
Free chlorine targets by CYA level
Stabilizer protects chlorine from sunlight but also slows it down, so the more CYA you have, the more free chlorine you need. These figures follow the FC/CYA relationship popularized by Trouble Free Pool (minimum FC ≈ 7.5% of CYA, shock level ≈ 40% of CYA).
| CYA (ppm) | Minimum FC | Shock (SLAM) FC |
|---|---|---|
| 0 | 1 ppm | 10 ppm |
| 20 | 1.5 ppm | 10 ppm |
| 30 | 2.3 ppm | 12 ppm |
| 40 | 3 ppm | 16 ppm |
| 50 | 3.8 ppm | 20 ppm |
| 60 | 4.5 ppm | 24 ppm |
| 70 | 5.3 ppm | 28 ppm |
| 80 | 6 ppm | 32 ppm |
Frequently asked questions
How much chlorine do I add to a 20,000 gallon pool?
To raise free chlorine by 1 ppm in 20,000 gallons you need about 20.5 fl oz of 12.5% liquid chlorine or 4.11 oz of 65% cal-hypo. A typical daily top-up of 3 ppm takes about 1.92 qt of 12.5% liquid chlorine.
How much bleach do I put in my pool?
Plain, unscented household bleach works the same way as pool chlorine, just weaker. At 6% you need about 21.3 fl oz per 10,000 gallons for each 1 ppm; at 8.25% about 15.5 fl oz. Avoid "splashless" or scented bleach, which contains thickeners and fragrance.
What should my free chlorine level be?
It depends on your stabilizer (CYA). A widely used rule is to keep free chlorine at no less than 7.5% of CYA: at CYA 40, at least 3 ppm. Without stabilizer, 1 to 3 ppm is enough. See the table on this page for each CYA level.
Is liquid chlorine or tablets better?
Liquid chlorine adds nothing but chlorine and a little salt, so it is easy to keep balanced. Trichlor tablets are convenient but each 1 ppm of chlorine also adds about 0.6 ppm of CYA, which builds up over a season and weakens chlorine. Many owners use tablets early in the season and switch to liquid once CYA reaches the target.
Does liquid chlorine lose strength?
Yes. Sodium hypochlorite slowly breaks down, much faster when warm and in sunlight. A jug stored in a hot garage for a few weeks can be noticeably weaker than its label. Buy fresh, store it cool and shaded, and if doses consistently fall short, lower the strength in the calculator.
What is the difference between free chlorine and total chlorine?
Free chlorine (FC) is the chlorine still available to sanitize. Total chlorine (TC) also includes combined chlorine (CC), which has already reacted with contaminants. CC = TC − FC. If CC is above about 0.5 ppm, it is time to shock; use the pool shock calculator.
Related calculators
- Pool Shock CalculatorShock dose from target FC or CYA-based SLAM level.
- Pool Stabilizer (CYA) CalculatorStabilizer to add, or water to drain to lower CYA.
- Pool pH CalculatorAcid or soda ash to hit your target pH.
- Pool Volume CalculatorGallons or liters for any shape, including sloped floors.
- Pool Salt CalculatorPounds or kg of salt and number of bags.
Related guides
See all Pool Water Chemistry Calculators.